Unit 3: Ecology of communities

Community Structure and Organization

A community is an assemblage of all the populations of different species living and interacting in a particular area (e.g., the "forest community" includes all the trees, birds, insects, and fungi).

Community structure is defined by:

• Species Richness: The total number of different species in the community.
• Species Abundance: The number of individuals per species.
• Species Diversity: A measure that combines both richness and evenness (the relative abundance of species).

Keystone Species

A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance.

They are not necessarily the most abundant species, but their removal can cause the entire ecosystem to change drastically, or even collapse.

Classic Example: The sea otter in the Pacific kelp forests.

• Sea otters (keystone predator) eat sea urchins.
• Sea urchins eat kelp (a type of large algae).
• If sea otters are removed, the sea urchin population explodes, devours all the kelp, and destroys the entire kelp forest ecosystem, which was a habitat for many other species.

Other examples include wolves (control deer populations), beavers (create wetlands), and starfish.

Ecotone and Edge Effect

Ecotone

An ecotone is a transition area between two adjacent, different ecosystems or communities (e.g., the area between a forest and a grassland, or between a river and its bank).

Edge Effect

The edge effect refers to the changes in population or community structures that occur at the boundary (ecotone) of two habitats.
Key characteristics of an edge:

• It has a unique setof abiotic conditions (e.g., more sunlight, more wind than the forest interior).
• It often contains species from both adjacent communities, plus some species that are specific to the edge itself.
• This often results in higher species richness and density in the ecotone than in either community alone.

Species Interactions

Interactions between species are the driving force of community ecology. They can be positive (+), negative (-), or neutral (0) for the species involved.

Ecological Succession and Climax Community

Ecological succession is the predictable, gradual process of change in the species structure of a community over time, often following a disturbance.

Types of Succession

• Primary Succession: Occurs on a lifeless surface where no soil exists. (e.g., a new volcanic lava flow, a bare rock exposed by a retreating glacier).
  1. Pioneer Species (e.g., lichens, mosses) colonize the bare rock.
  2. They break down the rock and create the first soil.
  3. Grasses and herbs move in, followed by shrubs, and finally trees.
  4. This process can take thousands of years.
• Secondary Succession: Occurs in an area where an existing community has been disturbed or destroyed, but the soil remains intact. (e.g., after a forest fire, an abandoned farm field).
  1. Pioneer species are annual plants (weeds).
  2. Grasses and perennials move in, followed by shrubs and fast-growing trees (e.g., pine).
  3. Finally, slower-growing hardwood trees (e.g., oak, hickory) establish.
  4. This process is much faster than primary succession.

Climax Community

The climax community is the traditional term for the final, stable, mature, and self-perpetuating community that develops at the end of succession.