Chapter 53 Community Ecology
Overview: What Is a Community?
• A community is defined as an assemblage of species living close
enough together for potential interaction.
• Communities differ in their species richness, the number of species
they contain, and the relative abundance of different species.
Concept 53.1 A community’s interactions include competition,
predation, herbivory, symbiosis, and disease
• There are a number of possible interspecific interactions that link the
species of a community.
• Interspecific interactions can be symbolized by the positive (+) or
negative (?) effects of the interaction on the individual populations.
• 0 indicates that a population is not affected by the interaction.
• The effect of an interaction between two species may change
as circumstances change.
• Interspecific competition can occur when species compete for a
specific limiting resource.
• When two species compete for a resource, the result is
detrimental to one or both species (?/?)
• Strong competition can lead to the local elimination of one of the two
competing species, a process called competitive exclusion.
• The competitive exclusion principle states that two species with
similar needs for the same limiting resources cannot coexist in
the same place.
• The ecological niche is the sum total of a species’ use of abiotic and
biotic resources in the environment.
• In the analogy stated by ecologist Eugene Odum, an
organism’s habitat is its “address,” and the niche is the
organism’s “profession.” • For example, the niche of a tropical tree lizard includes the
temperature range it tolerates, the size of branches it perches
on, the time of day when it is active, and the kind of insects it
• The competitive exclusion principle can be restated to say that
two species cannot coexist in a community if their niches are
• However, ecologically similar species can coexist in a
community if their niches differ in one or more significant ways.
• A species’ fundamental niche is the niche potentially occupied by that
• The fundamental niche may differ from the realized niche, the
niche a species actually occupies in a particular environment.
• When competition between two species with identical niches does not
lead to the local extinction of either species, it is generally because
evolution by natural selection results in modification of the resources
used by one of the species.
• Resource partitioning is the differentiation of niches that
enables two similar species to coexist in a community.
• Character displacement is the tendency for characteristics to be
more divergent in sympatric populations of two species than in
allopatric populations of the same two species.
• Predation is a +/- interaction between species in which one species,
the predator, kills and eats the other, the prey.
• The term predation elicits images such as a lion attacking and eating
• This interaction also includes interactions such as seed
predation, in which seed-eating weevils eat plant seeds.
• Natural selection favors adaptations of predators and prey. • Predators have many feeding adaptations, including acute
senses and weaponry such as claws, fangs, stingers, or poison
to help catch and subdue prey.
• Predators that pursue prey are generally fast and agile; those
who lie in ambush are often camouflaged.
• Prey animals have evolved adaptations that help them avoid being
• Behavioral defenses include fleeing, hiding, and self-defense.
• Alarm calls may summon many individuals of the prey species
to mob the predator.
• Adaptive coloration has evolved repeatedly in animals.
• Camouflage or cryptic coloration makes prey difficult to
spot against the background.
• Some animals have mechanical or chemical defenses.
• Chemical defenses include odors and toxins.
• Animals with effecting chemical defenses often exhibit
bright warning aposematic coloration.
• Predators are cautious in approaching potential
prey with bright coloration.
• One prey species may gain protection by mimicking the appearance
of another prey species.
• In Batesian mimicry a harmless, palatable species mimics a
harmful, unpalatable model.
• In Müllerian mimicry, two or more unpalatable species resemble
• Each species gains an additional advantage because
predators are more likely to encounter an unpalatable
prey and learn to avoid prey with that appearance.
• Predators may also use mimicry. • Some snapping turtles have tongues resembling wiggling
worms to lure small fish.
• Herbivory is a +/- interaction in which an herbivore eats parts of a
plant or alga.
• Herbivores include large mammals and small invertebrates.
• Herbivores have specialized adaptations.
• Many herbivorous insects have chemical sensors on their
feet to recognize appropriate food plants.
• Mammalian herbivores have specialized dentition and
digestive systems to process vegetation.
• Plants may produce chemical toxins, which may act in combination
with spines and thorns to prevent herbivory.
• Parasitism is a +/? symbiotic interaction in which a parasite derives
its nourishment from a host, which is harmed in the process.
• Endoparasites live within the body of the host; ectoparasites
live and feed on the external surface of the host.
• Parasitoidism is a special type of parasitism in which an insect
(usually a wasp) lays eggs on or in living hosts.
• The larvae feed on the body of the host, eventually killing
• Many parasites have complex life cycles involving a number of
• Some parasites change the behavior of their hosts in ways that
increase the probability of the parasite being transferred from
one host to another.
• Parasites can have significant direct and indirect effects on the
survival, reproduction, and density of their host populations.
• Pathogens are disease-causing agents that have deleterious effects
on their hosts (+/?)
• Pathogens are typically bacteria, viruses, or protists. • Fungi and prions can also be pathogenic.
• Parasites are generally large, multicellular organisms, while most
pathogens are microscopic.
• Many pathogens are lethal.
• Mutualism is an interspecific symbiosis in which two species benefit
from their interaction (+/+).
• Examples of mutualism include nitrogen fixation by bacteria in
the root nodules of legumes; digestion of cellulose by
microorganisms in the guts of ruminant mammals; and the
exchange of nutrients in mycorrhizae, the association of fungi
and plant roots.
• Mutualistic interactions may result in the evolution of related
adaptations in both species.
• Commensalism is an interaction that benefits one species but neither
harms nor helps the other (+/0).
• Commensal interactions are difficult to document in nature
because any close association between species likely affects
both species, if only slightly.
• For example, “hitchhiking” species, such as the barnacles that
attach to whales, are sometimes considered commensal.
• The hitchhiking barnacles gain access to a substrate and
seem to have little effect on the whale.
• However, the barnacles may slightly reduce the host’s
efficiency of movement.
• Conversely, they may provide some camouflage.
• Coevolution refers to reciprocal evolutionary adaptations of two
• A change in one species acts as a selective force on another
species, whose adaptation in turn acts as a selective force on
the first species. • The linkage of adaptations requires that genetic change in one
of the interacting populations of the two species be tied to
genetic change in the other population.
• An example is the gene-for-gene recognition between a
plant species and a species of virulent pathogen.
• In contrast, the aposematic coloration of various tree frog
species and the aversion reactions of various predators
are not examples of coevolution.
• These are adaptations to other organisms in the
community rather than coupled genetic changes in
two interacting species.
Concept 53.2 Dominant and keystone species exert strong controls
on community structure
Species diversity is a fundamental aspect of community structure.
• A small number of species in the community exert strong control on
that community’s structure, especially on the composition, relative
abundance, and diversity of species.
• The species diversity of a community is the variety of different kinds
of organisms that make up the community.
• Species diversity has two components.
• Species richness is the total number of different species in the
• The relative abundance of the different species is the proportion
each species represents of the total individuals in the
• Species diversity is dependent on both species richness and
• Measuring species diversity may be difficult, but is essential for
understanding community structure and for conserving biodiversity.
Trophic structure is a key factor in community dynamics. • The trophic structure of a community is determined by the feeding
relationships between organisms.
• The transfer of food energy up the trophic levels from its source in
autotrophs (usually photosynthetic organisms) through herbivores
(primary consumers) and carnivores (secondary and tertiary
consumers) and eventually to decomposers is called a food chain.
• In the 1920s, Oxford University biologist Charles Elton recognized
that food chains are not isolated units but are linked together into
• A food web uses arrows to link species according to who eats
whom in a community.
• How are food chains linked into food webs?
• A given species may weave into the web at more than one
• Food webs can be simplified in two ways.
• We can group species in a given community into broad
• For example, phytoplankton can be grouped as primary
producers in an aquatic food web.
• A second way to simplify a food web is to isolate a portion of
the web that interacts little with the rest of the community.
• Each food chain within a food web is usually only a few links long.
• Charles Elton pointed out that the length of most food chains is
only four or five links.
• Why are food chains relatively short?
• The energetic hypothesis suggests that the length of a food
chain is limited by the inefficiency of energy transfer along the
chain. • Only about 10% of the energy stored in the organic matter
of each trophic level is converted to organic matter at the
next trophic level.
• The energetic hypothesis predicts that food chains should
be relatively longer in habitats with higher photosynthetic
• The dynamic stability hypothesis suggests that long food chains
are less stable than short chains.
• Population fluctuations at lower trophic levels are
magnified at higher levels, making top predators
vulnerable to extinction.
• In a variable environment, top predators must be
able to recover from environmental shocks that can
reduce the food supply all the way up the food
• The dynamic stability hypothesis predicts that food chains
should be shorter in unpredictable environments.
• Most of the available data supports the energetic hypothesis.
• Another factor that may limit the length of food chains is that, with the
exception of parasites, animals tend to be larger at successive trophic
• Certain species have an especially large impact on community
structure because they are highly abundant or because they play a
pivotal role in community dynamics.
• The exaggerated impact of these species may occur through
their trophic interactions or through their influences on the
• Dominant species are those species in a community that are most
abundant or have the highest biomass (the sum weight of all
individuals in a population).
• There is no single explanation for why a species becomes dominant
in a community. • One hypothesis suggests that dominant species are
competitively successful at exploiting limiting resources.
• Another hypothesis suggests that dominant species are most
successful at avoiding predation or disease.
• This could explain why invasive species can achieve such
high biomass in their new environments, in the absence
of their natural predators and pathogens.
• One way to investigate the impact of a dominant species is to remove
it from the community.
• Keystone species are not necessarily abundant in a community.
• They influence community structure by their key ecological
• If keystone species are removed, community structure is greatly
• Ecologist Robert Paine of the University of Washington first
developed the concept of keystone species.
• Paine removed the sea star Pisaster ochraceous from rocky
• Pisaster is a predator on mussels such as Mytilus
californianus, a superior competitor for space in the
• After Paine removed Pisaster, the mussels were able to
monopolize space and exclude other invertebrates and
algae from attachment sites.
• When sea stars were present, 15 to 20 species of
invertebrates and algae occurred in the intertidal zone.
• After experimental removal of Pisaster, species diversity
declined to fewer than 5 species.
• Pisaster thus acts as a keystone species, exerting an
influence on community structure that is disproportionate
to its abundance. • Some organisms exert their influence by causing physical
changes in the environment that affect community structure.
• An example of such a species is the beaver, which