Organisms in Their Environment 42. Concept 42.1 Ecological Systems Vary in Space and over Time Physical geographystudy of the distribution of Earths climates.

Organisms in Their Environment

42

Concept 42.1 Ecological Systems Vary in Space and over Time

Physical geography—study of the distribution of Earth’s climates and surface features

Biogeography—study of the distributions of organisms


Abiotic components of the environment— nonliving

Biotic component—living organisms

An ecological system—one or more organisms plus the external environment with which they interact


Ecology—term coined by Ernst Haeckel in 1866; made it a legitimate scientific subject and emphasized its relevance to evolution because ecological interactions drive natural selection.

System—a whole, comprising a set of interacting parts; neither the parts nor the whole can be understood without taking account of the interactions.


Ecological systems can include any part of the biological hierarchy from the individual to the biosphere.

Each level brings in new interacting parts at progressively larger spatial scales.

Figure 42.1 The Hierarchy of Ecological Systems


Population—group of individuals of the same species that live, interact, and interbreed in a particular area at the same time.

Community—assemblage of interacting populations of different species in a particular area.

Ecosystem—community plus its abiotic environment

Biosphere—all the organisms and environments of the planet


Generally, large ecological systems tend to be more complex and have more interacting parts.

But small systems can also be complex:

The human large intestine is densely populated with hundreds of microbial species.

The gut environment provides stable conditions and ample nutrients.

The microbial species interact with each other and with their environment in many complex ways.


At any given time, an ecological system is potentially unique.

In the human gut, the microbial species vary from person to person and with diet.

The host’s genotype and diet affect the gut environment from the bacterial point of view; and the bacteria influence their environment, which includes the host.

Some health disorders may be treatable by manipulating the gut bacterial community.

Figure 42.2 The Microbial Community of the Human Gut Depends on the Host’s Diet (Part 1)

Figure 42.2 The Microbial Community of the Human Gut Depends on the Host’s Diet (Part 2)

Concept 42.2 Climate and Topography ShapeEarth’s Physical Environments

Variation in physical environments results from atmosphere and ocean circulation patterns and geological processes.


Weather—the state of atmospheric conditions in a particular place at a particular time

Climate—average conditions and patterns of variation over longer periods

Adaptations to climate prepare organisms for expected weather patterns.


Earth receives uneven inputs of solar radiation due to its spherical shape and tilt of its axis as it orbits the sun.

Subsequent results in temperature variation:

• Air temperatures decrease from low to high latitudes.

• High latitudes experience more seasonality— greater fluctuation over the course of a year.

Figure 42.3 Solar Energy Input Varies with Latitude

Figure 42.4 The Tilt of Earth’s Axis of Rotation Causes the Seasons


Solar energy inputs are always greatest in the equatorial region, which drives global patterns of air circulation.

Hadley cells:

The tropical air is warmed, rises, and then cools adiabatically (an expanding gas cools).

The rising warm air is replaced by surface air flowing in from the north and south.

The cooling air sinks at 30°N and 30°S.

Figure 42.5 Global Atmospheric Circulation


Other circulation cells form at the mid-latitudes and at the poles.

The circulation patterns influence prevailing winds and precipitation patterns.

Rising warm tropical air releases lots of moisture as rainfall. The sinking air at 30°N and 30°S is dry—most of the great deserts are at these latitudes.

Prevailing winds are deflected by the rotation of the Earth—the Coriolis effect.

Figure 42.6 Direction of Prevailing Surface Winds


Prevailing winds drive the major ocean surface currents.

Example: northeast trade winds drive water to the west; when it reaches a continent it is deflected northward until the westerlies drive the water back to the east.

Figure 42.7 Ocean Currents


Deep ocean currents are driven by water density differences.

Colder, saltier water is more dense and sinks to form deep currents.

Deep currents regain the surface in areas of upwelling, completing a vertical ocean circulation.


Oceans and large lakes moderate climate because water has a high heat capacity.

Water temperature changes slowly as it exchanges heat with the air.

Poleward-flowing ocean currents carry heat from the tropics toward the poles, moderating climate at higher latitudes.

Example: the Gulf Stream warms northern Europe.


Climate diagram—superimposed graphs of average monthly temperature and precipitation throughout a year.

The axes are scaled so that precipitation is adequate for plant growth when the precipitation line is above the temperature line.

The growing season occurs when temperatures are above freezing and there is enough precipitation.

Figure 42.8 Walter Climate Diagrams Summarize Climate in an Ecologically Relevant Way


Earth’s topography also influences climate.

As you go up a mountain, air temperature drops by about 1°C for each 220 m of elevation.

When prevailing winds bump into mountain ranges, the air rises up, cools, and releases moisture. The now-dry air descends on the leeward side.

This results in a dry area on the leeward side, called a rain shadow.

Figure 42.9 A Rain Shadow


Topography also influences aquatic environments:

Flow velocity depends on slope.

Water depth determines gradients of many abiotic factors, including temperature, pressure, light penetration, and water movement.

Concept 42.3 Physical Geography Provides the Template for Biogeography

Organisms must be adapted to their physical environments.

For example, a plant that has no means of conserving water cannot thrive in a desert.

Species are found only in environments they can tolerate.


Early naturalist–explorers began to understand how the distribution of Earth’s physical environments shapes the distribution of organisms.

Their observations revealed a convergence in characteristics of vegetation found in similar climates around the world.


Biome—a distinct physical environment inhabited by ecologically similar organisms with similar adaptations.

Species in the same biome in geographically separate regions display convergent evolution of morphological, physiological, or behavioral traits.


Terrestrial biomes are distinguished by their characteristic vegetation.

Distribution of terrestrial biomes is broadly determined by annual patterns of temperature and precipitation.

These factors vary along both latitudinal and elevational gradients.

Figure 42.10 Temperature and Precipitation Gradients Determine Terrestrial Biomes

Figure 42.11 Global Terrestrial Biomes


Other factors, especially soil characteristics, interact with climate to influence vegetation.

Example: Southwestern Australia has Mediterranean climate with hot, dry summers and cool, moist winters. The vegetation is woodland/shrubland, but no succulent plants are here.

The soils are nutrient-poor, and there are frequent fires. Succulents are easily killed by fires.

Figure 42.12 Same Biome, Different Continents (Part 1)





The biome concept is also applied to aquatic environments.

Aquatic biomes are determined by physical factors such as water depth and current, temperature, pressure, salinity, and substrate characteristics.

Table 42.1 Major Aquatic Biomes


The primary distinction for aquatic biomes is salinity: freshwater, saltwater, and estuarine biomes.

Salinity determines what species can live in the biome, depending on their ability to osmoregulate.


In streams, current velocity is important. Organisms must have adaptations to withstand flow.

Current also impacts the substrate—whether rocky, sandy, silty, etc. Substrate also determines what species are present.


Still-water biomes (lakes and oceans) have zones related to water depth.

Nearshore regions (littoral or intertidal) are shallow, impacted by waves and fluctuating water levels. Distinct zonation of species is common.

Photic zone—depth to which light penetrates; photosynthetic organisms are restricted to this zone.

Figure 42.13 Water-Depth Zones (Part 1)

Figure 42.13 Water-Depth Zones (Part 2)


Aphotic zone is too deep for light penetration.

Benthic zone—lake or ocean bottom

Water pressure increases with depth. Organisms in the deepest oceans (abyssal zone) must have adaptations to deal with high pressure, low oxygen, and cold temperatures.

Concept 42.4 Geological History Has Shapedthe Distributions of Organisms

Alfred Russel Wallace studied species distributions in the Malay Archipelago and observed dramatically different bird faunas on two neighboring islands, Bali and Lombok.

The differences could not be explained by soil or climate.

Figure 42.14 Wallace’s Line


He suggested that the deep channel between the islands would have remained full of water (and a barrier to movement of terrestrial animals) during the Pleistocene glaciations when sea level dropped.

Thus, the faunas on either side of the channel evolved mostly in isolation over a long period of time.


Wallace’s observations led him to divide the world into six biogeographic regions.

They contain distinct assemblages of species, many of which are phylogenetically related.

Many of the boundaries correspond to geographic barriers to movement: bodies of water, extreme climates, mountain ranges.

Figure 42.15 Movement of the Continents Shaped Earth’s Biogeographic Regions


Boundaries of some biogeographic regions are related to continental drift.

Example: southern beeches (Nothofagus) are found in South America, New Zealand, Australia, and some south Pacific islands.

The genus originated on the southern supercontinent Gondwana during the Cretaceous and was carried along when Gondwana broke apart.

Figure 42.16 Distribution of Nothofagus (Part 1)

Figure 42.16 Distribution of Nothofagus (Part 2)


Biotas of the seven biogeographic regions developed in isolation throughout the Tertiary (65 to 1.8 mya), when extensive radiations of flowering plants and vertebrates took place.


Continental movement has recently eliminated some barriers, allowing biotic interchange.

Examples: when India collided with Asia about 45 mya, and when a land bridge formed between North and South America about 6 mya.


Biogeographers use phylogenetic information, along with the fossil record and geological history, to study modern distributions of species.

Geographic areas are superimposed on phylogenetic trees. The sequence and timing of splits in the phylogenetic tree are compared with sequence and timing of geographic separations or connections.

Apply the Concept, Ch. 42, p. 837

Concept 42.5 Human Activities Affect Ecological Systemson a Global Scale

Human activities are altering ecological systems on a global scale.

Some have suggested we are entering a new geological period, the “Anthropocene.”

We are changing the distributions of organisms, vegetation, and topography, as well as Earth’s climate.


Human-dominated ecosystems, such as croplands, pasturelands, and urban settlements now cover about half of Earth’s land area.

These ecosystems have fewer interacting species and are less complex.


In agricultural lands, monocultures replace species-rich natural communities.

Diversity of crops planted is also very low: 19 crops comprise 95% of total global food production.

Agricultural systems are more spatially and physically uniform than natural ecological systems.

Figure 42.17 Human Agricultural Practices Produce a Uniform Landscape


Human activities also reduce complexity in natural ecosystems:

• Damming and channelization of rivers

• Pollution and habitat fragmentation

• Overexploitation of wild species

• Introductions of new species


Humans move species throughout the globe, sometimes deliberately, sometimes inadvertently.

Human-assisted biotic interchange is homogenizing the biota of the planet, blurring the spatial heterogeneity in species composition that evolved during long periods of continental isolation.

Concept 42.6 Ecological Investigation Depends onNatural History Knowledge and Modeling

Natural history—observation of nature outside of a formal, hypothesis-testing investigation—provides important knowledge about ecosystems.

These observations are often the source of new questions and hypotheses and aid in design of ecological experiments.

Concept 42.6 Ecological Investigation Depends onNatural History Knowledge and Modeling

Computer models are important tools in the study of ecosystems.

Natural history knowledge is used to build these models.

Example: rangeland grasses are affected by other plants and herbivores, soil fertility, climate, and fire. To predict the effect of removing cattle to bring back grasses, all these interactions must be known.

Answer to Opening Question

Grasslands in different parts of the world differ in significant ways.

Grasslands occupy a range of climatic conditions from moist to very arid.

In Europe and eastern North America, pastures were once forests; “resting the range” and managing herd size can restore much of the ecosystem.

Figure 42.18 Harmonious Grazers

Answer to Opening Question

Temperate grasslands of midwestern North America, Eurasia, South America, and African savannas have long histories of evolution with grazing mammals.

It is not clear why removal of cattle has not restored grasslands in the U.S.–Mexico Borderlands; it is an area of active research.

Organisms in Their Environment 42. Concept 42.1 Ecological Systems Vary in Space and over Time Physical geographystudy of the distribution of Earths climates.

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