Pentathlon Science Resource Guide 2015-2016 1
AN INTRODUCTION TO ECOLOGY
2015–2016
PENTATHLON
SCIENCE RESOURCE
GUIDE
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INTRODUCTION
Natural ecosystems are driven by the interactions among the many organisms that live within
them as well as by the ways that the local climate, soils, and chemistry define the bounds of what
types of organisms can survive there. These interconnected relationships are the basis for the
field of ecology, which is our topic of study in this resource guide. Ecology is a broad and
continuously developing field. Scientists within this discipline study very diverse topics.
Population ecologists might be interested in the competitive relationships among multiple male
grouse for a mate; community ecologists may study the impacts of the reintroduction of a
predator, such as a wolf or grizzly bear, on other animals in the ecosystem; and landscape
ecologists might be concerned with the impacts that a widespread invasive species is having on
multiple habitat types for many wildlife species across vast areas.
A major emphasis of the discipline of ecology is that the many elements of an ecosystem are
naturally interconnected. Not only do organisms interact in ways that we can observe, such as
when trees grow rapidly and shade out competitors, as animals fight for territory, or as
herbivores graze on herbaceous species, but organisms are also connected by the ways that
energy, nutrients, and water cycle through different ecosystem components and regulate the
structure and function of the ecosystem. These relationships can also change over space and time
as energy sources are changed. Understanding of how complex ecological relationships vary
through space and time is an important and necessary step toward conserving biodiversity in our
natural world.
Finally, it is important to recognize that while humans have had an enormous impact on our
planet, we are simply part of the interconnected world in which we live. Humans have altered
atmospheric chemistry, changed species distributions and interactions, and influenced the
productivity of ecosystems. These changes often come full circle to impact the health, food
security, and well-being of the earth’s human inhabitants as well as that of the other 8.7 million
species with which we share our planet. In the course of this resource guide, we will examine
relationships between organisms and their environment, learn about the interconnectedness of all
life on the planet, and close with a look at how humans have changed these relationships.
NOTE TO STUDENTS: You will notice as you read through the Resource Guide that some key terms and
phrases are boldfaced. These terms are included in the glossary at the end of the Resource Guide.
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FIGURE 1.1
Levels of ecological organization. Ecology is the scientific study of the relationships between organisms and their environment.
German biologist Ernst Haeckel in 1860.Haeckel was the first to use the
term “oekologie” (ecology).
SECTION I:
INDIVIDUAL AND POPULATION ECOLOGY
An Introduction to Ecology What is Ecology?
Ecology is the scientific study of the relationships between organisms and their environment.
Ecologists, the scientists who study ecology, are often interested in the interactions that
determine the distribution (geographical location) and abundance (quantity) of a type of
organism. Some of these interactions are between the same or different kinds of organisms
(biotic interactions), and others are between an organism and its physical environment (abiotic
interactions). The more that these interactions are understood, the more we realize that in every
ecological system, organisms (including humans!) and their environment are interconnected, and
that altering one part of an ecosystem usually changes others as well. Ecologists are interested in
answering many types of research questions about these relationships: What accounts for
patterns of distribution and abundance? How do these patterns change over long timescales?
How have humans altered these patterns? And, will these patterns be impacted by a changing
climate?
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Ecological communities vary widely in scale and number of species, ranging from microscopic
communities of marine plankton to vast temperate forest communities and from uniform agricultural
communities to ecologically diverse regions, such as tropical rainforests.
A History of Ecology
Ecology as a study began relatively recently. The term “oekologie” (ecology) was first used by
the German biologist Ernst Haeckel (1834–1919) in 1866. The term is derived from the Greek
words oikos (meaning “house” or “dwelling”) and logos (“study of” or “science”). Haeckel
recognized that there was a strong relationship between ecology and the theory of natural
selection. The theory was promoted by Charles Darwin’s The Origin of Species in 1859.
Natural selection is the process by which individuals with better adapted inherited
characteristics tend to survive and reproduce more successfully than other individuals with less
well adapted characteristics. This framework provides a mechanism for the processes that control
the distribution and abundance of all organisms.
It is important to distinguish ecology from both environmental studies and environmentalism.
The field of environmental studies examines human impacts on physical, biological, and
chemical processes. Environmental studies tend to be very broad in scope, integrating ecology,
geology, economics, social studies, and philosophy. It has led to a social movement,
“environmentalism,” which seeks to take steps to minimize these human impacts on planet Earth.
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FIGURE 1.2
We can think about ecological interactions on multiple scales, from the individual organism to the entire planet.
Ecology, in contrast, is a broad science encompassing research questions on many spatial and
temporal scales. Sub-disciplines have arisen in more recent years, including Historical Ecology,
Wildlife Ecology, Plant Ecology, Human Ecology, Urban Ecology, Landscape Ecology, and
Disturbance Ecology, as well as many others.
The Ecological Hierarchy
We can think about ecological interactions on multiple scales, from the individual organism to
the entire planet. At each scale, a different set of patterns emerges, and different processes
determine the interactions that we see. Individuals often compete with one another for limiting
resources such as light, water, food, or nutrients. A population is a group of individuals of the
same species that live in a particular area and interact with each other. Populations of plants and
animals interact with one another in many ways, including competing for limiting resources.
Some populations are a food source for another population, while other populations may be
mutually beneficial, each providing a valuable resource or service to the other. The study of
populations of organisms, population ecology, often seeks answers to questions about how and
why the locations and numbers of populations change over space or time.
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FIGURE 1.3
Broad-scale patterns of climate and geology allow us to categorize similar landscapes and ecosystems into biomes (large-scale regions dominated by similar types of ecosystems). Collectively, all of the world’s ecosystems together make up the biosphere.
Two or more interacting populations of
plants and animals of different species
in the same area are referred to as
communities. Ecological communities
vary widely in scale and number of
species, ranging from microscopic
communities of marine plankton to vast
temperate forest communities and from
uniform agricultural communities to
ecologically diverse regions, such as
tropical rainforests. The study of the
interactions within and among these
communities is called community
ecology. Community ecologists might
ask research questions such as: “Why
do some places have more species than
others?”; “Why do high and low levels
of disturbance result in reduced species
diversity, but an intermediate level of
disturbance results in higher
diversity?”; and “How is community
structure altered when a top predator is
removed from the area?”
An ecosystem is a community of
organisms and the abiotic environment
(the non-living physical and chemical components) in which they live. Some examples of
ecosystems include tropical rainforests, intertidal regions, high-altitude deserts, and freshwater
marshes. We will discuss many of these ecosystems in depth a little later in this section.
Scientists who study ecosystem ecology are interested in the way climate alters the distribution
of biotic communities, the rates at which nutrients and water enter and cycle through a
community, and the way that soils and precipitation patterns alter the relationships between plant
communities.
Landscapes are patchworks of multiple communities and ecosystems, and are typically studied
at larger spatial scales, such as what one would view while standing on a mountaintop or from an
airplane. At the landscape scale, scientists are interested in researching topics including how
variability in soils and topography influences patterns of community composition and species
diversity. A major component of landscape ecology is the study of the effect of ecological
patterns (e.g., patches of habitat types, rivers, mountains, agricultural areas) on processes (e.g.,
wind and water movement, disturbance events, nutrient cycling).1
Broad-scale patterns of climate and geology allow us to classify similar landscapes and
ecosystems into biomes (large-scale regions dominated by similar types of ecosystems), such as
tropical grasslands, deserts, temperate forests, arctic tundra, etc. Collectively, all of the world’s
ecosystems together make up the biosphere, the highest level of biological organization. The
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FIGURE 1.4
The Scientific Method. Within each level of the ecological hierarchy, scientists use the scientific method to answer ecological research questions.
biosphere consists of all living organisms on the planet and their physical environment. At the
biome and biosphere level, scientists are interested in studying topics such as connectivity and
energy flow between biomes, global changes in climate, and large-scale extremes and norms of
carbon, energy, nutrients, and temperature.
The Scientific Method
Within each level of the ecological hierarchy, scientists use the scientific method to answer
ecological research questions. All scientific studies begin with observations of the natural world.
These observations lead to questions about the processes driving the phenomena observed. A
scientist then makes a hypothesis, or educated guess, based on prior experience and knowledge
about what is driving the observed process. A hypothesis should be very specific and should
include a statement of cause and effect and must also be formed in such a way that it is
quantifiable by the data to be collected. Testable predictions about the outcome of hypothesis
testing or experiments are then made. Then, experiments or observational studies are designed
and conducted, and data are collected and analyzed. The outcomes of experiments are interpreted
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to see whether they support or
reject the original hypothesis. If
the outcome of the experiment
supports the original hypothesis,
then conclusions are drawn about
the studied system, additional
questions or hypotheses may be
made to expand the scope of the
question, and additional data
collected. If the data collected are
not in support of the hypothesis,
then the original conceptual
relationships about how the
system works must be
reevaluated, and a new
hypothesis might be formed.
When a greater understanding is
gained as the result of the testing
of multiple related hypotheses, a
theory is formed to collectively
explain the results of a large
number of experimental
observations. The large body of
scientific literature and knowledge has come from centuries of scientists following the scientific
method to build upon our understanding of the natural world. Because ecological data are
constantly being collected at field sites and in laboratories across the world, our understanding of
ecology is always changing.
Geographic Ecology: The Abiotic Environment The physical environment sets the ultimate limits of an organism’s potential distribution. Long-
term trends in temperature, wind patterns, and precipitation, patterns of salinity or acidity, and
soil properties are among the important factors that limit where an organism can survive, grow,
and reproduce. The environment determines whether a particular area will be a tropical
rainforest, a montane lake, or a temperate grassland, setting the stage for all of the organisms that
will occupy the area. Within the bounds set by abiotic factors, competitive relationships between
organisms further limit the distribution of a species.
Temperature and Climate
We are all aware of the weather around us. The current temperature, humidity levels,
precipitation, and wind determine the clothing we choose, our method of transportation, and our
activity choices for the day. Over long periods of time, the long-term average weather pattern, or
climate, places limitations on the biological life that can successfully endure, grow, and
reproduce in an area. Variation in climate on large scales is due to the amount of solar radiation
that reaches the Earth’s surface at a given geographical location. We typically see maritime
climates (high humidity with little daily or seasonal fluctuations in temperature) in coastal
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FIGURE 1.6
A moving air mass picks up moisture as it travels over a body of water. As the air mass moves up the mountain range, it cools and condenses, releasing much of its moisture as rain or snow. The drier air mass then warms as it travels down the inland side of the mountain range.
The rain shadow effect is responsible for the temperate, rainy climates of the Pacific Northwest on the west side of the Cascade Mountains (left) and the arid deserts of the interior
Great Basin on the east side (right).
regions with an oceanic influence and continental climates (large seasonal and daily variation in
temperature) in the interiors of large land masses.
Topography often creates regional patterns in
temperature and precipitation, creating vastly
different ecosystem types within a small
geographic area. For example, we commonly
see moist, mild climates and tall, green
vegetation on the windward side of mountain
ranges near the ocean, and a harsher, arid
climate with drought-tolerant desert vegetation
on the inland (or leeward) side of mountains. A
moving air mass picks up moisture as it travels
over a body of water. As the air mass moves up
the mountain range, it cools and condenses,
releasing much of its moisture as rain or snow.
The drier air mass then warms as it travels down
the inland side of the mountain range. This
orographic or rain shadow effect is
responsible for the temperate, rainy climates of
the Pacific Northwest on the west side of the
Cascade Mountains, and the arid deserts of the interior Great Basin on the east side. Spatial
patterns in temperature and precipitation drive differences in vegetation (and their associated
animal) communities from the high-latitude tundra to temperate deciduous forests in the mid
latitudes to tropical rainforests at the equator.
Climate is variable in time as well as in space. Daily and seasonal patterns are driven by the
Earth’s rotation on its axis and its revolution around the sun. Longer term variation in climate is
impacted by large-scale cyclic patterns in ocean and atmospheric currents (e.g., El Niño/La Niña
events).
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FIGURE 1.7
The best-known fluctuation in high- and low-pressure cells in the Earth’s atmosphere is the El Niño Southern Oscillation (ENSO), in which unusually warm sea surface temperatures occur at the equator.
As Earth orbits the sun, its location,
relative to the sun, changes. This causes
different intensities of sunlight to reach the
Earth’s surface. The patterns of differential
solar radiation create seasonal climate
differences. Seasonal differences in
temperature are more noticeable in the
polar and temperate regions than in the
tropics.
Over time scales of years to decades,
climatic variation is caused by fluctuations
in high- and low-pressure cells in the
Earth’s atmosphere. The best-known of
these fluctuations is the El Niño Southern
Oscillation (ENSO), in which unusually
warm sea surface temperatures occur at the
equator. In North America, El Niño tends
to cause warmer and drier winters in the
Northwest and Midwest, and cooler and
wetter than average winters from Mexico
up to California. La Niña events produce
the opposite patterns, and usually follow El
Niño events. ENSO events are not well
understood, and are difficult to predict, but
generally occur at irregular intervals about
every three to seven years.
Over the past 500 million years, variation in Earth’s orbit has caused many episodes of cooling
and warming. Scientists speculate that these cooling and warming patterns are a result of
changing concentrations of greenhouse gases (atmospheric gases that absorb and reradiate the
earth’s radiation) in the earth’s atmosphere as well as due to variability in Earth’s orbit around
the sun. Clearly, these significant changes in climate over time have had a large impact on the
distributions of plants and animals. For example, although Antarctica is now covered with ice,
and plant and animal life is limited, fossil evidence shows it was once home to a diverse array of
plant and animal life.
Soils
Soil is another important part of the physical environment that helps to determine the distribution
of organisms. Soil is the medium in which plants grow. It provides physical support for plant
growth, assists air, nutrient, and water movement to plant roots, moderates temperature, and
protects from toxins. Soil properties influence the type of vegetation present, and thus, the
number and types of biological organisms that the ecosystem can support.
When we think about the plants that surround us, we often imagine the plant material consisting
of leaves, branches, stems, and flowers. Much of the plant, however, exists below ground. The
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FIGURE 1.8
Decomposition, the breakdown of complex organic compounds (i.e., those which make up plants and animals) into simpler ones, adds nutrient materials to the soil.
soil surrounding the roots anchors the plant and
provides support. Pores in the soil ventilate the soil:
roots take up oxygen (O2) and release carbon dioxide
(CO2), a process called respiration. Pores in the soil
also absorb water and hold it where it can later be
used by the plant roots. Plants need water
continuously to transport nutrients, maintain plant
tissues, regulate temperature, and photosynthesize.
Because precipitation is not continuous, it is
important that soils have enough water-holding
capacity to allow plants to survive during dry
periods. Soils also supply plants with the dissolved
mineral nutrients that are necessary for plant survival
and growth. Plants take up these nutrients through
their roots and incorporate them into the plant
tissues. Animals (including humans) then get their
nutrients by eating plant materials.
Soil is formed from the weathering and breaking
down of parent material, the underlying rock or
mineral substrate (usually bedrock or material that
has been transported by wind or water). Mechanical
weathering occurs as parent material starts to
physically break down. Processes such as
freeze/thaw and wet/dry cycles cause parent material to expand and contract, slowly breaking the
rock into smaller pieces over time and increasing the surface area. At the same time, the
influence of water, oxygen, and acids further break down rock materials in a process called
chemical weathering. Decomposition, the breakdown of complex organic compounds (i.e.,
those which make up plants and animals) into simpler ones, adds nutrient materials to the soil.
The processes of soil formation are very slow. It can take many thousands of years to develop a
complex organic soil that is capable of supporting diverse plant and animal communities.
Physical and chemical properties of soils vary. Physical properties such as color, texture,
moisture-holding capacity, and depth allow us to classify and map soils and predict their capacity
to support life. The color of a soil is easy to distinguish and tells us a lot about the composition.
Soils rich in organic matter are very dark. Reddish and yellow-brown soils indicate iron oxides,
manganese oxides give a purple hue, and quartz, gypsum, and calcium and magnesium
carbonates make the soil appear white. Soil particles can be classified into gravel, sand, silt, and
clay.
Soil texture refers to the amounts of different-sized soil particles found in soil. Soils are
considered to be clay, sand, silt, or a mixture of two or more of these types, depending on their
composition. Soil texture plays a major role in the soil’s water-holding capacity and is rated from
fine to coarse. Fine soils with high clay content hold water for much longer than sandy, well-
drained soils. Fine soils have much smaller pore spaces, which impacts the movement of air and
water and decreases the ability of plant roots to penetrate. Soil depth can vary widely across a
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FIGURE 1.9
Fine soils with high clay content hold water for much longer than sandy, well-drained soils. Fine soils have
much smaller pore spaces, which impacts the movement of air and water and decreases the ability
of plant roots to penetrate.
FIGURE 1.10
Over time soil becomes diversified with depth from the soil surface, becoming layered with the downward movement of organic material through the soil. This vertical layering is the soil profile, and each layer is referred to as a horizon.
landscape, depending on parent material,
slope, and vegetation type. Mountaintops
tend to have shallow, rocky soils, and level
ground at the base of slopes generally has
much more soil accumulation.
While a soil in a given area originates from
the same parent material, over time it begins
to become varied with depth from the soil
surface. It becomes layered with the
downward movement of organic material
through the soil. We call this vertical
layering the soil profile, and each layer
within the profile is referred to as a horizon.
The surface layer, or O horizon, consists of
organic material that accumulates from
decomposing plant materials such as needles,
twigs, and leaves. Just below this is the A
horizon, more commonly referred to as the
topsoil. While this layer is made up primarily
of weathered parent material, it tends to be
rich in organic materials due to the trickling
of materials downward from the O horizon.
The B horizon, sometimes called the
subsoil, has limited organic matter and often
has accumulated mineral particles due to leaching from
the topsoil. This horizon is usually quite dense, making
it difficult for many plants to extend roots down into
this layer. Finally, the C horizon lies beneath the
subsoil, and is comprised of unconsolidated materials.
There is low biological activity in this layer, and it
retains many characteristics of the parent material.
Below the C horizon is the bedrock or parent material
(sometimes called the R horizon). Very different soils
arise from regional differences in parent material,
climate, and vegetation communities.
Water and Light
Water is a critical component for all organisms, as its
presence in biotic tissues is needed to facilitate all
physiological functioning. The amount of water
present in an ecosystem is an important determinant of
what types of plants and animals can live there. As we
saw earlier, ocean and atmospheric currents largely
drive large-scale precipitation patterns, with regional
influences by topography. In addition, the moisture-
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FIGURE 1.11
The conversion of sunlight into carbon compounds by photosynthesis drives the production of energy for use by biological organisms.
holding capacity of an area’s soils impacts
how much of this precipitation is
effectively stored in the ecosystem for use
by biological organisms. Water availability
is also influenced by surface runoff
(overland flow of excess water), downward
percolation through pores in the soil,
evapotranspiration (the sum of the water
lost from evaporation plus transpiration—
the movement of water through a plant),
and groundwater flows. These processes
are the major pathways by which water
flows through an ecosystem. Extreme
conditions of very low or very high water
availability create unique challenges for
organisms that inhabit these areas, and
many have evolved unique strategies to
persist in these harsh environments.
The conversion of sunlight into carbon
compounds by photosynthesis drives the
production of energy for use by biological
organisms. Photosynthesis has two major
steps: the first is harvesting energy from
sunlight and the second is fixing carbon to
generate carbohydrates. The rate of
photosynthesis determines the supply of
energy available to organisms. This
impacts individual growth rates,
reproductive success, and, ultimately, the geographic range of a species. The availability of light,
then, is an important driver of ecosystem productivity.
In aquatic ecosystems, the greatest light availability is at the water surface. There is a rapid
decline of the absorption of light as the depth of the water increases. In terrestrial environments,
light is absorbed by the plants themselves, influencing the amount of light that can penetrate to
the Earth’s surface. In a dense forest, much of the light entering the ecosystem is absorbed by the
highest layers of the forest canopy, which is why it is often very dark in these types of forests,
even in the middle of the day. The amount of sunlight that does penetrate the canopy depends on
the type, quantity, and orientation of branches and leaves.
Seasonal changes in many ecosystems influence the amount of sunlight that is available at the
Earth’s surface. Temperate forests tend to have many deciduous trees, which lose their leaves in
the fall, allowing much more light to penetrate the canopy than in the summer months. In high-
latitude ecosystems, winter months bring much shorter days than summer months, reducing the
amount of time each day that sunlight is available for use by biological organisms. In tropical
ecosystems near the equator, day length, and thus light availability, is fairly constant year round.
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Some plants that live in high-salinity environments have roots that do not allow salt uptake, others have the ability to secrete excess salt through special glands on their leaves, and some isolate salt in internal organs.
Other Abiotic Factors: Wind, Salt, pH, Nutrients
Many other abiotic factors can limit or facilitate the successful survival, growth, or ability of an
organism to reproduce in an environment. Wind can cause significant erosion, either by
transporting existing particles, or by wearing down surfaces. Plants in areas prone to high wind
conditions have evolved strategies, such as flexible stems that bend without breaking, succulent
tissues that retain moisture, and narrow leaves (e.g., grasses, needles) and are often small
statured, to avoid the drying effects of high wind.
Salinity (a measure of the dissolved salt content in water) alters properties of water and can limit
an organism’s ability to absorb water. We usually think of the ocean when we talk about salt
water, as the Earth’s oceans contain 97 percent of its water. The salinity of the ocean varies. The
highest salinity in ocean water is found near the equator. Soils adjacent to oceans, such as those
in salt marshes, are similarly very saline. This creates very unique ecosystems that are capable
of persisting in these high-salinity environments. Some plants that live in high-salinity
environments have roots which exclude salt uptake, others have the ability to secrete excess salt
through special glands on their leaves, and some isolate salt in internal organs. Many animals
that live in estuary and tidal marsh habitats will move with the changing tide to maintain their
own salinity requirements. Some fish can drink salt water and excrete the salt through their gills.
Sea birds often excrete excess salt through specialized salt glands in their nasal cavities. Marine
mammals, while they live in high-salinity conditions, get most of the fresh water that they need
to survive from the food that they eat.
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FIGURE 1.12
Freshwater and marine ecosystems are linked as components of the hydrologic cycle, the process
by which water travels from the air to the earth and back to the atmosphere.
Nutrients are chemical elements that are required by all organisms for metabolism and growth.
Nutrients must occur in the environment for plants and animals to survive, but nutrient
concentrations vary considerably in different ecosystem types. Increases in some mineral
elements such as aluminum, hydrogen, and manganese can cause increased soil acidity, resulting
in an environment that is toxic to many organisms and sometimes restricts plant growth. Various
nutrient requirements and tolerances to each element determine the biotic life that can live and
thrive in these unique environments. On the other hand, certain necessary nutrients may become
limited, or in short supply, restricting growth of individual plants or preventing some species
from establishing in these limiting types of environments.
Geographic Ecology: Biomes
AQUATIC ENVIRONMENTS
Water covers about 75 percent of the Earth’s surface, which means that aquatic environments are
more widespread than terrestrial environments. The salinity of water greatly influences the
adaptations of organisms that live in these environments. Because of this, aquatic environments
are subdivided into two major groups: saltwater (or marine) and freshwater, each with unique
organisms and ecosystem processes. Freshwater and marine ecosystems are linked as
components of the hydrologic cycle, the process by which water travels from the air to the earth
and back to the atmosphere.
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FIGURE 1.13
Rocky intertidal zones lie at the boundary of the ocean and the beach and are influenced by the changing tide and the action of pounding waves.
Oceans cover about 71 percent of Earth’s surface and contain diverse marine ecosystems. Marine
ecosystems are often categorized by their location to shorelines and depth in the water column.
Differences in temperature, surface material, light availability, interactions with other organisms,
and water column pressure determine the distribution of marine organisms.
The area where the river meets the ocean is called an estuary and is characterized by variation in
salinity that fluctuates with the tides. Estuaries are very productive environments and are
important habitats for juvenile fish, shellfish, crabs, and sea grasses. Marshes are formed when
upland sediment is carried downriver and accumulates near ocean shorelines. They are
dominated by grasses, rushes, and forbs that are rooted underwater. Like estuaries, they are very
productive as nutrients are deposited in these areas, and they provide an important habitat for
many types of fish, crabs, birds, and mammals. Mangroves are coastal ecosystems inhabited by
salt-tolerant trees and shrubs. The plant roots trap mud and sediment, which build up and alter
the shoreline. They also are very productive and provide a habitat for many unique species across
the globe, such as monitor lizards, monkeys, manatees, and many fish and bird species.
Rocky intertidal zones (the rocky zone occupying the area between high and low tide) lie at the
boundary of the ocean and the beach and are influenced by the changing tide and the action of
pounding waves. Diverse plant and animal communities are tightly anchored to the rocky surface
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FIGURE 1.13
The open ocean, or pelagic zone, is stratified vertically. The benthic zone refers to the ecological zone at the bottom of a body of water. In the ocean, the benthic zone starts in the rocky intertidal area and continues along the ocean floor out to sea.
of these areas to keep from being washed away. Sessile (fixed) organisms (such as mussels) in
these communities must be able to cope with salinity, drying winds, wave action, and
temperature fluctuations. Mobile organisms (such as crabs and sea stars) often move to tide pools
during periods of low tide.
Sandy beaches may appear to have very little life associated with them. The changing tides and
constant wave action limit establishment by plants. However, beneath the sand surface small
organisms such as clams, crustaceans, and polychaete worms survive, protected from the wave
action and temperature and moisture fluctuations. These organisms feed on plankton and detritus
during high tide periods.
Coral reefs are usually found in warm, shallow coastal regions, generally at depths of less than
fifty meters. Corals are an aggregation of live organisms individually referred to as “coral
polyps,” which are animals closely related to jellyfish. Corals build their hard skeleton structure
by extracting calcium carbonate from seawater. These skeletons encase each individual polyp,
and the polyps live on top of the calcium carbonate skeletons of previous polyps, which
accumulate over time, building large structures called reefs. Corals live in close association with
algae. The algae live inside each polyp and provide energy through photosynthesis, while the
corals provide protection from grazing to the algae. The reefs provide structure and protection
for other organisms as well, making coral reefs much more diverse and productive than the open
ocean that surrounds them.
The open ocean, or pelagic zone, is
layered vertically. Light availability
decreases rapidly with depth in the water
column, which leads to rapidly changing
habitats. Throughout the pelagic zone,
the dominant organisms are tiny
phytoplankton (microscopic plants) and
zooplankton (microscopic animals and
juvenile stages of larger animals), with
the highest concentrations of
photosynthetic organisms in the layers
nearest to the water surface. These very
tiny organisms absorb nutrients directly
from the seawater and in turn are a food
source for much larger oceanic
organisms. Deeper in the water column,
light becomes limited, and biota become
few and far between. Small and
microscopic marine crustaceans feed on
the decaying material that falls from
layers above it. Fish, larger crustaceans,
octopuses, and squid are common deep
sea predators.
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Freshwater and marine ecosystems are linked as components of the hydrologic cycle, the process by which
water travels from the air to the earth and back to the atmosphere.
The benthic zone refers to the ecological zone at the bottom of a body of water. In the ocean, the
benthic zone starts in the rocky intertidal area and continues along the ocean floor out to sea. The
ocean bottom is sparsely populated by very unique organisms that often are not well studied due
to the difficulties of examining the ocean floor. The base of the benthic food chain is made up of
detritus from dead phytoplankton, marine mammals, birds, fish, and invertebrates. Polychaete
worms and crustaceans are diverse and abundant in these areas. Sea cucumbers and sea stars
graze on the organic matter on the ocean floor or filter food out of the water. Benthic predators
often use bioluminescence (the biochemical emission of visible light by living organisms) to
lure prey.
Freshwater ecosystems make up a small portion of the earth’s surface, but create important
linkages between terrestrial and marine environments. Rivers and streams transport nutrients and
material from terrestrial uplands downhill to the ocean. The smallest and highest elevation
streams are called first-order streams. When two first-order streams merge, they create a
second-order stream. The Amazon River in South America, the world’s largest river by
discharge, is a twelfth-order stream. Each stream has stretches of riffles, fast-moving portions
flowing over coarse surface, and pools, deep, slow-moving stretches with fine sediment. Fish
and other swimming organisms inhabit fast-moving portions of the main channels, and
invertebrates tend to live in or on the river substrate, feeding on dead organic matter.
The environments change from the high-elevation headwater streams to the lowland large rivers.
Headwater streams (orders 1–3) are often cold and well-shaded, and productivity is limited.
Dominant organisms tend to be aquatic insects that shred leaves and other organic matter,
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FIGURE 1.14
The smallest and highest elevation streams are called first-order streams. When two first-order streams merge, they create a second-order stream. FIGURE 1.15
Riparian ecosystem is a transition between the aquatic ecosystem and the adjacent terrestrial ecosystem. Riparian ecosystems occur at the boundary of land and water along rivers or other water channels.
breaking it down into small particles that
accumulate on the stream bottom. Medium-
sized creeks and smaller rivers (orders 4–6)
have more surface water exposed to the sun, so
water temperature is higher. As the slope
becomes less steep in the lower elevations, the
current slows down. Vegetation can more
easily establish and persist in these streams,
and there is increased production of algae.
Aquatic larvae feed on organic matter that has
been transported downstream as well as on
algae and rooted plants. Predators shift to
warmer-water species, and bottom-feeding fish
such as catfish and suckers are common. In
streams and rivers of orders 6 and above, the
channel becomes wide and slow-moving, with
increased flow volume. There is increased
sediment accumulation on the substrate, and
bottom-dwelling collectors of detritus are the
primary consumers. Most rivers eventually
meet the ocean, linking inputs from upland
terrestrial ecosystems to the open sea.
Riparian ecosystem is a transition between the
aquatic ecosystem and the adjacent terrestrial
ecosystem. Riparian ecosystems occur at the
boundary of land and water along rivers or
other water channels. Vegetation communities
are generally very different in these ecosystems
than in the adjacent upland areas due to greater
water availability. Riparian areas are very
important ecosystems for water and nutrient
cycling, energy flow, and wildlife habitat.
Freshwater wetlands are places where the land
is covered by shallow water, sometimes
seasonally, as in floodplains, swamps, ponds,
and the edges of rivers and lakes. Wetland soils
are typically saturated with water, and the
vegetation that grows in them is characteristic
of aquatic plants, dominated by flora such as
submerged water plants, floating vegetation,
and cattails, often surrounded by trees and
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FIGURE 1.16
This map shows the location (lighter areas) of the major deserts of the world.
shrubs. Many fish, amphibians, reptiles, mammals, and invertebrates are dependent on the
world’s freshwater wetland ecosystems.
TERRISTRIAL ENVIRONMENTS
Terrestrial biomes are classified in terms of temperature and precipitation gradients as well as by
the major plant life form that they contain (trees, grasses, and/or shrubs). They vary
considerably from warm, moist tropical forests to cold, dry polar regions. Plant growth forms are
indicative of the types of environments in which they live, from evergreen, broadleaved trees in
the tropics that are able to photosynthesize year round to cacti and shrubs with succulent (thick,
heavy foliage that can store water) leaves or stems that can store water for long periods of time in
desert regions to deciduous (annual shedding of leaves) forests in temperate regions that have
seasonal patterns of wet and dry. We will briefly discuss several of the major types of terrestrial
biomes in terms of their unique climate, seasonality, plant communities, and physical
characteristics.
Tropical Rainforests Tropical rainforests are found near the equator in areas where rainfall exceeds 2000 mm per year.
There is little seasonality or variability in the constant warm temperatures and persistent
moisture, causing continuous growth by plant species. These are some of the most productive
ecosystems in the world, and they contain a disproportionate amount of the planet’s biological
diversity. Both evergreen and deciduous trees are found in tropical rainforests, and the
availability of light determines the structure of the vegetation. Plant species must either be very
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FIGURE 1.17
Temperature and precipitation drive differences in vegetation (and their associated animal) communities.
competitive to use the sunlight in the upper
canopy, or they must adapt to the lower
light environment beneath dense layers of
vegetation.
Tropical Seasonal Forests and Savannas At latitudes nearer the Tropics of Capricorn
(23.5°S) and Cancer (23.5°N), rainfall
becomes much more seasonal, with distinct
wet and dry periods. Vegetation responses
to this seasonality include lower tree
densities, shorter-statured vegetation, loss
of leaves on deciduous trees during the dry
season, and a greater abundance of
understory plants (shrubs, grasses) relative
to rainforests. Frequent fires during the dry
periods and seasonal flooding both
contribute to the development of
woodlands or savannas (vegetation
communities in tropical and subtropical
ecosystems with trees and shrubs
intermixed with a dense grass understory).
Hot Deserts Deserts are sparsely populated with plants and animals, reflecting the harsh conditions of high
temperatures and long periods without precipitation. Many desert plants are succulent, allowing
them to store water in their tissues for long periods of time so that the plant can survive the
period of low water availability. Some desert annuals carry out their entire life cycle in just a
couple of weeks. After a period of precipitation, they germinate, flower, produce seeds, and then
die. The major deserts of the world include the Sahara, the Arabian deserts, the Atacama Desert,
the Chihuahuan Desert, the Sonoran Desert, and the Mojave Desert.
Temperate Grasslands Large expanses of grassland exist in both the Northern and Southern hemispheres at temperate
latitudes. These areas have great seasonal variability in temperature, with long periods of
freezing temperatures during a cold, dry winter, and warm, moist summers. Fire and grazing by
large herbivores are important natural disturbance processes in these ecosystems, limiting the
establishment of woody tree and shrub species and favoring an ecosystem dominated by grasses
and small herbs.
Temperate Shrublands and Woodlands Shrublands and woodlands develop in temperate regions with a winter rainy season. The
vegetation in these ecosystems tends to be small statured, with thick, stiff evergreen leaves. They
are generally well adapted to long dry periods and will slowly grow and photosynthesize in a
moisture-limited environment. Some coastal temperate shrublands include the fynbos of
Australia and the chaparral of North America. Inland shrublands and woodlands areas are
associated with the seasonally cold climates and often fall in the rain shadow of mountain ranges.
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Temperate deciduous forests occur where cold periods are prolonged enough to make ongoing photosynthesis inefficient, but where the growing season is long enough and the soils nutrient rich
enough to make regrowth possible in the spring.
The Great Basin of North America sits between the Cascade/Sierra Nevada crest to the west and
the Rocky Mountains to the east and is characterized by shrubs and infrequent, small trees.
Temperate Deciduous Forests Temperate deciduous forests occur where cold periods are prolonged enough to make ongoing photosynthesis inefficient, but where the growing season is long enough and the soils nutrient
rich enough to make regrowth possible in the spring. These forests occur in eastern North
America and on both the eastern and western edges of Eurasia. There are multiple vertical layers
to these forests, with a subcanopy of trees as well as shrubs and forbs below the upper canopy.
Temperate Evergreen Forests There is quite a bit of variability in the environmental conditions that support evergreen forest
growth in the temperate regions, from warm coastal areas to cool inland ecosystems. At the high
precipitation end of the spectrum, these forests are sometimes referred to as temperate
rainforests, and at the low precipitation end, drier forest types support frequent fire return
intervals of ten to twenty years, which promote the persistence of the species present. Soils in
temperate evergreen forests tend to be nutrient poor, in part because of the acidity in the leaves
that becomes incorporated into the soil profile. The diversity of these forests is usually lower
than that of either deciduous or tropical forest types. Dominant tree species in North America are
needle-leaved conifers such as pines, firs, hemlocks, and junipers.
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Boreal forests are primarily composed of coniferous species that resist damage from hard winter freezes.
The tundra in Greenland. The tundra is found in the Arctic as well as at the edges of the Antarctic
Peninsula.
Boreal Forests Continuous temperatures below freezing for up to six months are common in continental forests
above 50° latitude, presenting significant challenges for vegetation persistence. The air
temperatures can reach –50°C, and the soils may freeze for long periods of time, obstructing
water drainage. The forests in these regions are primarily composed of coniferous species that
resist damage from hard winter freezes. Decomposition of plant material is very slow due to the
cold temperatures, and the rate of plant growth exceeds that of decomposition, leading to a large
buildup of organic material on the forest floor. During seasonal summer droughts, fires can burn
slowly in this organic layer for many months or even sometimes years.
Tundra Above about 65° latitude, trees no longer dominate vegetation structure. In the tundra, there is a
permanently frozen subsurface soil layer called permafrost. Above the permafrost, the soil
thaws each summer and is frozen each winter. Permafrost keeps this soil layer chilled, even
during the growing season, limiting plant growth, microbial activity, and nutrient accumulation.
Low statured sedges, grasses, forbs, and shrubs persist, and mosses and lichens make up an
important component of the ecosystem. Vegetation development is simplified, normally with few
species and short-statured growth forms. The tundra is found in the Arctic as well as at the edges
of the Antarctic Peninsula.
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The Hawaiian hoary bat (Lasiurus cinereus) is the only species of terrestrial mammal native to the Hawaiian Islands. No other mammals have been able to cross the Pacific Ocean to disperse to the
Hawaiian Islands on their own.
THE ORGANISM AND ITS ENVIRONMENT
As we have seen, environment largely determines which species can live and successfully
reproduce in the unique abiotic conditions of an area. A range is the geographical area where a
particular species can be found. A species’ physical and behavioral characteristics reflect its
adaptation to a particular environment. Different environments provide a differing set of
challenges, and the set of characteristics that make a species well adapted to one environment
often limit its success in another. The diversity of life represents many evolutionary strategies
that allow different organisms to successfully survive, grow, and reproduce in their respective
environments.
Limits to Dispersal: Physical and Chemical Limits
All species have a niche, a set of optimal environmental conditions that are most conducive to
their successful photosynthesis, growth, survival, and reproduction and that define the way that
the species fits into an ecological community or ecosystem. This ecological niche is a result of
natural selection, as a species’ composition and behavior become ever more adapted to its
environment. As conditions move away from this set of optimal conditions, rates of these
processes decrease. Stress, then, is an environmental condition (which may be temporary) that
limits biological processes, lowering an organism’s rate of growth, survival, or reproduction. At
the limits of a population’s range, the physical environment limits an organism’s ability to obtain
the resources needed to grow and reproduce, thereby limiting the population within a fixed
geographical area (fundamental niche). Extreme conditions such as temperature or water stress
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(e.g., flooding, drought) or the presence of toxic chemicals can surpass an organism’s tolerance
levels, even within its normal geographic range.
Limits to Dispersal: Biotic Limits
Biotic interactions with other organisms can further limit the geographic distribution of a species
(realized niche). Competition between two or more species, herbivory, or predation may limit a
species’ distribution, even under optimal abiotic conditions. Similarly, two species may require
that they have overlapping distributions. For example, some pollinators visit specific flowers in
search of pollen and nectar and in the process pollinate the flowers. Both the plants containing
the flowers and the pollinators are limited by the range of the other organism. Organisms can
also be excluded from an area due to pathogens or parasites.
Adaptation and Natural Selection
Differential success is the idea that those organisms best adapted to a given environment will be
most likely to survive to reproductive age and have offspring of their own. Organisms that are
successful in their environments will be more likely to be successful in reproduction, and
therefore the better-adapted organisms will reproduce at a greater rate than the less well-adapted
organisms. The differential success of individuals within a population of organisms occurs when
individuals with particular heritable traits consistently have more offspring than individuals with
other heritable traits. For example, more favorable traits such as camouflage, rapid movement, or
large size might allow an organism to escape predation, thus living longer and having more
opportunities to reproduce. This process of natural selection is the key mechanism driving the
evolution (directional change) in biological populations over time. Evolution occurs as a species
gradually becomes increasingly different from its ancestors and better adapted to its (often
changing) environment.
Evolution is often described in terms of genetic change. The genes in all living organisms are
composed of deoxyribonucleic acid (DNA), and they specify how to build the proteins that are
the building blocks of biotic life. Any given gene can have two or more different forms (called
alleles) that result in the production of different versions of the protein. During sexual
reproduction, a single copy of each gene is inherited from an organism’s mother and another
from its father. Multiple combinations of alleles together designate an organism’s genetic
makeup, or genotype. The physical manifestation of that genetic makeup is called the
phenotype.
Over time, the frequencies of alleles in a population can shift toward phenotypes that make a
population better adapted to its environment. Natural selection serves to sort individuals in a
population, favoring those that have more favorable heritable traits over others. In The Origin of
Species, Charles Darwin outlined that natural selection is a product of two major conditions: 1)
variation in some heritable characteristic occurs among individuals within a population, and 2)
this variation results in differential survival and reproductive rates among individuals.
Population Dynamics
A population is a group of individuals of the same species that live in the same geographical area
at the same time and interact with one another. The relationships between individuals in space
and time determine, in part, the distribution of the species. These population dynamics
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determine the population size (the number of individuals in the population) and/or abundance
(the number of individuals in a given area).
TABLE 1: Factors that Limit Populations
Factors that cause a population to
increase
Factors that cause a population to
decrease
Abiotic
Favorable light
Favorable temperature
Favorable chemical environment
Too much or too little light
Too cold or too warm
Unfavorable chemical environment
Biotic
Sufficient food
Low number or low effectiveness of
predators
Few or weak diseases and parasites
Ability to compete for resources
Insufficient food
High number or high effectiveness of
predators
Many or strong diseases and parasites
Inability to successfully compete for
resources
Species Interactions
Biotic interactions can occur between members of the same species (intraspecific interactions)
or between two different types of organisms (interspecific interactions). In the framework of
population dynamics, we will discuss these processes as they relate to interactions between two
or more members of the same species, but it is important to note that the terminology and
processes are the same in the framework of community ecology, when multiple species interact
(and which will be discussed in the next section).
When resources (for example, nutrients, light, water, and food) are insufficient to satisfy all of
the organisms that depend on them, they are distributed at a disadvantage to at least some of the
individuals in the ecosystem. In some cases, a resource limitation results in a reduction of that
resource to all organisms equally, often resulting in a decline of the species and sometimes local
extinction. More commonly, resource limitation results in some dominant individuals continuing
to utilize the resources that are needed to the disadvantage of other individuals, negatively
impacting growth and development, and even leading to mortality. This intraspecific competition
in animals can result in limiting reproduction, altering behavior patterns, and increasing the
likelihood of disease and parasites. In plants, competition often results in reduced growth and
seed production.
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Section I Summary Ecology is the scientific study of the relationships between organisms and their environment.
Natural selection is the process by which individuals with better adapted heritable
characteristics tend to survive and reproduce more successfully than other individuals.
Ecological interactions take place on multiple scales, from the individual organism to the
entire planet. A population is a group of individuals of the same species that live in a
particular area and interacts with each other. Populations of plants and animals interact with
one another in many ways, including competing for limiting resources. Two or more
interacting populations of plants and animals of different species in the same area are referred
to as communities. An ecosystem is a community of organisms and the physical environment
in which they live. Landscapes are patchworks of multiple communities and ecosystems and
are typically studied at larger spatial scales.
Broad-scale patterns of climate and geology allow us to categorize similar landscapes and
ecosystems into biomes—large-scale regions dominated by similar types of ecosystems.
The scientific method is used by scientists to answer ecological research questions. All
scientific studies begin with observations that lead to questions about the processes driving
the phenomena observed. A hypothesis is then made about what is driving the observation.
Data are then collected and analyzed, and the hypothesis is revisited. When a greater
understanding is gained from an ecosystem as the result of the testing of multiple related
hypotheses, a theory is formed to collectively explain the results of a large number of
experimental observations.
The physical environment sets the ultimate bounds of an organism’s distribution. Long-term
trends in temperature, wind patterns, and precipitation, patterns of salinity or acidity, and soil
properties are among the important determinants that limit where an organism can survive,
grow, and reproduce. The environment determines whether a particular area will be a tropical
rainforest, a montane lake, or a temperate grassland.
Climate is the long-term average weather pattern for an area that places constraints on the
type of biological life that can survive in the area. Within a climatic regime, topography often
creates regional patterns in temperature and precipitation, creating vastly different ecosystem
types within a small geographic area.
Soil is an important component of the physical environment that helps to determine the
distribution of biological organisms. Soil provides physical support for plant growth,
facilitates air, nutrient, and water movement to plant roots, moderates temperature, and
protects plants from toxins. Soil properties help to determine the type of vegetation present
and the number and types of biological organisms that the ecosystem can support.
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Water is a critical component for all organisms, as its presence in biotic tissues is needed to
facilitate all physiological functioning. The amount of water present in an ecosystem is an
important determinant of what types of plants and animals can live there.
The conversion of sunlight into chemical energy by photosynthesis drives the production of
energy for use by biological organisms. Photosynthesis has two major steps: harvesting
energy from sunlight and fixing carbon to generate carbohydrates.
Water covers about 75 percent of the Earth’s surface, making it the planet’s dominant
environment. The salinity of water greatly influences the adaptations of organisms that live in
these environments. Because of this, aquatic environments are subdivided into two major
groups: saltwater (or marine) and freshwater, each with unique organisms and ecosystem
processes. Freshwater and marine ecosystems are linked as components of the hydrologic
cycle, the process by which water travels from the air to the Earth and back to the
atmosphere.
Terrestrial biomes are classified in terms of temperature and precipitation gradients as well as
by the major plant life form that they contain—trees, grasses, and/or shrubs.
A range is the geographical area in which a particular species can be found. Within that
range, individuals of that species are dispersed depending on resource availability and
physical and biological constraints to individual establishment or reproduction.
There are many ways that organisms of the same species interact with each other in the same
geographical area, constraining the number of organisms in a population. These population
dynamics determine the population size (the number of individuals in the population) and or
abundance (the number of individuals in a given area). Within populations, the dispersion of
individuals depends on the availability of resources, dispersal, and biotic interactions.
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SECTION II:
COMMUNITY ECOLOGY
In any ecosystem or habitat, a community is a unique collection of plants, animals, bacteria, and
fungi that interact with one another and with their environment in the same place and at the same
time. The community’s physical environment is usually loosely defined as a limited geographical
region. For example, a community might be all of the organisms that live within a particular
lake, sand dune, forest, or desert. These organisms may compete with one another for limiting
resources such as light, nutrients, food, and water, may rely on each other for food, or may
interact in a way that is mutually beneficial. Collectively, these interspecific interactions form
the field of ecology known as community ecology.
Community ecologists seek to understand how the number of species, their spatial arrangements,
and the interactions among them form the structure of the ecosystem. A simple measure of this
structure is the number of different species occurring in a defined geographical area, referred to
as species richness. While richness gives us an idea of the complexity of a community, there are
not equal numbers of each species represented. The evenness is the percentage that the
individuals of each species contribute to the total number of organisms of all species present, and
it gives us an idea of the “rareness” or “commonness” of any given species of interest. Species
diversity combines the species richness and evenness measures to give a measure of the
variability and variety of living organisms in an ecosystem.
TABLE 2: Species Richness and Evenness Two communities with the same number of species and total organisms can be divided in numerous ways. Consider the following
example of two simplistic ecological communities. Each community has only two species present, so the species richness is the
same. However, community 1 is much more homogeneous. In contrast, community 2 has a much greater evenness among the
numbers of species present.
TABLE 2:Species Richness and Evenness
Community 1 Community 2
Species 1 97 individuals 25 individuals
Species 2 1 individuals 25 individuals
Species 3 1 individual 25 individuals
Species 4 1 individual 25 individuals
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FIGURE 2.1
The term biodiversity can refer to variability at multiple scales, from genes to species to ecosystems.
BIODIVERSITY
Types of Biodiversity
Species diversity is one type of biodiversity, the variation of life. While we generally think of
biodiversity and species diversity as being synonymous, it is important to recognize that the term
biodiversity can refer to variability at multiple scales, from genes to species to ecosystems.
Genetic diversity is the variation in alleles present in a population that results in individual
differences in appearance, function, and behavior. These genetic differences among individuals
within a species increase the chance that the species will continue, as individuals differ in their
ability to grow, reproduce, and continue through multiple changing environmental conditions.
Species diversity tells us the variety of species that are found in an ecological community.
Ecosystem diversity refers to the different ecological communities that are found within a fixed
area. For the purposes of this discussion, we will be referring to biodiversity as the diversity of
species found within an ecological community.
Global Patterns of Biodiversity
Globally, scientists have estimated that there may be somewhere between 5 and 30 million
different species, although these numbers are just guesses, as only about 1.7 million species have
been identified and described.
However, these species are not
evenly distributed across the
planet. There is a huge amount
of variety of life from the very
equatorial tropics to the
species-poor polar regions.
In North America, there is a
trend of greater variety of tree
species in the east, and less
variety in the west, with a
hotspot of tree diversity in the
U.S. southern states. In
contrast, over the same area,
there is a trend of greater bird
diversity in the west, and fewer
species in the eastern part of the
continent. Small island
ecosystems have lower
biodiversity than large islands
or mainland ecosystems, with
very distant islands like the
Hawaiian archipelago being
relatively species poor.
Additionally, there are places of
high biodiversity that have been
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greatly threatened by human activity, called biodiversity hotspots. There are currently thirty-
five areas that have been designated as biodiversity hotspots on the planet, and they are often a
focus of biological conservation efforts.
The sub-discipline of community ecology seeks to answer questions about why these
distributions of diversity exist such as the following: Why are there more species in the tropics
than in the temperate regions? Why do we usually find a few dominant species and many less
common species in any one ecosystem? Why do woody species typically replace grasses and
herbs over time? As we start to understand more about an ecosystem, it becomes possible to start
to answer some of these questions.
Causes of Biodiversity
What drives these patterns of biodiversity? Clearly this is a difficult question with multiple
theories that explain these patterns. Some hypotheses for the observed patterns of diversity are
as follows:
1. The evolutionary speed hypothesis says that there are more species in some areas (like the
tropics) because speciation (the formation of new and distinct species by evolution) happens
faster in these areas or has been happening longer. This can occur if increased temperature
increases the rate of speciation, allowing species to diversify more rapidly than in more
temperate regions. Places with a long evolutionary history (like many places in the tropics)
will be more diverse than places with a shorter evolutionary history (for example, the
Hawaiian Islands, which are geologically very young, very distant from the mainland, and
have low species richness).
Biodiversity Hotspots of India
There are two biodiversity hotspots in India: the Eastern Himalaya hotspot and the Western Ghat
hotspot. The Eastern Himalaya hotspot contains the northwestern and northeastern states of India
as well as northern Pakistan, Nepal, and Bhutan, and includes Mt. Everest, the world’s tallest
mountain. Because these mountains rise very abruptly, they contain many unique ecosystems in
a relatively small area, from grasslands to subtropical forests to alpine meadows. There are an
estimated 10,000 plant species, almost a thousand bird species, and about three hundred mammal
species. Logging for agriculture, livestock grazing, and settlement has fragmented habitats in the
area, degraded many ecosystems, and caused large-scale erosion on steep slopes.
The Western Ghat hotspot spreads across six Indian states, encompassing a mountain region that
parallels the country’s western coast. The Western Ghat hotspot contains an estimated five
thousand plant species, over five hundred bird and one hundred mammal species, 179 amphibian
species, and 288 freshwater fish species. Clearcutting for tea, coffee, and teak plantations has
significantly fragmented habitats and threatened the biodiversity in this region.
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2. The geographic area hypothesis says that large areas have more diversity than smaller land
areas due to increased room for speciation and buffering against extinction. Larger areas have
more diversity of habitats that support a greater variety of species. Since the tropics contain a
much larger area than temperate zones, they have more species per unit area.
3. The interspecific interactions hypothesis says that the high species diversity in the tropics
is associated with greater competition and higher predation rates than are found in other
regions. In higher latitude regions, where physical conditions are generally more severe than
in tropical regions, abiotic factors (for example, extreme ranges in temperature associated
with seasons) mainly control natural selection. The species most adapted to the severe
seasonal extremes continues. In milder tropical regions, abiotic conditions are generally more
favorable for life; thus, predation and competition are bigger drivers of natural selection in
those areas. In areas with intense competition, natural selection should lead to highly
specialized species niches (the specific habitat requirements and functional role of an
organism in its community).
4. Climate is determined by the solar radiation, water, and temperature of a region, resulting in
varying energy availability in different regions of the globe. The ambient energy hypothesis
says that where there is more energy, there will be greater biodiversity. Global patterns of
biodiversity generally support this hypothesis, as we see higher biodiversity in areas with
climates that are more favorable for plant growth.
5. The productivity hypothesis says that greater production (the rate of generation of biomass
in an ecosystem) should result in greater biological diversity. This hypothesis, however,
seems to be disputed by most available data. Some of the world’s most diverse plant
communities, the fynbos in South Africa and the heath scrublands of Australia, for example,
are fairly low productivity sites. These communities, however, are near sites that occur on
better soils and have more productive vegetation, but are characterized by lower species
diversity.
6. Finally, the intermediate disturbance hypothesis describes smaller scale patterns of
biological diversity. This hypothesis says that in the absence of disturbance (the disruption of
an ecosystem, community, or population that changes the surface and resource availability,
and/or the physical environment), the few most competitive species should eliminate less
dominant competing species. The disturbance should result in a loss in species diversity.
However, when there are small periodic disturbances, such as herbivory, disease, flooding, or
fires, these disturbances open up areas of habitat. The absence of dominant species leads to
greater overall biological diversity.
These factors interact in complex ways to contribute to the patterns of biodiversity that we see
across the globe, and there are many exceptions to these patterns. A community ecologist’s task
is to describe and explain these interactions for a particular ecosystem being studied.
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TABLE 3: Causal Factors Driving Patterns of Biodiversity
(adapted from Currie 1991, Krebs 2008, and Willig et. al 2003)
Factor Reason
Evolutionary Speed More time and faster evolution leads to the evolution of new
species
Geographical Area Large, complex habitats provide more evolutionary niches
Interspecific Interactions Competition leads to resource partitioning (dividing a niche), and
predation allows similar species to coexist
Ambient energy Fewer species can tolerate climate extremes
Productivity Richness is limited by the division of energy among species
Disturbance Moderate disturbance slows competitive exclusion
Interspecific Interactions
We will look at the interactions between two or more individuals of different species of in greater
detail here. The different types of interactions are divided into what we will call negative species
interactions and positive species interactions. It is important to note that this is not synonymous
with something ecologically “good” or “bad.” Negative species interactions refer to all
interactions where one of the interacting species benefits from the relationship, and the other
species suffers, or where both species suffer from the association. In contrast, positive species
interactions occur when one or both (or all) species benefit from the association, but no species is
negatively impacted. Additionally, two or more species can coexist in an area but not interact by
utilizing different resources.
NEGATIVE SPECIES INTERACTIONS
Predation We can all picture a mountain lion preying on a deer, or a hawk on a field mouse, but perhaps
not everyone has considered that a Venus flytrap consuming an insect or a ladybug consuming
an aphid are also examples of predation. Predation occurs when one species (a predator) kills
and eats another organism, its prey. In this relationship, one species benefits, and the other is
harmed. Predation can have a strong influence on the population sizes of both predator and prey
species. Most commonly, increasing the number of prey species will result in an increase in the
numbers of the predator as well because there is an increase in its food source. On the other
hand, if predator numbers increase, the result is the decline in prey species as they are consumed.
And, when those prey species decline, eventually the predator abundance will decline due to
limited food supplies. Because it takes time for the population of one species to change in
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The process of herbivory differs from predation on animals in that herbivory
usually does not kill the plant, rather just removes part of it.
response to the other species population change, we often see predator-prey cycles, or regularly
spaced intervals of increases and decreases in the population sizes of the predator and prey.
Herbivory Animals such as grazers (which eat herbaceous plants)
and browsers (which eat woody plants) are herbivores,
animals that consume part of living plants or algae, often
feeding on many individuals at a time. The process of
herbivory differs from predation on animals in that
herbivory usually does not kill the plant, rather just
removes part of it. The removal of plant tissue may
impact the individual plant’s ability to survive or put it
at a competitive disadvantage with the surrounding
vegetation. Since most plants cannot move to escape
herbivory, many have evolved other strategies to protect
their tissues from herbivory. Some plants have structural
defenses, such as thorns, hairs, or spines to discourage
consumption, and others have chemicals to deter
animals or insects from consuming them. Both predation
and herbivory can have profound effects on the
distribution and abundance of their prey species, often
altering the structure and composition of ecological
communities and even sometimes causing shifts in
community types.
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Interspecific competition occurs when there is a limiting resource that both species require. Lions and hyenas occupy the same ecological niche and compete for the same prey.
Competition Interspecific competition negatively impacts two or more species. Interspecific competition
occurs when there is a limiting resource that both species require, such as water, nutrients, or
light, that is in short supply. There are two major types of competition: interference competition
and exploitation competition.
Interference competition occurs when one species actively attempts to exclude another species,
as when a blue jay chases other birds from a birdfeeder. Exploitation competition occurs when
one species more efficiently utilizes a limiting resource than another species does. Some
common examples of exploitation competition include lion and cheetah populations competing
for gazelles at the same time, or two species of shrubs competing for soil nutrients and water.
Interspecific competition, particularly exploitation competition, can alter the structure of the
community and can temporarily or sometimes permanently impact the availability of the limiting
resource.
Parasitism At least half of the species on our planet are symbionts, or organisms that live in conjunction with another organism. The majority of these relationships are parasitic, where the parasite consumes
the tissues or robs the resources of the host organism. In some cases, the parasite is a pathogen,
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Clownfish are protected from predator fish by the stinging tentacles of the sea anemone. In return, the clownfish chases away polyp-eating fish and fertilizes the anemone.
causing disease in the host species. There are many common examples of parasites impacting a
human host such as head and body lice, tapeworms, scabies, and ticks. Parasitic species that live
on the surface of their host are called ectoparasites. Many fungal and insect ectoparasites attack
crop and horticultural species, causing significant amounts of damage. Athlete’s foot, mites,
fleas, and ticks are commonly found living on the surface tissues of animals.
Endoparasites are parasitic species that live inside their host organism and feed on the host
organism or rob the host of nutrients. Many human bacterial diseases are the result of parasites,
including tuberculosis, which is caused by the species Myobacterium tuberculosis, and the
bubonic plague, caused by the bacterium Yersinia pestis. Endoparasites are mostly protected
from the external environment by their host species. Although they are provided with a constant
source of food (usually the host’s tissue), they are at constant risk from attack by the host organism’s immune system. Both types of parasitism and any accompanying disease can greatly impact the ecological
function of a species. Parasites can serve as population regulation controls, removing the weakest
members in a population, and can weaken the defenses of an organism such that it loses a
competitive advantage or can make an organism, population, or species more vulnerable to
predation or additional disease.
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A common obligate mutualistic relationship is the interaction between a fungus and algae that form a lichen.
Amensalism A relationship between organisms of two different species in which one is unaffected and the
other is negatively impacted by the association is called amensalism. Amensalisms can occur
either through competition, when a larger or stronger organism eliminates a smaller or weaker
organism from obtaining necessary resources, or by antibiosis, when one organism is damaged
or killed by a chemical that the other organism secretes. An example of antibiosis is the black
walnut tree, which secretes a toxin called juglone that injures or kills many other plants that grow
in the root zone of the tree.
POSITIVE SPECIES INTERACTIONS
So far we have been discussing interactions in which one or more species are negatively
impacted by another; however, other symbiotic relationships have evolved such that one species
is positively impacted by the other, or even both species are positively affected by the
relationship.
Mutualism
A relationship between members of two species in
which both members benefit from the association is
known as mutualism. We can find examples of these
relationships in plants and animals (including humans!).
There are two major types of mutualism. One type is
known as obligatory mutualism, where one species
cannot survive without the other species. A common
obligate mutualistic relationship is the interaction
between a fungus and algae that form a lichen. The
fungus would not survive without its algal partner,
which makes food for the fungus. The fungus in turn
provides protection for the algae and obtains nutrients
and water for the lichen. The second type of mutualism
is facultative relationships, where the organisms both
benefit from being together, but it is not required for
their survival. One of the most commonly used
examples demonstrating a facultative mutualistic
relationship is that of the clownfish and the sea
anemone. Clownfish are protected from predator fish by
the stinging tentacles of the sea anemone. In return, the clownfish chases away polyp-eating fish
and fertilizes the anemone.
Commensalism
A relationship between two organisms of different species in which one organism benefits from
the association and the other is neither benefitted nor harmed is known as commensalism. The
relationship between cattle egrets and cattle (or other domestic livestock) is one of the most
frequently used examples: Cattle egrets forage for insects and other prey items in fields where
cattle are grazing. As cattle move around, they stir up the insects, making it easier for the cattle
egrets to capture their prey. The cattle egrets benefit from this relationship, but the cows are not
impacted either positively or negatively.
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Commensalism in India: The Trickster in Folklore and in Nature Indian folklore tells the story of a hungry jackal that went out one night in search of food in a
local village near his jungle home. A pack of village dogs chased the jackal away, and he fell
into a tub of indigo dye, turning himself blue. The blue jackal continued into the jungle and
encountered a lion, the King of the Jungle. The jackal introduced himself as Chandru, the
protector of all jungle animals. He told the lion that he would only protect the jungle in exchange
for food and shelter. For a time, all of the jungle animals brought
him the best of food, but when the monsoon rains came, they
washed all of the blue dye off of Chandru, and the animals saw that
they had been tricked and that he was just a simple jackal. The
animals chased the trickster into the jungle, and he was never seen
again.
As in the story, in India, the golden jackal functions as a bit of a
trickster in the commensal relationship it shares with the tiger. The
jackal often lives just outside of towns and villages and only is active at night. Lone jackals are
scavengers and will trail a single tiger and feed on the remains of the tiger’s prey. The tigers
generally ignore the jackals and neither benefit nor are negatively impacted by the jackal’s
presence.
COMMUNITY ORGANIZATION AND STRUCTURE The processes of predation, herbivory, parasitism, mutualism, and commensalism are strategies
that an individual organism uses to acquire the resources that it needs for metabolism, growth,
and reproduction. All of the interactions among species in an ecological community serve to
provide order to the community. In this section, we will look at some of the ways that a
community is structured by transferring this energy among the organisms in the ecological
community.
Trophic Cascades
We can look at the way that energy flows through an ecosystem by two major pathways. Energy
flow may be controlled by the rate at which energy enters an ecosystem and is converted to
organic matter (“bottom-up” control) or can be controlled by the rate at which top predators in an
ecosystem consume organic matter, influencing the composition and abundance of organisms
lower on the food chain (“top-down” control). Changes in the distribution or abundance of
organisms at one trophic level can have profound impacts on organisms at other trophic levels.
The reintroduction of wolves (the top predator) to Yellowstone National Park in 1995 allowed
researchers to study the trophic cascade involving wolves, elk, and aspen. In the absence of
wolves (before 1995), elk population numbers in the park had been at very high levels, resulting
in high herbivory of the aspen stands. When wolves were released back into the park, they began
to regulate numbers of elk, thus releasing some of the browsing pressure on the aspen.2
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FIGURE 2.3
Trophic levels of a terrestrial and an aquatic food web.
The complex, interwoven set of relationships between trophic levels within an ecosystem is
referred to as a food web.
Food Webs
The example just discussed described one set
of relationships between aspen, elk, and
wolves, but in reality, the ecosystem is much
more complex. There are other plants that elk
browse on, such as willows and cottonwoods,
other animals that browse on aspen (deer), and
other animals that wolves prey upon (moose,
deer). This more complex, interwoven set of
relationships between trophic levels within an
ecosystem is referred to as a food web. The
species in a food web are either a basal species,
an intermediate species, or a top predator. A
basal species is one that does not feed upon
any other species but is eaten by other species.
These species include plants and algae. An
intermediate species is both a food source for
others as well as one that eats other organisms,
such as herbivores. A top predator eats other
species but is not a food source for any other
species. In the Yellowstone example, the
aspen, willows, and cottonwoods are basal
species, the elk, deer, and moose are
intermediate species, and the wolves are the
top predators.
Interactions between species in a food web are
often more complex than simply the predator-
prey relationship. When a predator consumes
one species of prey, this may alter the
competitive relationships between that type
of prey and other associated species. These
indirect effects of predation may be very
important to ecosystem dynamics and should
be considered when trying to understand the
complex interactions at play.
Keystone Species
Some species fill roles in a community that
are critically important to the functioning of
the community. These keystone species
largely determine ecosystem structure, and in
their absence the ecosystem would
dramatically change.3 Keystone species are
often not particularly common in the system,
but their impact on system structure and
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Sea stars were one of the first organisms to be recognized as a keystone species.
function is disproportionately large. Sea stars were one of the first organisms to be recognized as
a keystone species. They are critical to the survival of many other species in the intertidal zone
because they are predators of the mussels and barnacles that would otherwise eat most of the
organisms in the tide pools.
DISTURBANCES
In the past several years, we have seen unusually high numbers of large wildfires in the western
United States. Periods of prolonged drought, a legacy of suppression of fire, and a warming and
drying climate have brought large fires to the forefront of the media. Because of the negative
attention that these extreme fire events are getting in recent years, it is easy to forget that fire and
other disturbances are natural parts of the way that ecosystems function and are often required
for the persistence of some species in an ecological community. More generally, a disturbance
is a discrete event in time that disrupts ecosystem, community, or population structure and
changes substrate and resource availability as well as changes the physical environment.
Types of Disturbances
Disturbance can be natural or anthropogenic (human-caused) and is either exogenous
(originating outside of the ecosystem), such as fire, drought, or wind, or endogenous (originating
inside the ecosystem), such as native diseases and pathogens. Fire is probably what most of us
think of first when we mention disturbance, but wind, flooding, insect herbivory, disease,
earthquakes, and volcanic eruptions are also common natural disturbance processes. In addition,
humans can initiate disturbances that have dramatic impacts on natural ecosystem functioning.
Some that are particularly destructive include logging, industrial pollution, livestock grazing,
introduction of invasive species, and conversion of land for urban areas or agriculture.
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A ponderosa pine ecosystem is characterized as having a frequent, low intensity fire regime.
Measures of Disturbance
We can characterize a disturbance by both spatial and temporal components including size, type
(e.g., flood, fire), frequency (how often the disturbance returns; e.g., 100-year flood, 30-year fire
return interval), intensity (degree of impact to the community; e.g., high-intensity fire),
seasonality (e.g., summer wildfire, winter floods), etc. The disturbance system, or regime,
describes the way that these components combine to create the natural patterns of disturbance in
a particular ecological community. For example, a ponderosa pine ecosystem is characterized as
having a frequent, low-intensity fire regime. In this ecosystem, fire typically returns about every
ten to twenty years and consumes only the understory vegetation, leaving the trees intact. These
low-intensity surface fires maintain the ponderosa pine forests in an open park-like ecosystem,
with large canopy trees, little understory vegetation, and a litter layer of pine needles.
Adaptations to Disturbance
Plants and animals that share a long evolutionary history with the disturbance regime of their
ecosystem have adaptations to survive or regenerate following a disturbance event. Plants are
either adapted to survive a disturbance event, or to quickly colonize after the disturbance has
passed. As we just discussed, ponderosa pine are adapted to survive low-intensity surface fire.
They have very thick bark, needles with high moisture content, and insulated buds, allowing
them to persist though repeated fire events. Many cacti have evolved to sustain periods of
prolonged drought. Their tissues are capable of storing water for long periods of time, and they
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The seeds of the ohia tree are very small and can travel by wind very long distances and germinate in small cracks in the bare lava where pockets of moisture are trapped.
have very shallow roots that enable them to utilize very
small amounts of precipitation. Other plants can quickly
germinate after a disturbance. The native Hawaiian Ohia tree
is one of the first species to colonize the new lava flows on
the active Kilauea volcano. The seeds of this tree are very
small and can travel by wind very long distances and
germinate in small cracks in the bare lava where pockets of
moisture are trapped.
Animals similarly can survive the disturbance regime of an
area by utilizing one or more mechanisms. Many animals
simply flee the disturbance area, by moving away from a
burn area or upland from a flood zone. Burrowing animals
can survive a fire, windfall, or tornado by staying beneath a
protective layer of soil. Lungfish can survive periods of
drought by burrowing in the mud and secreting a mucus coat
that hardens and protects them until there is water available
again.
Stability and Resilience
The adaptations that plants and animals have evolved that
allow them to persist in areas where disturbance is a natural
part of the ecosystem contribute to the stability of the
biological system. Stability is the ability of an ecosystem to recover after a disturbance. It is
usually thought that increased biodiversity leads to increased stability (though there are
exceptions), and thus stability is often measured in terms of numbers or biomass. However, it can
also be measured by an ecosystem’s resistance to change, or resilience. A resilient community is
capable of undergoing disturbance and change and will still maintain the same function,
structure, diversity, and ecological interactions that were present before disturbance.
Plantago or Indian Psyllium Desert plants have unique strategies that allow them to survive and persist
in very harsh arid environments. Species of Plantago or Indian psyllium are
native to the arid regions of North India. They have evolved a special
mucilage (a thick, gelatinous substance) that makes up about 30 percent of
their seed coat. This coating swells when wetted and retains water,
protecting the seed from desiccation while germinating.
People in India often combine the seeds with fruit juice or stewed fruit.
Crushed seeds are added to oil and vinegar as a treatment for rheumatism or
other problems with swelling. The seeds are commonly exported for manufacturing of Metamucil, a fiber
laxative. The same water-holding mucilage that protects desert seeds also absorbs water and toxins in the
human digestive tract and acts as a soothing lubricant.
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FIGURE 2.4
This diagram depicts forest succession. Succession tends to move from small-statured, short-lived herbaceous
plants toward taller, longer-lived woody species.
SUCCESSION
Disturbance events are often the starting point for ecological succession, the directional change
in species composition, structure, and resource availability over time that is driven by biotic
activity and interactions as well as changes in the physical environment.
Community Change
At the core of succession is change. Changes can be compositional as the relative numbers of
species present changes, and structural, as forests slowly take over areas that were previously
grassland. The mechanisms driving that change are the colonization and extinction of species in
response to abiotic processes and biological interactions. The theoretical endpoint of succession
is a climax community. It is the stable endpoint that experiences very little ongoing change,
until a large disturbance resets the successional clock. In reality, however, at any given time
there are areas on a landscape that are in a climax stage and others at different stages of
succession.
Very generally, succession tends to move from small-statured, short-lived herbaceous plants
(often called pioneer species, the first species to establish after a disturbance) toward taller,
longer-lived woody species. FIGURE 2.4 depicts a landscape with areas at different times since
fire. The newly burned areas show the starting point for succession. In the early post-fire
environment, grasses, wildflowers, and weedy species often establish within the first year or two.
Several years after a fire, longer-lived species such as shrubs and perennial forbs may start to
dominate the plant community, and seedlings of the canopy species will begin to germinate.
Mechanisms of Succession
Species occurrence following succession is a result of 1) the species that can get there and
establish first (dispersal and colonization) and 2) species that can persist and reproduce
(competition). In addition, there is an element of chance as to which species will start the
succession, depending on which species got there first.
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FIGURE 2.5
Diagram depicted forest succession.
Ecological Climax, Stability, and Alternative Stable States
Classic ecological theory says that succession is moving directionally toward a stable climax
community. In nature, we see this occurring in many areas, where a disturbance resets the
successional clock. The ecosystem moves through different successional stages in a predictable
way from grassland to shrubland to hardwood forest to evergreen forest. The directional process
of succession toward a climax community assumes that succession is predictable and repeatable.
However, in reality, sometimes different combinations of species develop in the same area under
similar environmental conditions. We call these other situations alternative stable states. Multiple
stable states can occur when there is an addition or exclusion of a dominant or strongly
interacting species during any phase of succession. In many cases, human actions have been the
cause of the shift toward an alternative stable state.
Gap Dynamics
We often think of disturbances as being large-scale events, but it is important to recognize that
they can also occur on much smaller scales, such as when a heavy wind event knocks down one
or a few trees, or when heavy rainfall causes erosion on a part of a hill slope. These small-scale
disturbances create gaps in the climax vegetation, and succession occurs within these gaps as it
does on a larger scale. These small scale successional dynamics are often referred to as gap
phase dynamics and occur between larger disturbance events, allowing increased light to enter
the ecosystem and serving to diversify the community.
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Section II Summary
A community is a unique collection of species that interact with one another and with their
environment in the same place and at the same time. These organisms may compete with one
another for limiting resources such as light, nutrients, food, and water; may rely on each
other for food; or may interact in a way that is mutually beneficial. Collectively, these
interspecific interactions form the field of ecology known as community ecology.
Species diversity, the variety of species found in an ecological community, is one type of
biodiversity. While we generally think of biodiversity and species diversity as being
synonymous, it is important to recognize that the term biodiversity can refer to variability at
multiple scales, from genes to species to ecosystems.
There is a major gradient of diversity from the very diverse equatorial tropics to the species-
poor polar regions, as well as other generalized patterns of diversity. Community ecology
seeks to understand these patterns and generally attributes them to many interacting causal
factors: evolutionary speed, geographic area, interspecific interactions, ambient energy,
productivity, and patterns of disturbance.
Interactions among species can benefit one, both, or neither of the species involved.
Predation occurs when one species (a predator) kills and eats another organism, its prey.
Herbivory is the process of an animal or insect consuming part or all of a plant, and is
sometimes considered to be a special type of predation. Competition occurs when there is a
limiting resource that both species require, such as water, nutrients, or light that is in short
supply, and both species are negatively impacted. Parasitism occurs when one species lives
in or on another species and feeds on its host or uses its resources.
Mutualism is a relationship between members of two species in which both members benefit
from the association. Commensalism is a relationship between two organisms of different
species in which one organism benefits from the association and the other is neither
benefitted nor harmed. All of the interactions among species in an ecological community
serve to provide order to the community.
Energy flow may be controlled by the rate at which energy enters an ecosystem and is
converted to organic matter (“bottom-up” control) or can be controlled by the rate at which
top predators in an ecosystem consume organic matter, influencing the composition and
abundance of organisms lower on the food chain (“top-down” control). Changes in the
distribution or abundance of organisms at one trophic level can have profound impacts on
organisms at other trophic levels.
The complex, interwoven set of relationships between trophic levels within an ecosystem is
referred to as a food web.
Keystone species fill roles in a community that are critically important to the functioning of
the community. These species may not be particularly numerous, but they largely determine
ecosystem structure, and in their absence the ecosystem would dramatically change.
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Disturbances can be natural or anthropogenic and are characterized by their type, size,
frequency, intensity, and seasonality. Plants and animals that share a long evolutionary
history with a disturbance regime have adaptations to survive or regenerate following a
disturbance event. The adaptations that plants and animals have evolved that allow them to
persist in areas where disturbance is a natural part of the ecosystem contribute to the stability
of the biological system.
Disturbance events are often the starting point for ecological succession, the directional
change in species composition, structure, and resource availability over time that is driven by
biotic activity and interactions as well as changes in the physical environment. Generally,
succession tends to move from small-statured, short-lived herbaceous plants toward taller,
longer-lived woody species.
Species occurrence following succession is a result of 1) the species that can get there and
establish first and 2) species that can persist and reproduce. Multiple stable states can occur
when there is an addition or exclusion of a dominant or strongly interacting species during
any phase of succession, altering community dynamics and the successional pathway. In
many cases, human actions have been the cause of the shift toward an alternative stable state.
Small-scale disturbances create gaps in the climax vegetation, and succession occurs on them
as it does on a larger scale. These small-scale successional dynamics are often referred to as
gap phase dynamics and occur between larger disturbance events.
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FIGURE 3.1
The sun’s heat warms the atmosphere, drives the Earth’s water cycle, moves air
and water currents, and is transformed into chemical energy that drives the
production of carbohydrates and other carbon-based compounds that are the
building blocks of organic life.
SECTION III: ECOSYSTEM, LANDSCAPE, AND
GLOBAL ECOLOGY
Ecosystem ecology studies the links between multiple organisms and their physical environment
as an integrated system. An ecosystem level approach is critical in the management of the
planet’s natural resources since it considers the interconnected relationships between multiple
biotic systems, including humans and their abiotic environment. An ecosystem approach takes a
big picture view of multiple plant and animal communities and of how energy and material
cycles through and between them.
Ecosystem level studies
often seek to understand
the factors that determine
the pools (amount or
quantity) and fluxes
(transfers or flows) of
material and energy in an
ecosystem. Some of the
materials that move
through ecosystems
include naturally derived
materials such as carbon,
water, nutrients, and
minerals as well as
materials derived by
humans such as
pesticides, herbicides, and
chemical contaminants.
Ecosystem processes
(such as photosynthesis
and decomposition) are
ways that energy and materials are transferred from one pool to another.
Energy Cycling
Sunlight is the ultimate source of energy that keeps Earth’s ecosystems functioning. The sun’s
heat warms the atmosphere, drives the Earth’s water cycle, moves air and water currents, and is
transformed into chemical energy that drives the production of carbohydrates and other carbon-
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FIGURE 3.2
The flow of energy through an ecosystem is the story of how carbon cycles through the food chain.
based compounds that are the building blocks of organic life. We can think about energy cycling
in an ecosystem largely as the movement of these carbon-based compounds through the tissues
of live and dead plants and animals, from the microscopic to the macroscopic.
There are two forms of energy: kinetic energy, the energy of an object in motion, and potential
energy, stored energy that is available for performing work. There are two fundamental laws that
govern the use and storage of all energy. The first law of thermodynamics states that matter
cannot be created nor destroyed. Matter can change forms—as, for example, when a solid
changes to a liquid with the addition of heat, or to a gas with the addition of even more heat—but
it never disappears completely. The second law of thermodynamics says that energy disperses
from being localized to spread out unless it is prevented from doing so. For example, when you
turn off the burner on a stove, it is still very hot, but without the continuous source of energy
(heat), the warmth present in the burner will begin to disperse, warming the area around it, and
the burner will begin to cool down.
These same principles apply to our ecological systems. Energy and matter are not created out of
thin air, nor are they destroyed. They simply change form. A plant is consumed and an herbivore
grows. An herbivore dies, decomposes, and replenishes the organic matter in the soil. As energy
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FIGURE 3.3
a) A trophic pyramid; b) A food web
is transferred from one organism to another, a portion of that energy is stored in living tissue, and
a part is dissipated as heat to the surrounding air. In this way, the flow of energy through an
ecosystem is the story of how carbon cycles through the food chain.
Primary Production
You may remember from earlier that the conversion of sunlight into carbon compounds by
photosynthesis drives the production of energy for use by biological organisms. Photosynthesis
has two major steps: the first is the harvesting of energy from sunlight and the second is the
fixing of carbon to generate carbohydrates. The rate of photosynthesis determines the supply of
energy available to organisms.
The rate that sunlight is converted by autotrophic organisms (organisms which produce organic
compounds; for example, plants on land and algae in water) via photosynthesis into organic
compounds is referred to as primary productivity. Gross primary productivity (GPP) is the
total rate of photosynthesis, or the total energy obtained by autotrophs. However, autotrophs
must use some energy in the process, reducing the total productivity rate. The rate of energy
stored after accounting for the energy expended is referred to as net primary productivity
(NPP). This productivity of an ecosystem is also sometimes referred to as a rate of production
and is measured in units of energy per unit area per unit time (such as grams per square meter per
year). The stored energy found at a given area at a given time is often referred to as biomass,
which is simply the amount of organic material that can be found at a given area at any given
time. We can measure the total amount of biomass in an agricultural pasture or in a body of
water if we are interested in the amount of organic matter that is stored in that area.
In terrestrial ecosystems, temperature, heat, and nutrients control rates of primary productivity.
Generally, NPP increases with increasing mean annual precipitation, mean annual temperature,
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and a longer growing season. Places that are very warm and moist, such as tropical rainforests,
have extremely high rates of NPP. In contrast, places that are warm but dry, such as deserts, have
low rates of productivity.
In aquatic ecosystems, the major controls on primary productivity are temperature, light, and
nutrient availability. As we discussed earlier, light availability decreases with depth in the water
column; therefore, primary production also decreases with depth. Microscopic phytoplankton
(free-floating algae, protists, and cyanobacteria) perform the majority of the ocean’s primary
production, form the basis of the oceans’ food web, and fix large amounts of carbon. Oceanic
plants, like their terrestrial counterparts, need nutrients for growth. The two most important
nutrients for phytoplankton growth are nitrate and phosphate.
In some conditions, there is no sunlight available to support photosynthesis. Certain bacteria
have evolved to these conditions and are able to synthesize energy from the oxidation of
inorganic materials. We find this method of energy production, called chemosynthesis, in many
places where there is no light availability, such as deep sea communities, in hot springs, in the
soil, and in mammalian intestines. Some scientists have hypothesized that chemosynthesis may
have played a more significant role early in Earth’s evolution than what we see today.
Primary production is not constant over time. Rates of photosynthesis and plant growth can vary
greatly with season and plant age. Places with distinct cold seasons or dry periods have a period
of plant dormancy, where primary productivity pauses. In warmer temperate and moist tropical
regions, there is little difference in primary productivity between seasons. Differences in annual
temperature and precipitation can affect the rates of plant growth as well. As we discussed,
temperature and moisture are major controls on primary productivity, so we expect warm years
with increased moisture to be more productive than years that are cold and dry. In forested
ecosystems, productivity is impacted by the age of the dominant trees. Young trees
photosynthesize and grow very rapidly, but older trees put more of their energy into the
structural components of the stem and branches and reduce growth rates overall.
NUTRIENT CYCLING
We talked about the way that carbon is fixed by primary producers and travels through the food
chain. Similarly, other essential elements are incorporated into living tissues and move through
an ecosystem. These cycles of nutrient movement through ecosystem components are called
biogeochemical cycles. The origin of these elements is either the atmosphere (like in the carbon
cycle) or from the weathering of rocks and minerals. They are incorporated into soil or water and
are taken up by plants, thus becoming fixed in living tissues and traveling a path through the
food chain. When plants and animals die, the elements are returned to the soil as dead organic
matter and are used by various decomposers, which in turn return the elements to their mineral
form, where they again become available for plant uptake. In this way, minerals are recycled
through an ecosystem.
Decomposition
The key to nutrient recycling through an ecosystem is decomposition, the breakdown of organic
material by decomposer organisms and the release of simple, soluble organic and inorganic
nutrients by those organisms as waste materials. Decomposers are bacteria and fungi that feed on
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FIGURE 3.5
The cycles of nutrient movement through ecosystem components are called biogeochemical cycles.
dead organic matter or detritus. Bacteria are the dominant decomposers of dead animal material,
and fungi are the dominant decomposers of decaying plants. The undecomposed organic matter
on the soil surface is known as litter and is abundant in most ecosystems. Large soil animals
such as earthworms, nematodes, and insects break up this litter into progressively smaller
particle sizes, increasing the surface area available for further chemical breakdown. This
chemical process, known as mineralization, breaks the bonds between carbon molecules and
inorganic nutrients, making them available for uptake by plant roots and starting the nutrient
cycle again. Like most biological processes, rates of decomposition are greatly impacted by
climate. Decomposition proceeds faster in areas with warmer temperatures and greater moisture
availability.
HUMAN ECOLOGY An exponentially growing human population is currently impacting our planet more than any
other species has done in Earth’s history. Our hunter-gatherer ancestors depended on plant
productivity and an abundance of animals for subsistence. With the initial development of
agriculture about 12,000 years ago, our dependence on planetary resources shifted from natural
systems to agricultural systems. In the mid-eighteenth century, humans’ relationship with
natural resource use changed dramatically with the Industrial Revolution. A significant shift in
the use of human and animal labor to mechanized labor provided the means for large-scale
production of food, but also a much larger and more destructive ecological footprint. Our current
system of agriculture, manufacturing, transport, and storage of the food needed for a human
population that exceeds 7 billion has had many profound impacts on our planet.
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Our current system of agriculture, manufacturing, transport, and storage of the food needed for a human population that exceeds 7 billion has had many profound impacts on our planet.
Human Population Growth
The root cause of all of the adverse impacts that humans are having on ecosystems is population
growth—there are simply too many of us for the resources available, and every day over 228,000
more people are added.4 At the time when agriculture was first initiated, the planet supported
about 5 million people and was growing at a rate of about 0.05 percent per year. During the mid-
1700s, the Industrial Revolution caused increases in global population growth rates. Around
1800, world population reached one billion people. The second billion was reached 130 years
later (1930), the third 30 years later (1959), the fourth 15 years later (1974), the fifth 13 years
later (1987), the sixth in 12 years (1999), and the seventh 12 years later (2011). We are expected
to reach 8 billion people in the year 2030 and nine billion by 2050. You can see that in recent
years the growth rate has slowed considerably, as people have become more aware of human
impact on the environment, yet our population continues to grow rapidly.
Endangered Species and Ecosystems
The field of conservation biology is concerned with the decline of biological populations and
the critical ecosystems in which they occur. In most cases, their decline is a direct result of
human impacts to habitats due to fragmentation, land cover conversion, and depletion or
pollution of resources such as food and water. Conservation biologists identify species or
populations that are already at a small population size or that are at risk of becoming endangered
in the future due to declining numbers. Once a species is recognized as at risk of becoming
endangered or extinct, the potential causes of its decline must be recognized and remedied.
Aggressive actions need to be taken to reverse the factors driving species to extinction.
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A male Asiatic lion. The Asiatic lion as well as the Bengal tiger and leopards are in rapid decline in India due to
frequent poaching.
Threats to Biodiversity
Overkill Ongoing fishing, hunting, or gathering beyond a rate from which a species can rebound is referred to as overkill. Overkill and can quickly threaten a species, particularly one that has a low rate of natural increase. Due to frequent poaching, the Asiatic lion, Bengal tiger, and leopard are in rapid decline in India and are found in restricted habitat patches. Similarly, the Hawaiian sandalwood tree has been overharvested across the island chain and some species are now endangered, and all are in decline. We see this story repeated in commercial fisheries around the globe. Marine fishery numbers are difficult to estimate, and overharvesting has caused the collapse of many fish stocks. Habitat Fragmentation and Land Cover Change In several of the previous sections we
have discussed how human activities
have threatened ecosystems, often
causing the extinction of some of the
organisms that depend on them for
habitat. Habitat destruction is the
leading cause of species extinctions.
The primary reason for these habitat
transformations is to expand
agricultural land to meet the dietary
needs of a rapidly expanding human population. The tropical biomes are often the focus of
discussions on habitat loss as the most rapid rates of forest conversion for agriculture still occur
in these areas.
Biotic Invasions Humans have intentionally or unintentionally introduced countless species of plants and animals
into areas outside of their historical range. Not all introduced species will do well in a new
environment, but the few that do are often superior competitors because they are no longer
limited by their native competitors, predators, and diseases. Animal invaders impact native
species through direct competition, grazing on native plants, and alteration of habitat. Plant
invaders outcompete native plants and alter nutrient cycling, hydrology, and fire regimes, further
degrading an area. One of the most problematic plant invaders in the American West is
cheatgrass, an annual grass that has spread extensively over sagebrush steppe ecosystems,
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replacing vegetation. Cheatgrass is extremely flammable and has decreased the time between
successive fire events in many areas.
In aquatic ecosystems, invasive species also have profound impacts. The Great Lakes region has
been particularly impacted due to its function as a major shipping port. A notorious invader to
this region has been the zebra mussel, which has been linked to the decline of native clams and
mussels by competition and to the demise of many thousands of sea birds, as they carry avian
botulism.
Pollution
Pollution is present in terrestrial, marine, and aquatic systems across the globe. In many cases
pollutants cause physiological stress to individual plants and animals as well as degrade habitats.
Pollutants can overload nutrient levels in rivers and streams, lead to respiratory, heart, and lung
disease, and can be found in the bloodstream and organs of animals that live far from the original
source. There are several major types of pollution: air pollution (e.g., emissions of harmful gases
or particles), water pollution (e.g., dumping of waste materials, chemicals, or microorganisms
into a water body), and land pollution (e.g., improper storage and leaching of domestic or
industrial waste materials). Many times the various types of pollution interact and occur
simultaneously, such as the dumping of solid waste and subsequent leaching into nearby
waterways and groundwater.
Climate Change
Global climate change has become perhaps the greatest ecological challenge of our time. Over
geological time, changes in the planet’s climate have profoundly impacted the types of biological
organisms that have existed in different time periods. Currently, we are undergoing a period of
change in our climate that is unprecedented. While historical warming and cooler patterns have
occurred, they were driven by variability in the tilt of the Earth. The changes that we are
experiencing today are a direct result of human activities. The burning of fossil fuels is adding
heat-trapping greenhouse gases to the atmosphere, causing warmer temperatures, stronger
storms, melting of polar regions, a rise in the sea level, and longer periods of drought. These
changes are impacting the environment as well as human quality of life in many ways.
Water shortages are becoming common. In places where water is limited, agriculture suffers and
hydroelectric plants have a reduced capacity to generate electricity. Plants and animals that live
in and near water bodies are not always able to adapt to the reduced flow levels and warmer
water temperatures. Increased incidences of drought and flooding damage crops and food storage
capabilities. Disease may spread more readily as temperatures increase. Many disease-carrying
insects such as mosquitos and ticks cannot survive very cold periods. As winters get warmer,
these insects’ capacity to survive year round is increased. This short list gives just a few of the
projected results of a warming climate. As you can imagine, they are certain to be detrimental to
humans as well as the other species with which we share the planet. It is critical that we take
rapid and deliberate steps to slow or reverse these trends and build a future based on
sustainability of our natural resources.
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Sustainability
Sustainable stewardship of planetary resources requires that humans do not use resources at a
faster rate than they can be replaced by biological processes. This requires us to understand
ecological processes such as groundwater recharge to prevent our exhaustion of the water
supply; to understand the complex successional patterns of regrowth of woody species after a
timber harvest; and for us to reduce fossil fuel extraction and use fossil fuels at a level that no
longer results in an increase in atmospheric greenhouse gases.
You have probably seen the common bumper sticker or heard the slogan “Think Globally, Act
Locally,” which has been used in many ways to urge people to make small decisions daily on a
personal level that consider the health of the planet. On an individual level, this can mean
making decisions to reduce water use, purchasing things that are made from recycled content and
minimal packaging materials, choosing foods grown without agricultural chemicals, walking,
bike, or take public transportation rather than drive, and to elect policymakers who share a vision
for environmental sustainability.
Globally, large changes will need to be made in the coming years to reduce emissions, shift to
alternative sources of energy, and cut waste in energy, water, and other resource use. Individual
decisions, community adaptations, and international cooperation will all be necessary to mitigate
the human-caused damage that has already been done to planetary natural resources and to
conserve them for future generations.
Section III Summary
Ecosystem ecology studies the links between multiple organisms and their physical
environment as an integrated system. An ecosystems approach takes a big picture view of
multiple plant and animal communities and examines the ways that energy and material cycle
through and between them.
Ecosystem level studies often seek to understand the factors that determine the pools (amount
or quantity) and fluxes (transfers or flows) of material and energy in an ecosystem.
Ecosystem processes (such as photosynthesis and decomposition) are ways that energy and
materials are transferred from one pool to another.
Sunlight is the ultimate source of energy that keeps Earth’s ecosystems functioning. Energy
cycling in an ecosystem is the movement of carbon-based compounds through the tissues of
live and dead plants and animals.
There are two forms of energy: kinetic energy, the energy of an object in motion, and
potential energy, stored energy that is available for performing work.
There are two fundamental laws that govern the use and storage of all energy. The first law
of thermodynamics states that matter cannot be created nor destroyed. The second law of
thermodynamics says that energy disperses from being localized to spread out unless it is
prevented from doing so.
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The rate that sunlight is converted by autotrophic organisms via photosynthesis into organic
compounds is referred to as primary productivity. Gross primary productivity (GPP) is the
total rate of photosynthesis, or the total energy obtained by autotrophs. The rate of energy
stored after accounting for the energy expended is referred to as net primary productivity
(NPP).
In terrestrial ecosystems, temperature, heat, and nutrients control rates of primary
productivity.
In aquatic ecosystems, the major controls on primary productivity are temperature, light, and
nutrient availability. Microscopic phytoplankton perform the majority of the ocean’s primary
production, form the basis of the oceans’ food web, and fix large amounts of carbon.
Chemosynthesis is the synthesis of organic compounds using the energy released from
chemical reactions instead of the energy of sunlight, and it occurs where there is no light
available, such as in deep sea communities, in hot springs, in the soil, and in mammalian
intestines.
Rates of photosynthesis and plant growth can vary greatly with season and plant age. Places
with distinct cold seasons or dry periods have a period of plant dormancy, where primary
productivity pauses. In warmer temperate and moist tropical regions, there is little difference
in primary productivity between seasons. Differences in annual temperature and precipitation
can affect the rates of plant growth as well.
Organisms that cannot make their own food rely on primary producers as their energy source.
All animals, fungi, and most bacteria are heterotrophs, consuming plant or animal material
for maintenance and growth. The generation of biomass of heterotrophic organisms is called
secondary production.
Cycles of nutrient movement through ecosystem components are called biogeochemical
cycles. The origin of these nutrients is either the atmosphere or from the weathering of rocks
and minerals. These nutrients are then incorporated into soil or water and are taken up by
plants, thus becoming fixed in living tissues and traveling a path through the food chain.
The key to nutrient recycling through an ecosystem is decomposition—the breakdown of
organic material by decomposer organisms, and the release of simple, soluble organic and
inorganic nutrients by those organisms as waste materials.
An exponentially growing human population is currently impacting our planet more than any
other species has done in Earth’s history.
The field of conservation biology is concerned with the decline of biological populations and
the critical ecosystems in which they occur. In most cases, their decline is a direct result of
human impacts to habitats due to fragmentation, land cover conversion, and depletion or
pollution of resources such as food and water.
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The major threats to biodiversity include overkill, habitat fragmentation, biological
invasions, pollution, and climate change. Global climate change has become perhaps the
greatest ecological challenge of our time. Currently, we are undergoing a period of change in
our climate that is unprecedented. While historical warming and cooling patterns have
occurred, they were driven by variability in the tilt of the Earth. The changes that we are
experiencing today are a direct result of human activities. It is critical that we take rapid and
deliberate steps to slow or reverse these trends and build a future based on sustainability of
our natural resources.
Sustainable stewardship of planetary resources requires that humans do not use resources at a
faster rate than they can be replaced by biological processes. Individual decisions,
community adaptations, and international cooperation will all be necessary to mitigate the
human-caused damage that has already been done to planetary natural resources and to
conserve them for future generations.
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CONCLUSION
“When we try to pick out anything by itself, we find it hitched to everything else in the universe.”
~John Muir (1838–1914)
Often new advances in the science of ecology reveal a new linkage between organisms and their
abiotic environment. The more we learn, the more we are forced to appreciate that no species
goes unaffected by outside processes, but each species is interdependent upon the world around
it. In this resource guide, we have discussed the way that climate, soils, water, nutrients, and
topography define the bounds of organisms’ distribution. Within those bounds, the relationships
between individuals of the same species determine which ones will persist and provide the
genetic material for the next generation of organisms. Interactions between members of different
species along with abiotic factors determine the geographic distribution of a species.
Once we start to understand the biotic and abiotic relationships of multiple ecosystems, and
understand how energy, water, and nutrients flow through and between them, we gain a
landscape-level perspective on the interconnectedness of the different communities and
ecosystems that a landscape encompasses. Globally, the same pattern persists. Multiple
landscapes are pieced together to form a planet that is connected by flows of energy, water,
carbon, and other elements and is variable in space and time. These connections are not simple—
they are generally far more complicated than we yet understand. To begin to understand them, it
is important that ecologists study the planet as an integrated whole, and one in which humans are
just a single species among the many others with which we coexist.
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GLOSSARY
A horizon – the topsoil; this layer of soil is made up primarily of weathered parent material, but
is relatively rich in organic materials due to leaching from the O horizon above it
Abiotic – of or pertaining to the physical environment
Abundance – the number of individuals of a species in a given area, or the relative amount of
species in a particular ecosystem
Age structure – the proportion of the population in each age class
Allele – one of two or more different possible forms of the same gene, resulting in genetic
variability within a population
Ambient energy hypothesis – says that where there is more energy, there will be greater
biodiversity
Amensalism – a relationship between organisms of two different species in which one is
unaffected and the other is negatively impacted by the association
Antibiosis – a relationship between organisms of two different species in which one is
negatively impacted by a substance produced by the other; a specific type of amensalism
Autotrophic organisms – organisms that produce their own nutritional organic compounds
B horizon – sometimes called the subsoil, contains limited organic matter and often has
accumulated mineral particles due to leaching from the topsoil
Basal species – species that do not feed upon any other species but are eaten by other species
Benthic zone – the ecological zone at the bottom of a body of water
Biodiversity – the variation of life
Biodiversity hotspots – places of high biodiversity that have been greatly threatened by human
activity
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Biogeochemical cycles – cycles of nutrient movement through ecosystem components
Bioluminescence – the biochemical emission of visible light by organisms
Biomass – the amount of organic material that can be found at a given area at any given time
Biomes – large regions dominated by similar types of ecosystems
Biosphere – the highest level of biological organization encompassing all of the world’s
ecosystems
Biotic – of or having to do with life or living organisms, particularly in their ecological
relationships
C horizon – the soil layer that lies beneath the subsoil and is comprised of unconsolidated
materials; there is low biological activity in this layer, and it retains many characteristics of the
parent material.
Carrying capacity – the maximum population size of a species that a given ecosystem can
sustain
Chemical weathering – the breakdown of rock materials by water, oxygen, and acids
Chemosynthesis – the process by which microbes create energy by converting carbon molecules
and nutrients into organic matter in the absence of sunlight
Climate – the long-term average weather pattern for an area, which places constraints on the life
that can successfully endure, grow, and reproduce in an area
Climax community – the theoretical stable endpoint in the successional trajectory that
experiences very little ongoing change until disturbance resets the successional clock
Commensalism – a relationship between two organisms of different species in which one
organism benefits from the association and the other is neither benefitted nor harmed
Community – two or more interacting populations of plants and animals of different species in
the same area
Community ecology – the study of the interactions within and among ecological communities
Conservation biology – the study of the decline of biological populations and the critical
ecosystems in which they occur
Continental climates – climates characterized by large seasonal and daily variation in
temperature
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Deciduous – describes a type of woody vegetation that loses its leaves in the fall, allowing much
more light to penetrate the canopy than in the summer months
Decomposition – the breakdown of complex organic compounds into simpler ones, which add
nutrient materials to the soil
Density-dependent factors – factors that impact growth, death, and birth rates differentially
depending on the initial size of the population for a given area
Density-independent factors – factors that impact birth and death rates proportionally with
population size, regardless of how many individuals were initially present
Disturbance – a discrete event in time that disrupts ecosystem, community, or population
structure and changes substrate and resource availability and changes the physical environment
Ecological niche – a set of optimal environmental conditions that are most conducive to the
successful photosynthesis, growth, survival, and reproduction of a species, and that defines the
way that a species fits into an ecological community or ecosystem
Ecology – the scientific study of the relationships between organisms and their environment
Ecosystem – a community of organisms and the physical environment in which they live
Ecosystem diversity – the diversity of ecological communities that are found within a fixed area
Ecosystem ecology – the study of the way climate alters the distribution of biotic communities,
the rates at which nutrients and water enter a community, and the way that soils and precipitation
patterns alter the relationships between plant communities and other organisms
Ecosystem processes – the ways that energy and materials are transferred from one pool to
another
Ectoparasites – parasitic species that live on the surface of their host
Endogenous – originating inside the ecosystem
Endoparasites – parasitic species that live inside their host organism and feed on the host
organism or rob the host of nutrients or other resources
Environmental studies – a field of study that examines human impacts on physical, biological,
and chemical processes
Estuary – the ecosystem where the river meets the ocean
Evapotranspiration – the sum of the water lost from evaporation plus transpiration
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Evenness – the percentage that the individuals of each species contributes to the total number of
organisms of all species present
Evolution – directional change in populations over time
Evolutionary speed hypothesis – says that there are more species in some areas because
speciation happens faster in these areas or has been happening longer
Exogenous – originating outside of the ecosystem
Exploitation competition – occurs when one species utilizes more of a limiting resource or uses
a limiting resource more efficiently than another species
Facultative mutualism – a type of mutualistic relationship where the organisms both benefit
from being together, but it is not required for their survival
First law of thermodynamics – states that matter cannot be created nor destroyed
First-order streams – the smallest and highest elevation streams
Food web – an interwoven set of relationships between trophic levels within an ecosystem
Fundamental niche – niche based on environmental factors
Gap phase dynamics – small-scale successional dynamics that occur between larger disturbance
events, allowing increased light to enter the ecosystem and serving to diversify the community
Genetic diversity – variation in alleles present in a population that results in individual
differences in appearance, function, and behavior
Genetic drift – change in the genetic composition of a population due to chance or random
events rather than by natural selection, resulting in changes in allele frequencies over time
Genotype – the genetic makeup of a particular organism
Geographic area hypothesis – states that large areas have more diversity than smaller land
areas
Greenhouse gases – atmospheric gases that absorb and reradiate the earth’s radiation
Gross primary productivity (GPP) – the total rate of photosynthesis or the total energy
obtained by autotrophs
Heterotrophs – organisms that consume plant or animal material for maintenance and growth
Horizon – each layer within the soil profile
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Hydrologic cycle – the process by which water travels from the air to the Earth and back to the
atmosphere
Hypothesis – an educated guess based on prior experience and knowledge about what is driving
an observation that is phrased in such a way that it is scientifically testable
Interference competition – occurs when one species actively attempts to exclude another
species
Intermediate disturbance hypothesis – says that in the absence of disturbance, a few most
competitive species become dominant; when there are small periodic disturbances, biological
diversity is greater.
Intermediate species – a species that is both a food source for others as well as a consumer of
other species
Interspecific interactions – biotic interactions between members of different species
Interspecific interactions hypothesis – high species diversity in the tropics is associated with
greater competition and higher predation rates
Intraspecific interactions – biotic interactions between members of the same species
Keystone species – species that fill roles that are critically important to the functioning of the
community
Kinetic energy – the energy of an object in motion
Landscape ecology – a field of ecology that examines large-scale spatial patterns and their
relationship to ecological functioning
Landscapes – patchworks of multiple communities and ecosystems, typically studied at larger
spatial scales
Litter – the fresh, undecomposed organic matter on the soil surface
Mangroves – coastal ecosystems inhabited by salt-tolerant trees and shrubs
Maritime climates – climates characterized by high humidity with little daily or seasonal
fluctuations in temperature
Marsh – an ecosystem type that is formed when upland sediment is carried downriver and
accumulates near ocean shorelines; dominated by grasses, rushes, and forbs that are rooted
underwater
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Mechanical weathering – the physical breakdown of parent material by processes such as
freeze/thaw and wet/dry cycles that cause parent material to expand and contract, slowly
breaking the rock into smaller pieces over time and increasing the surface area
Metapopulations – interacting groups of populations of the same species that are dispersed
among patchy habitats but have occasional migration and interaction
Mineralization – the process where bonds between carbon molecules and inorganic nutrients are
broken, making them available for uptake by plant roots
Mutualism – a relationship between members of two species in which both members benefit
from the association
Natural selection – the process by which individuals with better adapted heritable characteristics
tend to survive and reproduce more successfully than other individuals
Net primary productivity (NPP) – the rate of energy stored after accounting for the energy
expended
Niche – a set of optimal environmental conditions that are most conducive to the successful
photosynthesis, growth, survival, and reproduction of a species and that define the way that the
species fits into an ecological community or ecosystem
Nitrogen fixation – the assimilation of nitrogen into organic compounds
O horizon – the surface layer of soil, consisting of organic material that accumulates from
decomposing plant materials
Obligatory mutualism – a type of mutualistic relationship whereby one species cannot survive
without the other species
Orographic or rain shadow effect – A moving air mass picks up moisture as it travels over a
body of water. As the air mass moves up a mountain range, it cools and condenses, releasing
much of its moisture as rain or snow. The drier air mass then warms as it travels down the inland
side of the mountain range.
Overkill – ongoing fishing, hunting, or gathering beyond a rate from which a species can
rebound
Parasite – an organism that consumes the tissues of the host organism or robs it of its food or
other resources
Parent material – the rock or mineral substrate that underlies the soil profile
Pathogen – an organism that causes disease in a host species
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Pelagic zone – the open ocean
Phenotype – the physical manifestation of an organism’s genetic makeup
Photosynthesis – the conversion of sunlight into carbon compounds, which drives the
production of energy for use by primary producers
Pioneer species – the first species to establish after a disturbance
Pools – 1) deep, slow-moving stretches of streams with fine sediment; 2) the amount of material
or energy in an ecosystem
Population – a group of individuals of the same species that live in a particular area and interact
with one another
Population dynamics – ways in which organisms of the same species interact with each other in
the same geographical area
Population ecology – the study of populations of organisms, which often seeks to find answers
to questions about how and why the locations and numbers of populations change over time
Population size – the number of individuals in the population
Potential energy – stored energy that is available for performing work
Predation – an interspecific relationship by which one species, a predator, kills and eats another
organism, its prey
Predator-prey cycles – regularly spaced intervals of increases and decreases in the population
sizes of the predator and prey
Primary productivity – the rate that sunlight is converted by autotrophic organisms via
photosynthesis into organic compounds
Production – the rate of generation of biomass in an ecosystem
Productivity hypothesis – says that greater production should result in greater biological
diversity
Range – the geographical area in which a particular species can be found
Realized niche – a niche based on environmental factors and the presence of other species
Resilience – an ecosystem’s resistance to change
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Respiration – the process by which organisms take up oxygen (O2) and produce carbon dioxide
(CO2)
Riffles – fast-moving portions of a stream that are flowing over coarse substrate
Riparian – the narrow ecosystem that parallels streams, rivers, and other water channels
Rocky intertidal – the rocky zone occupying the area between high and low tide
Salinity – a measure of the dissolved salt content in water
Savannas – a grassland ecosystem occurring in tropical and subtropical ecosystems with trees
and shrubs intermixed with a dense grass understory
Scientific method – the process by which scientific inquiry takes place, including the following
defined steps: ask a question, state a measureable hypothesis, conduct an experiment, analyze the
results, and make a conclusion; based on these conclusions, a hypothesis is then either rejected or
not.
Second law of thermodynamics – energy disperses from being localized to spread out unless it
is prevented from doing so
Secondary production – generation of biomass of heterotrophic organisms
Soil profile – vertical layering in the soil column
Soil texture – the relative proportions of different sized sediment grains
Speciation – the formation of new and distinct species by evolution
Species diversity – a measure of the variability and variety of living organisms in an ecosystem
Species richness – the number of different species occurring in a defined geographical area
Stability – ability of an ecosystem to recover after a disturbance
Stress – an environmental condition that constrains physiological processes, lowering an
organism’s rate of growth, survival, or reproduction
Succession – the directional change in species composition, structure, and resource availability
of an area over time that is driven by biotic activity and interactions as well as changes in the
physical environment and the dominating species
Succulent – plants such as cacti that have thick, heavy foliage for water storage
Surface runoff – overland flow of excess water
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Symbiont – organism that lives in association with another organism
Theory – formed to collectively explain the results of a large number of experimental
observations
Top predator – eats other species but is not a food source for any other species
Upwelling – periods when deep, cold, nutrient-rich ocean waters are driven to the surface to
replace the warmer, nutrient-poor surface waters
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NOTES
1. Turner, M.G. 1989. Landscape Ecology: The effect of pattern on process. Annual Review of
Ecology, Evolution and Systematics. Vol. 20: 171–197.
2. Ripple, W.J. and R.L. Beschta. 2012. Tropic cascades in Yellowstone: The first 15 years after
wolf reintroduction. Biological Conservation. Vol. 145(1): 205–213.
3. Mills, L.S., M.E. Soule, and D.F. Doak. 1993. The keystone species concept in ecology and
conservation. Bioscience Vol43(4): 219.
4. Population Reference Bureau.
BIBLIOGRAPHY
Cain, M.L., W.D. Bowman & S.D. Hacker, 2011. Ecology. 2nd
Edition. p. 648.
Currie, D.J. 1991. Energy and large-scale patterns of animal and plant species richness.
American Naturalist Vol. 137: 27–49.
Krebs, C. 2008. The Ecological World View. p. 574.
Mills, L.S., M.E. Soule, and D.F. Doak. 1993. The keystone species concept in ecology and
conservation. Bioscience Vol43(4): 219.
Ripple, W.J. and R.L. Beschta. 2012. Tropic cascades in Yellowstone: The first 15 years after
wolf reintroduction. Biological Conservation. Vol. 145(1): 205–213.
Smith, T.M. and R.L. Smith. 2009. Elements of Ecology, 7th
Edition, p. 649.
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