© 2011 Pearson Education, Inc. Chapter 4 Populations and Communities.

© 2011 Pearson Education, Inc.

Chapter 4

Populations and Communities


Dynamics of natural populations• Population: a group of members of the same

species living in an area• Community: populations of different species living

together in an area• Populations grow with births and immigration• They decline with deaths and emigration

(Births + Immigration) – (Deaths + Emigration)

= Change in population number


Population growth• Population growth: change in population• Equilibrium: births + immigration are equal to

deaths + emigration• Often, population growth is not zero

• Population growth rate: amount the population has changed divided by the time it had to change

• Population growth curves: graph how populations grow; used to find• How fast a population could grow• How many individuals there are now• What the future population size could be


Population growth curves• Constant population growth rate: adding a

constant number of individuals over each time period• The simplest type of growth to model• Is an unlikely pattern and is not generally found in

nature• But it’s a good comparison to other growth

patterns

• Starting with 2 individuals, and a constant growth rate of 2, at the end of 24 weeks you would have 50 individualsPopulation number at the start + (A constant * Time)

= Population number at the end


Three models of population growth


Exponential growth• Each species can increase its population• With favorable conditions

• Exponential increase: does not add a constant number of individuals for each time period• The doubling time remains constant• For example, it takes 2 days to go from 8 to 16

individuals, as well as from 1,000 to 2,000 individuals• Such growth is called an “explosion”

• J-curve: the curve of exponential growth


Exponential growth of a population• Carrying capacity (K): the maximum population of a

species that a given habitat can support without being degraded• This is the upper limit of population growth


Outcomes of population increase• The population continues to grow and then dies off• Logistic growth: some process slows growth so it

levels off near carrying capacity• Results in an S-shaped curve• It levels off at K

• As the population approaches K, growth slows• The population remains steady and growth = 0• The maximum rate of population growth occurs

halfway to K


Populations show logistic growth• J-shaped explosions are often followed by crashes• J-shaped growth results from unusual disturbances• Introduction of a foreign species, a suddenly changed

habitat, the elimination of a predator, arrival in a new habitat

• Other populations show an S-curve• Followed by cycles of lower and higher numbers

around K• Or shoot above K, eventually cycling around K


Real-life growth


Biotic potential vs. environmental resistance• Biotic potential: the number of offspring (live births,

eggs, or plant seeds and spores) produced under ideal situations• Measured by r (the rate at which organisms

reproduce)• Varies tremendously from less than 1 birth/year

(some mammals) to millions/year (plants, invertebrates)


Biotic potential vs. environmental resistance• Recruitment: survival through early growth stages

to become part of the breeding population• Young must survive and reproduce to have any effect

on population size


Environmental resistance• Abiotic and biotic factors cause mortality (death)• Prevents unlimited population growth

• Environmental resistance: the biotic and abiotic factors that may limit a population’s increase• Biotic: predators, parasites, competitors, lack of food• Abiotic: unusual temperatures, moisture, light, salinity,

pH, lack of nutrients, fire


Environmental resistance

• Environmental resistance can also lower reproduction• Loss of suitable habitat, pollution• Changed migratory habits of animals


Biotic potential and environmental resistance


Reproductive strategies: r-strategists• The interplay of environmental resistance and

biotic potential drives the success of two reproductive strategies


r-strategists

• r-strategists (r-selected species): produce lots of young, but leave their survival to nature• Results in low recruitment• Rapid reproduction, rapid movement, short life span• Adapted to a rapidly changing environment• “Boom-and-bust” populations • “Weedy” or “opportunistic” species• For example, housefly


Reproductive strategies: K-strategists• K-strategists (K-selected species): lower biotic

potential• Care for and protect young• Live in a stable environment already populated by

the species• Larger, longer lived, well-adapted to normal

environmental fluctuations• Their populations fluctuate around carrying capacity• For example, elephant, California condor


Life histories• Life history: progression of changes in an organism’s life • Age at first reproduction, length of life, etc.• Visualized in a survivorship graph

• Type I survivorship: low mortality in early life• Most live the bulk of their life span (e.g., humans)

• Type III survivorship: many offspring that die young• Few live to the end of their life (oysters, dandelions)

• Type II survivorship: intermediate survivorship pattern (squirrels, coral)

• K-strategists have a Type I pattern; r-strategists show Type III


Survivorship curves


Predictable pattern in species• There is a predictable pattern to the way human

activities affect species• r-strategists become pests when humans change an

area• Houseflies, dandelions, cockroaches increase

• K-strategists become rarer or extinct with change• Eagles, bears, and oaks decline


Why does a population grow?• A population’s size depends on the interplay

between its biotic potential and environmental resistance

• A population’s biotic potential remains constant• Environmental resistance changes

• Population balance is a dynamic balance• Additions (births, immigration) and subtractions

(death, emigration) occur continually• The population may fluctuate widely or very little


Limits on populations• The carrying capacity may not explain what limits a

population• Population density: number of individuals per unit

area• The higher the density, the more likely a factor (e.g.,

crowding or disease) affects the population


Density dependence and independence• Density-dependent factor: increases with

increased population density• Predation, disease, food shortage


Density dependence and independence

• Density-independent factor: one whose effects are independent of the density of the population• Spring freeze, fire• Is not involved in maintaining population equilibrium in

the logistic growth


Regulating a population• Only density-dependent factors can regulate a

population (keep it in equilibrium)• Top-down regulation: control of a population (species)

by predation• Bottom-up regulation: control of a population occurs

as a result of scarcity of a resource (food)


Top-down and bottom-up control


Critical number• Critical number: the minimum population base

allowing the survival and recovery of a population• A pack of wolves, flock of birds, school of fish

• The group is necessary to provide protection and support

• If a population falls below this number• Surviving members become more vulnerable• Breeding fails• Extinction is almost inevitable


Humans are responsible for biodiversity loss• Human activities are responsible for the decline and

extinction of species• Humans change habitats, introduce alien species,

pollute, hunt, etc.

• The Endangered Species Act calls for recovery of• Threatened species: populations are declining rapidly• Endangered species: populations are near the critical

number


Community interactions• Relationships between species may be positive (helpful),

negative (harmful), or neutral for each species• Predation: one member benefits, the other is harmed (+−)

• Includes parasitism

• Competition: both species are harmed (−−)• Interspecific competition: between different species• Intraspecific competition: between the same species

• Mutualism: both species benefit (++)• Commensalism: One species benefits, the other is not

affected (+0)


Major types of interactions between species


Predation• Predator: the organism that does the feeding• Prey: the organism that is fed upon• Predator-prey interaction: carnivores (meat eaters)

eat herbivores (plant eaters)• Herbivores feed on plants• Parasites feed on hosts• Parasite: an organism (plant or animal) that feeds on

its “prey,” usually without killing it• Host: the organism that is being fed upon


Parasitic organisms• Include tapeworms, disease-causing bacteria,

viruses, fungi• Parasites affect host populations in a density-

dependent way• Increased population density makes it easier for

parasites and their vectors (carriers of the parasite) to find new hosts

• Pathogens: bacteria and viruses that cause disease• No real ecological difference from other parasites• Highly specialized parasites


Several types of parasites


Regulation of prey• Predators can regulate herbivore numbers (top-down

control)• Moose crossed to Isle Royale, in Lake Superior• Several years later, wolves also reached the island

• Without wolves, the moose population grew rapidly• Low environmental resistance for the moose

• The wolf population also increased and preyed on moose• Low environmental resistance for the wolves

• Fewer moose (high environmental resistance) resulted in fewer wolves (high environmental resistance)


Predator-prey relationships


Other factors affect populations• Wolf predation is not the only factor affecting moose• Deep snow limited access to food• A tick infestation caused significant mortality

• A sharp decline in moose keeps wolf populations low• Not enough calves to catch• Wolves can’t catch a mature moose in good condition

• Predator-prey relationships involve top-down (on the prey) and bottom-up (on the predator) regulation


Overgrazing• If herbivores eat plants faster than they can grow,

plants are depleted and animals eventually suffer• Reindeer were brought to St. Matthew Island• No population can escape the ultimate limitations set

by environmental resistance


Plant-herbivore interactions


Predator removal• Eliminating predators upsets plant-herbivore

relationships• White-tailed deer in the U.S. were originally

controlled by wolves, mountain lion, bears• But these were killed because they were thought to

threaten livestock and humans• Deer populations can get so large they overgraze the

area• Humans control numbers through hunting


Keystone species• Removal of one species can create a cascade of

effects• Impacting far more than just the other species they

interact with

• Sea stars eat mussels in rocky intertidal zones• Removing sea stars allows mussels to crowd out all

other species, reducing diversity

• Keystone species: play a crucial role in maintaining ecosystem biotic structure• Moderate other species that would take over• Allow other, less-competitive species to flourish


Sea stars: a keystone species


Competition• Interactions where both species are harmed• They compete for a scarce resource

• Species that compete have overlapping niches• Over time, there is pressure to reduce the overlap


Competition• Intraspecific competition: competition between

members of the same species• Occurs over resources

• Territory: an area defended by an individual or group• Most defense is intimidation—serious fights are rare• Organisms fight to protect an area for nesting,

establishing a harem, or food resources


Territoriality


Territoriality as an advantage• It protects a population from the possibility of

everyone getting some resources, but nobody getting enough to survive

• Without territoriality, when resources are scarce• All members would get only a part of what they need

and could die

• Competition lowers fitness and production of offspring• Territoriality lowers the direct effects of competition


Not everyone gets a territory• Individuals unable to claim a territory• May disperse, opening new habitats to the species• May die

• Territoriality is most likely in K-strategists• An adaptation that helps organisms disperse and

stabilizes populations


Impact on the species• Survival of the fittest: one of the forces in nature

leading to evolutionary changes in a species• Those individuals in a competing group that can

survive and reproduce demonstrate superior fitness to the environment

• Every factor of environmental resistance is a selective pressure• Individuals who survive and reproduce have the

genetic endowment to better cope with their environment


Interspecific competition• Competitive exclusion principle: species cannot

survive competition if they occupy the same niche• But species do occupy the same area without

becoming extinct• Abiotic conditions in an environment vary in space

and time

• Adaptations of species to specific conditions allows it to thrive and overcome its competitors in one location or time, but not in another


Environments are heterogeneous• Competitors in the same habitat have different

niches• Woodpeckers eat insects; other birds eat seeds• Space can also be a resource

• Resource partitioning: the division of a resource and specialization in different parts of it


Resource partitioning in warblers


Resource partitioning in plants


Adaptations limit competition• Adaptations, including behavior, limit competition• Organisms can put energy into reproduction

• Competition changes some characteristics, so organisms use only a part of their niche• For example, two closely related finches on the same

island have different sized beaks to eat different seeds

• Character displacement: a physical change that decreases competition when two species co-occur


Character displacement in finches


Mutualism• An arrangement between two species where both

benefit• Examples of mutualism• Pollinators (e.g., bees) receive nutrition while plants

receive pollination• Fungi on roots: the fungus gets nutrition; the plant

gets easier intake of soil nutrients• Lichens are made of a fungus and an alga


Mutualistic relationships


Commensalism and amensalism• Commensalism: one species benefits; the other is

unaffected• More rare• Buffalo stir up insects that cattle egrets eat• Orchids live on trees but do not harm or feed off of

them• Symbiosis: two species live close to each other• Can be beneficial (mutualism) or harmful (parasitism)


Commensalism


Genes and traits

• Every individual within a population has a set of gene.

• Each gene is responsible for a trait of the individual.• An offspring acquires traits from each parent due to

the set of genes it inherits. • Sometimes in the process of inheritance a gene may

change or mutate.


Gene Pool

• The set of all genes of all individuals in a population is called the gene pool.

• A unique form of a single gene is called an allele. Eye color is determined by which allele an offspring inherits (blue, green, brown, etc)


Adaptation of foxes to changing conditions• If there are plenty of rabbits, everybody eats well

and no real selective pressure and there is a very diverse gene pool, but…

• What if there is grass killing disease?• Rabbit pop?• Selective pressure


Natural Selection

• When rabbit pop declines, weak and slow rabbits are easily caught by foxes

• What trait then becomes beneficial to fox individuals?

• Who lives longer and reproduces more frequently?


Evolution

• What happens to the gene pool?

• This is called microevolution.


Selective pressures• Selective pressures: environmental resistance

factors affect which individuals survive and reproduce• Predators, parasites, drought, lack of food

• Animals with traits that protect them or allow them to escape can survive and reproduce• Predators function as a selective pressure• Food is a selective pressure


Natural selection and evolution• Natural selection: the process of specific traits

favoring survival of certain individuals• Charles Darwin• Biological evolution: modification of the gene pool of

a species by natural selection over generations


Adaptations to the environment• Fitness: features (traits) that adapt an organism for

survival and reproduction

• Adaptations needed for coping with abiotic factors • Obtaining nutrients, energy, defense against

predation • Finding and attracting mates• Migrating and dispersal


Adaptation for survival and reproduction


The limits of change• Does the trait increase survival and reproduction?• If “yes,” the trait is maintained through natural selection

• Various organisms have evolved different traits• To accomplish the same function (e.g., avoid

predation)

• A species has three alternatives when faced with a new selective pressure• Adaptation: through natural selection• Migration• Extinction


Factors that determine whether a species can adapt• Some individuals have traits (alleles: variations or

new combinations of genes) that allow them to survive and reproduce under the new conditions

• There must be enough survivors to maintain a viable breeding population

• Natural selection should lead to increased adaptations over successive generations


The California condor• By the early 1980s, only 20 birds remained • Deaths due to poaching, lead poisoning, habitat

destruction

• Wildlife biologists captured all wild-living birds and started a captive breeding program

• By 2008, 320 condors survived• 180 in captivity• This is still not a viable breeding population, but wild

condors are forming pairs and nesting


Vulnerable and highly adaptive species


The evolution of species• Biodiversity is decreasing because species are

going extinct faster than new species are appearing• Mutations and natural selection adapt a species to

the biotic community and environment in which it exists

• The process of speciation: the final result of adaptation may be a population so different from the original population it is considered a new species


How two species may develop from one• They must be reproductively isolated: the original

populations separate into nonbreeding populations• Or else genes will continue to mix, keeping it one species

• Separate populations must be exposed to different selective pressures• As they respond to these pressures, they become different• They can’t interbreed, even if they come together again

• Arctic and gray foxes became separated by glaciation• Selective pressure on the northern population resulted in

heavier fur, shorter limbs, and white fur


Evolution of new species


Darwin’s finches• Charles Darwin speculated that differences in beak

size and structure in finches on islands in the Galápagos happened after subpopulations became isolated on separate islands

• Birds blown west from South America landed on islands• Intraspecific competition caused birds to disperse• Different selective pressures caused birds to become

specialized for feeding on different things• When populations re-encountered each other, they

were so different, they could not interbreed


Some of Darwin’s finches


Drifting continents• Geographic isolation of populations is fundamental

to speciation• Populations became isolated through the slow

movement of Earth’s continents

• Plate tectonics: continents were once connected into a great land mass (Pangaea), which then separated

• Tectonic plates: in the Earth’s crust, huge slabs of rock that float on an elastic layer beneath them


Drifting continents and plate tectonics


Movement of tectonic plates• Tectonic plates ride atop convection currents caused

by rising hot interior material and sinking cooler material

• Adjacent tectonic plates move• Separating (in mid-ocean ridges)• Sliding past each other (at fault lines)• Colliding (creating mountain ranges)


Movement of tectonic plates (cont)

• The 2004 tsunami (tidal wave) in Indonesia • The Indo-Australian plate slid under the Eurasian

plate• Displaced enormous quantities of water and caused

waves that killed 225,000 people


Tectonic movement affects climate

• As continents move, their climate changes accordingly

• Continental movement changes the direction and flow of ocean currents, which affects climate

• Uplifting of mountains changes movements of air currents


Tectonic movement affects diversity

• Changes in Earth’s crust have increased diversity• Moving continents isolated populations into six

zoogeographic regions

• Australia is on the Indo-Australian tectonic plate • Its ancient separation led to many unique species


Introduced species

• Rabbits were introduced into Australia for sport shooting• Without natural enemies, the population exploded,

devastating the environment• After being eradicated on Philip Island, the island’s

vegetation was dramatically restored


Rabbit eradication on Philip Island


Plants• Introduced plants can have a devastating effect on

plants and animals• The chestnut blight fungus has killed almost every

American chestnut tree • Kudzu, a vine introduced to control erosion and for

cattle fodder, has invaded and climbed over forests• Purple loosestrife has invaded wetlands and

replaced edible wildlife plants


Introduced plant species


Pests• Most insect pests in croplands and forests are

introduced• Japanese beetles, fire ants, gypsy moths• Goats and pigs on islands• Domestic cats diminish songbird populations


CHAPTER 4

Populations and Communities

Active Lecture Questions


(Births plus ________) minus (________ plus Emigration) = Change in population number

a. Emigration; Birthsb. Immigration; Deathsc. Immigration; Birthsd. Emigration; Deaths

Review Question-1


(Births plus ________) minus (________ plus Emigration) = Change in population number

a. Emigration; Birthsb. Immigration; Deathsc. Immigration; Birthsd. Emigration; Deaths

Review Question-1 Answer


True or False: Natural selection results in a modification of the gene pool toward traits that enhance survival and reproduction of a population.

a. Trueb. False

Review Question-2


True or False: Natural selection results in a modification of the gene pool toward traits that enhance survival and reproduction of a population.

a. Trueb. False



The rate at which members of a species reproduce if there are no limiting factors is referred to as

a. K-strategy.b. r-strategy.c. environmental resistance.d. biotic potential.

Review Question-3


The rate at which members of a species reproduce if there are no limiting factors is referred to as

a. K-strategy.b. r-strategy.c. environmental resistance.d. biotic potential.



A species that has a critical role in maintaining an ecosystem’s biotic structure is a

a. J-curve species.b. K-strategist.c. keystone species.d. biotic potential.

Review Question-4


A species that has a critical role in maintaining an ecosystem’s biotic structure is a

a. J-curve species.b. K-strategist.c. keystone species.d. biotic potential.



The theory that has helped us understand earthquakes, volcanic activity, and the geographic distribution of present-day biota is

a. the law of limiting factors.b. the Laws of Thermodynamics.c. the theory of evolution.d. plate tectonics.

Review Question-5


The theory that has helped us understand earthquakes, volcanic activity, and the geographic distribution of present-day biota is

a. the law of limiting factors.b. the Laws of Thermodynamics.c. the theory of evolution.d. plate tectonics.



According to Fig. 4-2, the population growth curve that represents logistic growth is

a. the J-curve.b. the S-curve.

Interpreting Graphs and Data-1


According to Fig. 4-2, the population growth curve that represents logistic growth is

a. the J-curve.b. the S-curve.

Interpreting Graphs and Data-1 Answer


According to Fig. 4-8, the approximate date that this population reached its greatest level of environmental resistance was

a. 1944.b. 1955.c. 1963.d. 1991.

Interpreting Graphs and Data-2


According to Fig. 4-8, the approximate date that this population reached its greatest level of environmental resistance was

a. 1944.b. 1955.c. 1963.d. 1991.

Interpreting Graphs and Data-2 Answer


A K-strategist’s population size typically fluctuates around

a. the midpoint of the J-curve.b. carrying capacity.c. density-dependent factors.d. density-independent factors.

Thinking Environmentally-1


A K-strategist’s population size typically fluctuates around

a. the midpoint of the J-curve.b. carrying capacity.c. density-dependent factors.d. density-independent factors.

Thinking Environmentally-1 Answer


A sudden hard freeze that kills members of an ecosystem is an example of

a. a critical number.b. a density-dependent factor.c. a density-independent factor.d. density-driven resistance.

Thinking Environmentally-2


A sudden hard freeze that kills members of an ecosystem is an example of

a. a critical number.b. a density-dependent factor.c. a density-independent factor.d. density-driven resistance.

Thinking Environmentally-2 Answer

© 2011 Pearson Education, Inc. Chapter 4 Populations and Communities.

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