CHAPTER
47
COMMUNITY ECOLOGY
This chapter describes the various types of symbiotic relationships that exist in biological communities. Many
theories describing the communal interactions, and much terminology is included. A number of landmark studies
leading to various theories are described. An Ecology Focus box discusses “Interactions and Coevolution.”
Chapter Outline
47.1 Concept of the Community

A community is a group of populations that interact with one another in the same environment.
1. Communities vary in size and may have boundaries that are difficult to determine.
2. A fallen log supports a community but a passing bird can eat one of its members.
3. A forest may appear distinct but it gradually fades into the surrounding areas.
A. Community Composition and Diversity
1. The composition of a community is a listing of the species within a community; it does not reveal the
relative abundance of organisms.
2. Diversity of a community includes not only a listing of the species in the community, but also the
abundance of each species.
a. The greater the diversity, the greater the number and the more even the distribution of the species.
B. Models Regarding Composition and Diversity
1. The number of species in a community increases as we move from the poles to the equator.
2. The individualistic model states that each population is there because of its adaptations.
a. A species range is based on its tolerance for abiotic factors including light, water, salinity, etc.
b. Species will have independent distributions; boundaries between communities will not be distinct.
3. The interactive model of community structure proposes:
a. A community is simply a higher level of organization beginning from cell to tissue to organism;
b. Just as cells are adapted to each other, a community had species adapted to each other;
c. A community remains stable because of homeostatic mechanisms;
d. The same species will recur in communities whose boundaries are distinct from one another.
4. Modern ecological data supports the individualistic model.
a. F. H. Talbot and co-workers built artificial reefs and set them in a uniform tropical lagoon.
b. Of the 42 species that colonized the reefs, there was only a 32% similarity reef-to-reef.
c. From month to month, 20–40% of the species changed.
d. The reef species composition appeared to depend on chance migrations.
e. Certain animals occurred near their food source and this determined their range.
f. Most likely, community structure depends on both abiotic and biotic factors.
C. Island Biogeography
1. Robert MacArthur and E. O. Wilson developed the general theory of island biogeography.
2. Nearby islands have more species because immigration is easier.
3. Larger islands have more species because a large island has more resources.
4. “Islands” can also include patches of forest surrounded by cropland, housing developments, etc.
a. The spatial heterogeneity model describes the patchiness of an environment.
b. The greater the number of habitat patches, the greater the diversity.
5. Stratification is an increase in vertical living spaces; a tree canopy provides a high-rise habitat and
vertical patchiness.
6. An equilibrium point is reached when the rate of species immigration matches the rate of species
extinction.
7. An equilibrium point can be dynamic with many species arriving and departing, or steady unless
disturbed.
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47.2 Structure of the Community
1. Interactions include: competition for resources, predator-prey interaction, and parasite-host
interactions.
2. Competition between two species for limited resources negatively affects the population size of both
species.
3. Predation and parasitism increase the predator population at the expense of the prey and host
populations.
4. In parasitism, one species is benefitted, the other is harmed.
5. In commensalism, one species is benefitted, the other is neither benefitted nor harmed.
6. In mutualism, both species benefit.
A. Habitat and Ecological Niche
1. A habitat is where an organism lives and reproduces in the environment.
2. The ecological niche is the role an organism plays in its community, including its habitat and its
interactions with other organisms.
a. The fundamental niche is the range of conditions under which it can survive and reproduce.
b. The realized niche is the set of conditions under which it exists in nature.
3. Generalist species (e.g., raccoons, roaches, humans) have a broad range of niches.
a. They have a survival advantage when environmental conditions are apt to change.
4. Specialist species (e.g., pandas, spotted owls, freshwater dolphins) have a narrow range of niches.
a. They have a survival advantage in stable environments.
B. Competition Between Populations
1. Interspecific competition occurs when different species utilize a resource (e.g., light, nutrient) that is
in limited supply.
2. If the resource is not in limited supply, there is no competition.
3. Lotka and Volterra (1920s) developed a formula: competition favors one species and can eliminate the
other.
4. Gause grew two species of Paramecium in one test tube; only one survived if they were grown
together.
5. Competitive exclusion principle: no two species can indefinitely occupy the same niche at the same
time.
6. Over time, either one population replaces the other or the two species evolve to occupy different
niches.
7. If it appears two species occupy the same niche, there must be slight differences; Gause found two
species of paramecium coexisted if one fed on bacteria at the bottom of the tube and the other fed on
suspended bacteria.
8. Resource partitioning occurs when species shift niches; they no longer directly compete.
a. Three species of Galapagos Island finches have three sizes of beaks for small, medium, and large
seeds.
b. When species live on separate islands, their beak sizes are intermediate; when they live together,
their beak sizes are diversified; this is character displacement.
c. Five species of warblers in the same tree spent time in different tree zones to avoid competition;
they had different niches.
d. Swallows, swifts, and martins fly in mixed flocks eating aerial insects but have different nesting
sites, etc.
e. The above examples are deduced from already completed partitioning.
f. Joseph Connell studied the competition occurring in barnacles that consistently shift to match
shoreline tidal zones.
1) By removing the larger Balanus barnacles from the lower zone, the smaller barnacles easily
moved in.
2) The smaller barnacle is more resistant to drying out; but the larger one can overgrow it.
C. Predator-Prey Interactions
1. Predation occurs when one organism (predator) feeds on another (prey).
2. In a broad sense, it includes not only single predator-prey kills, but also filter feeding whales that strain
krill, parasitic ticks that suck blood, and even herbivorous deer that eat leaves.
3. Predator-Prey Population Dynamics
a. Some predators reduce the densities of their prey.
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1) When Gause reared the protozoans Paramecium caudatum and Didinium nasutum together in
culture, Didinium ate all the Paramecium and then died of starvation.
2) When prickly-pear cactus was introduced to Australia from South America, it spread wildly
without competition on the desert; a natural predator moth from South America was
introduced and the cactus and moth populations plummeted dramatically.
b. Natural predator-prey relationships allow persistent populations of both predator and prey
populations, though both may fluctuate over time.
1) Often a graph of predator-prey population densities shows regular peaks and valleys with the
predator population lagging slightly behind the prey; two reasons are possible.
a) The biotic potential of the predator may be great enough to overconsume the prey; the
prey population declines and the predator population then follows.
b) Or the biotic potential of the prey is unable to keep pace and the prey population
overshoots the carrying capacity and suffers a crash.
4. The Classic Case of the Snowshoe Hare and the Canadian Lynx
a. Careful records of pelts of both animals for over a century have demonstrated regular fluctuations.
b. To test whether the lynx or hare food supply was causing the cycling, three experiments were
done.
1) A hare population was given a constant supply of food and predators were excluded; the
cycling ceased.
2) The hare populations were given a constant food supply but predators were not excluded; the
cycling continued.
3) Predators were then excluded but no extra food was added; the cycling continued.
c. The interpretation of these results is that both a hare-food cycle and a predator-hare cycle combine
to produce the overall effect.
d. The grouse population also cycle, perhaps because the lynx switches to grouse when the hare
populations decline; thus predators and prey do not normally exist as simple two-species systems.
D. Prey Defenses and Other Interactions
1. Prey have evolved a variety of antipredator defenses.
2. Plant adaptations for discouraging predation include:
a. sharp spines,
b. tough leathery leaves,
c. poisonous chemicals in their tissues, and
d. chemicals that act as hormone analogues to interfere with insect larval development.
3. Animals have defenses that include:
a. camouflage for concealment; this also requires behavior (stillness),
b. cryptic coloration to blend into the surroundings,
c. fright of the predator,
d. warning coloration,
e. vigilance and association with other prey for better warning.
E. Mimicry
1. Mimicry occurs if one species (the mimic) resembles another species (the model) possessing an
antipredator defense.
2. Batesian mimicry, named for Henry Bates, is a form of mimicry in which one species that lacks
defense mimics another that has successful defenses.
3. Mullerian mimicry, named for Fritz Mhller, is where several different species with the protective
defenses mimic one another (e.g., stinging insects all share same black and yellow color bands).
F. Symbiotic Relationships
1. Symbiosis is a close relationship between members of two populations.
2. Parasitism
a. Parasitism is similar to predation; the parasite derives nourishment from the host.
b. Viruses are always parasitic; parasites occur in all kingdoms of life.
c. Endoparasites are small and live inside the host.
d. Ectoparasites are larger and remain attached to the body of hosts by specialized organs or
appendages.
e. Many parasites have two hosts.
1) The primary host is the main source of nutrition.
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2) The secondary host may serve to transport (vector) the parasite to other hosts.
Parasites are specific and require certain species as hosts.
Lyme Disease
1) The bacterium Borrelia burgdorfei causes arthritis-like symptoms in humans.
2) The bacterium primarily lives in white-tailed deer mice.
3) The larval deer ticks of Ixodes dammini or I. ricinus feed on deer mice and can transfer the
bacteria to humans.
3. Commensalism
a. In commensalism, one species benefits and the other is neither harmed nor benefitted.
b. It is difficult to determine true commensalism because it is difficult to ensure that the host is not
harmed.
c. Barnacles secure a home by attaching themselves to the backs of whales and the shells of
horseshoe crabs.
d. Remora fish attach themselves to the bellies of sharks, securing a free ride and the remains of the
shark’s meals.
e. Epiphytes (Spanish moss) grow in the branches of trees to receive light but take no nourishment
from the tree.
f. Clownfish live within the tentacles of sea anemones for protection.
g. Some relationships are so loose that it is difficult to know if they are true commensalism.
1) Cattle egrets feed near cattle because the egrets flush insects as they graze.
2) Baboons and antelopes forage together for added protection.
4. Mutualism
a. In mutualism, both species benefit.
b. Mutualism can be found among organisms in all kingdoms of life.
c. Examples include the following:
1) Bacteria in the human intestinal tract are provided with some of our food but also provide us
with vitamins.
2) Termites can only feed on wood because their gut contains the protozoa that digest cellulose.
3) Mycorrhizae are symbiotic associations between the roots of fungal hyphae and plants.
4) Flowers and insect pollinators represent a shift from insects eating pollen to eating nectar.
5) Lichens are made of algae that produce food and fungi that preserve water, although the algae
can survive alone.
d. Classic Example of the Ant and the Acacia Tree
1) In tropical America, the bullhorn acacia provides a home for ants in its hollow thorns.
2) The acacia also provides ants with food from its nectaries, and protein in nodules called
Beltian bodies.
3) In return, the ant protects the plant from herbivores and other plants that might shade it.
4) When ants on an experimental tree were killed with insecticide, the tree also died.
e. Tree-Ant-Caterpillar Complex
1) Trees in the genus Croton also have nectaries that feed ants.
2) Ants have a mutualistic relationship with Thisbe caterpillars that feed on Croton saplings.
3) Thisbe caterpillars offer nourishment to ants, keeping them nearby.
4) The caterpillar releases the same chemical that causes the ants to defend an ant colony.
5) The result is that the caterpillars are protected while feeding on the trees.
f. Cleaning Symbiosis
1) Crustacea, fish, and birds act as cleaners to a variety of vertebrate clients; this is called
cleaning symbiosis.
2) Large fish in coral reefs line up at cleaning stations and wait their turn to be cleaned by small
fish.
3) The possibility of feeding on host tissues as well as on ectoparasites complicates this case of
mutualism.
47.3 Community Development
f.
g.
 Communities change over both short and long intervals of time due to continental drift, glaciation, etc.
A. Ecological Succession
1. Ecological succession is a change involving a series of species replacements in a community
following a disturbance.
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2.
3.
Primary succession begins in a habitat lacking soil; this might occur following a volcanic eruption.
Secondary succession begins when soil is already present but it has been disturbed and returns to a
natural state, as in an abandoned cornfield.
a. In the first years, wild grasses and other pioneer species (plants that are invaders of disturbed
areas) invade.
b. Soon sedges and shrubs invade.
c. Later, there is a mixture of shrubs and trees.
4. In 1916, Clements proposed the climax-pattern model of succession: that succession leads to a climax
community that is characteristic for an area.
a. A climax community has a community composition that depends on climate.
1) Dry climates eventually produce deserts.
2) Wet climates proceed to forests.
3) Intermediate moisture will result in grasslands, shrubs, etc.
4) Soils will also influence the developing community.
b. Each stage facilitates the occurrence of the next stage (called the facilitation model).
1) Shrubs cannot grow on dunes until the dune grass has developed the soil.
2) Therefore the grass-shrub-forest must occur sequentially.
5. The inhibition model challenged Clements’s view of succession.
a. Colonists hold onto their space and inhibit the growth of other plants until the colonists die.
b. Death releases resources that allow different, longer-lived species to invade.
6. The tolerance model provides yet another view of succession.
a. Sheer chance may determine which seeds arrive first; in this case, the successional stages may
merely reflect the maturation time.
b. Trees merely take more time to develop; however, both facilitation and inhibition of growth may
be taking place.
7. All models are probably involved and succession may not often reach the same final potential natural
community.
47.4 Community Biodiversity
1. Community stability is seen in three ways: persistence through time, resistance to change, and
recovery once a disturbance occurred.
2. A forest may remain unchanged year after year; that is persistence.
3. A deciduous forest resists change by regrowing its leaves after an insect infestation.
4. A chaparral community is resilient to fire and quickly recovers to its normal state.
A. The Intermediate Disturbance Hypothesis
1. The intermediate disturbance hypothesis states a moderate level of disturbance yields the highest
community diversity.
2. Fire, wind, severe weather, and water erosion are abiotic and external factors that cause such
disturbances.
3. If disturbances affect one type of patch and not another, the effect of patchiness is to provide overall
stability.
4. If widespread disturbances occur frequently, diversity is limited and a community will be dominated
by rapid growth and short life span (r-strategist) colonizers.
5. When disturbances are less widespread and infrequent, species with slow growth rates and long life
spans will (K-strategists) dominate.
6. Therefore, too much disturbance, or not enough, may threaten the diversity of tropical rain forests and
coral reefs.
7. Archeological remains show that the Maya cultivated huge areas from 300 to 900 AD; the civilization
collapsed, and 1,200 years later the community composition still remains different from a local tropical
rainforest.
B. Predation, Competition, and Biodiversity
1. Predation by a particular species can reduce competition and increase diversity.
a. Robert Payne removed the starfish Pisaster from test areas along the coast of North America.
b. In the control area, there was no change in numbers of species.
c. In the removal area, the mussel Mytilus increased in number and excluded other invertebrates and
algae from attachment sites.
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2.
Such predators that regulate competition and maintain diversity are called keystone species; thus,
keystone species are organisms that play a greater role in maintaining the function and diversity of an
ecosystem than would be predicted by their abundance.
3. The elephant may be a keystone species in the savannah where it feeds on shrubs and trees and causes
the woodlands to become grassy savannas.
C. Island Biogeography and Biodiversity
1. Predation has changed the Barro Colorado Island.
a. This island was formed in Panama from damming a river in the 1910s.
b. Island biogeography predicts fewer species can survive on islands; the jaguar, puma and ocelot are
now gone.
c. As a result, the medium-sized coatimundi increased in numbers; it is a predator of bird eggs.
d. Thus, the numbers of bird species is less on the island than is expected for its size.
2. Therefore preserves must be larger in order to conserve the k-strategists, especially top predators.
D. Exotic Species and Biodiversity
1. Introduction of exotic species is devastating if they are not held in check by predators and competitors.
2. African honeybees introduced into Brazil displaced domestic honeybees.
3. The brown tree snake introduced to Guam has devastated the bird population.
4. The red fox was introduced in Australia to prey on the introduced European rabbit; it was successful
but has also reduced the populations of native small mammals.
Lecture Enrichment Ideas
Experience Base: Many students will be familiar with a backyard garden and know what happens when it is left
untilled. However, others will require media to illustrate this concepts, as well as the coral reef commensals and
keystone predators. Understanding of the exotics and keystone species may also require illustration. The zebra
mussel problem will be recognized by students in the East and Midwest. The African honeybee problem will be
familiar to students along the southern U.S. border.
1.
Use local examples to describe how any two species in the same community must occupy at least slightly
different niches. In the U.S. Midwest, corn has many insect herbivores: corn earworm, corn stem borer,
corn rootworm. How can they all survive in the same field?
2.
Discuss how organisms that have a host-parasite relationship must actually coevolve. Why would there
necessarily be a relationship between the phylogenies worked out for loons and loon lice? What is the fate
of a loon louse if its only host goes extinct? One way to help determine species relationships, such as
among birds, is to examine the relationship of the parasites that baby birds get from their parents in the
nest.
3.
Discuss disease as a parasitic relationship between the host and the disease-producing organism, and
consider the “new diseases,” such as AIDS, which kill the host, as opposed to a disease such as a cold,
which does not.
4.
The evolution of viral and bacterial diseases can also be explained as a case of cycling. The numbers of
individuals who come down with the current strain of influenza peaks, then most are resistant until another
strain evolves. Explain how this differs in time and mechanism from predator-prey cycling.
5.
Frederick Clements saw the changes in vegetation that occurred in Nebraska following a tornado or prairie
fire, but he was the first to see a pattern to it and call the phenomenon “succession.” His error was in
considering the community a “super-organism” that acted as a group. This might be a good place to briefly
discuss the recent Gaia hypothesis that is sometimes used to promote the whole world as a super-organism.
Currently scientists are debating the same issue: is the whole more than the sum of its parts?
6.
Successional changes are often in response to human-made disturbance. Indeed, there is little virgin
timberland or prairie left in the United States. Nearly all of Europe and populated China represents
surfaces that have been intensely cultivated for centuries. Animals likewise are heavily responsible for the
sequence of succession, through plowing of soil, distribution of seeds, etc. The term “climax” is being
replaced with the term “potential natural community” to reflect the difference between the current disturbed
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community and what might potentially be present in the absence of the human activity, without alluding to
a super-organism effect.
7.
Lecture question: For a farmer trying to predict how much damage will occur to crops, which model of
community structure would make it easiest to predict future outbreaks, interactive or individualistic? The
individualistic model, with insects' numbers dependent upon their own abiotic limitations, would be easier
to predict than community interactions.
8.
Frederick Clements first described succession of plant communities and went on to discuss plant
communities as a superorganism that moved in to replace earlier communities and was self-healing.
Gleason opposed this view and contended succession was a plant-by-plant struggle for resources that
eventually resulted in a community change. MacArthur investigated this in the field and proved Gleason
correct. This is of current interest because the concept of Gaia is often presented in a similar “world is a
superorganism and is self-healing” manner.
9.
Succession is usually presented as a sequence of changes in plant communities and consumer animals are
just left to follow along behind. Victor Shelford described the succession of the Indiana lakeshore dunes
including insect life. Recent work has shown animals to be critical in dispersing most plants and in soil
formation.
10.
Lecture questions: Why are exotic pests often more serious problems when they enter a new continent than
they were “back home.” What is happening when we plant an exotic in our garden and it cannot survive
outside the garden without our care and “weeding”?
Critical Thinking
Question 1. When researchers repeat the two paramecium/only one true medium experiment to test the
only-one-species in-a-niche hypothesis, often either one can be the surviving species—it is not always the same
species that dominates. Why might this be true?
Answer: While the environment and media are uniform, the species themselves have reproductive cycles and
different metabolic wastes. Starting the competition experiment at different stages in each organism’s cycle could
place one at a disadvantage. In addition, different numbers of each species or proportions used at the beginning
could tip the balance either way as different wastes are excreted.
Question 2. Rodents in Vietnam live with the plague bacteria without getting sick; rodents in India are moderately
resistant and rodents in Europe are killed rapidly. In contrast, peoples of European origin may become ill with
tuberculosis but rarely die of it, while the same bacterial tuberculosis is a much more virulent disease in Asia.
Medical researchers believe that plague historically originated in southeast Asia but tuberculosis was only recently
introduced into Asia. Why?
Answer: If a disease and a host have had long periods of time together, susceptible individuals have been
eliminated or left far fewer offspring while resistant hosts have survived and left more offspring. Therefore, the
current patterns of resistance reveal the time period the host and parasite have been together. Actually, there are also
historical records for the more recent introduction of tuberculosis to Asia.
Question 3. Why would succession today perhaps not lead to the same climax community that Clements first saw a
century ago?
Answer: Succession is a change in living communities over time, determined by the buildup of soil and
competition for resources such as light, water, and nutrients, and based on the available soil, seedbank, and plant
dispersal systems. It is based on the available species that are able to withstand the local physical and climatic
conditions. Primary succession begins on bare rock and requires much time for pioneer plants to build up the soil
layer to allow grasses, shrubs, and eventually forests to grow. Secondary succession begins with soil already
present; it usually has been moved back to an early stage by flood, fire, cultivation, glaciers, etc. and can rapidly
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move to a climax community if seeds are available. If the modern disturbances have greatly altered the
microclimate, the soil, or other factors, then the potential natural community may never be reached because the
community no longer has the organisms or tolerance to return to the earlier climax community.
Question 4. In the case of the bad-tasting monarch butterfly and the good-tasting viceroy that is a Batesian mimic,
what happens if there is a larger number of mimics? How would this differ in the case of a Mullerian mimic?
Answer: The effectiveness of the mimicry relies on predators learning the model is bad tasting or stings, etc. The
orange-and-black viceroy is harmless but achieves protection by resembling the bad-tasting monarch. If the Batesian
mimic (viceroy) becomes too common, it waters down the effectiveness of the warning pattern; too many birds like
eating the majority ov viceroys they catch. However, a Mullerian mimic also has anti-predator defenses and copies
others with similar warning coloration, thereby strengthening the effect of the warning coloration.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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