07 Communities and Ecological Succession

How communities are organized

A biological community is made up of populations of different species that live and interact in the same area. Community structure describes which species are present, how abundant they are, and how they are distributed and connected by interactions such as feeding and competition.

An ecosystem includes the community as well as nonliving conditions such as soil, water, and climate. This distinction helps separate the living relationships within an area from the broader system that includes its physical environment.

Species occupy niches: the conditions they tolerate, the resources they use, and the interactions that shape their way of life. When two species have strongly overlapping resource needs, they may compete. They may also coexist by partitioning resources—for example, by feeding at different times or in different parts of a habitat.

Some species have especially large effects on community structure. A keystone species can influence many other populations even if it is not abundant. A foundation species, such as reef-building coral, creates or defines habitat that other organisms use.

Takeaway: Community structure depends on the species present, their niches, their relative abundances, and their interactions.

How species affect one another

Species interactions can be described by their average effects on each partner:

• Competition: both species are negatively affected as they contend for resources, such as plants competing for light and water $(-/-)$.

• Predation or herbivory: one species benefits by consuming another organism or its tissues, as when a hawk eats a mouse or a caterpillar eats leaves $(+/-)$.

• Parasitism: a parasite benefits while its host is harmed, as when a tick feeds on a mammal $(+/-)$.

• Mutualism: both species benefit, as when a pollinator gets nectar while helping a plant reproduce $(+/+)$.

• Commensalism: one species benefits while the other is not measurably affected $(+/0)$.

These effects can vary with circumstances. Predators and herbivores can limit prey or plant populations, while competition can change access to resources. Interactions may also have indirect effects: a predator that reduces herbivore numbers may allow plant abundance to increase.

Takeaway: Interactions influence populations directly and can also change other parts of a community indirectly.

How communities change over time

Ecological succession is change in a community’s species composition over time. It often follows a disturbance, but it is not a fixed progression toward one inevitable final community. The course of succession depends on local conditions, the disturbance, which organisms arrive, and how species change conditions for one another.

Primary succession starts where soil is absent or very limited, such as on rock exposed after a glacier retreats. Early colonists can help break down rock and add organic matter. As conditions change, other species may be able to establish.

Secondary succession follows a disturbance where soil remains, such as a fire or an abandoned field. Seeds, roots, and other surviving organisms can help the community recover, so secondary succession often proceeds faster than primary succession.

For example, hardy plants and other organisms may first colonize newly exposed ground near a retreating glacier. As soil develops, additional plants can establish and change shade, nutrients, and habitat for animals. Which species appear, and when, varies from place to place; succession does not always lead to a stable, unchanging endpoint.

Takeaway: The presence or absence of soil distinguishes primary from secondary succession, while local conditions shape the particular sequence of change.

Describing biodiversity

Biodiversity can be described at genetic, species, and ecosystem levels. To compare communities, ecologists often consider species richness—the number of species—and evenness—how similar their abundances are.

Two communities can have the same species richness but different evenness. In one, the species may have similar numbers of individuals; in another, a single species may dominate. Counting species alone would not show this difference in abundance.

Physical conditions, available resources, disturbance, dispersal, and interactions among species all shape biodiversity. Succession may change diversity, but biodiversity does not necessarily increase steadily at every stage. Because species are linked through food webs, competition, and habitat relationships, changes in one population can affect others.

Takeaway: Richness and evenness describe different aspects of community diversity, and neither alone explains all the relationships that shape a community.

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