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Chapter 8 Summary
8-1 Community Structure and Species Diversity
Ecologists describe the structure of a community in terms of its physical appearance, species
diversity, and niche structure.
The diversity of terrestrial species declines with distance from the equator (latitude). In marine
communities, species diversity varies with depth and pollution levels.
MacArthur and Wilson's species equilibrium model, also called the theory of island
biogeography, predicts that species diversity will increase with island size and with proximity to
a mainland. This model also applies to habitat islands, such as national parks.
8-2 Types of Species
Many native species have suffered as a result of the introduction of nonnative species or alien
species. Birds are excellent indicator species and can serve as an early warning of damage to a
community. The decline of a keystone species can cause a ripple effect through an ecosystem.
Foundation species shape communities by creating and enhancing habitat that benefits other
species.
8-3 Species Interactions: Competition and Predation
Ecologists have identified five basic types of interactions between species: interspecific
competition, predation, parasitism, mutualism, and commensalism. Over time, resource
partitioning may allow similar species to coexist.
Predation benefits a predator at the expense of its prey. Carnivores feed on mobile prey, and
herbivores feed on plants. Predators have evolved a variety of ways to increase their chances of
finding food, and prey have evolved various ways to defend themselves against or avoid
predators.
8-4 Species Interactions: Parasitism, Mutualism, and Commensalism
In parasitism, the parasite benefits and the host is harmed. Mutualism benefits both species.
Commensalism benefits one species and leaves the other unaffected.
8-5 Ecological Succession: Communities in Transition
The two types of ecological succession are primary succession and secondary succession. With
primary succession, pioneer species colonize an area. Over time, they may be replaced by early
successional plant species, which may be replaced by midsuccessional plant species. These
species can give way to late successional plant species. However, research indicates that we
cannot predict the course of succession or view it as progress toward an ideally adapted climax
community.
Compared to primary succession, secondary succession occurs very rapidly.
A disturbance or change in environmental conditions disrupts an ecosystem or community and
affect its species diversity and stage of succession. According to the intermediate-disturbance
hypothesis, communities that experience fairly frequent but moderate disturbances have the
greatest species diversity.
8-6 Ecological Stability, Complexity, and Sustainability
The stability of communities is maintained by constant dynamic change in response to changing
environmental conditions. Three aspects of sustainability in living systems are inertia or
persistence, constancy, and resilience. The exact relationship between species diversity and these
aspects of ecosystem stability is still under investigation. However, the precautionary principle
suggests that we should prevent damage to the environment, even if some cause-and-effect
relationships are not fully understood.