Unit 3: Community Ecology

1. Community Structure and Organization

A community is an assemblage of different populations (multiple species) living and interacting in the same area. Community ecology studies the factors that influence the structure, organization, and diversity of species within a community.

Key attributes of a community include:

Species Composition: The list of all species present.
Species Diversity: The variety and abundance of different species.
Dominant Species: Species that are the most abundant or have the highest biomass.
Physiognomy: The overall physical structure of the community (e.g., forest vs. grassland).
Trophic Structure: The feeding relationships (food web).

2. Keystone Species

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

If a keystone species is removed, the entire community structure can change dramatically. They are "key" to holding the ecosystem together.

Examples:

Sea Otter (Predator): Sea otters eat sea urchins, which eat kelp. When otters are present, they control urchins, allowing kelp forests to thrive.
Beaver (Ecosystem Engineer): Beavers build dams, creating wetlands that provide habitat for many other species.
Fig Trees (Resource Provider): In tropical forests, fig trees provide a critical, year-round food source for many animals.

Exam Tip: Do not confuse keystone species with dominant species.

Dominant Species: High impact because it has high abundance (e.g., oak trees).
Keystone Species: High impact despite having low abundance (e.g., sea otters).

3. Ecotone and Edge Effect

Ecotone

An ecotone is a transition area or boundary between two different ecosystems (e.g., the area between a forest and a grassland).

Edge Effect

The edge effect is the tendency for ecotones to have a higher density and diversity of species than either of the two adjacent ecosystems.

This is because the ecotone contains species from both adjacent habitats, plus unique "edge species" adapted to the transition zone.

Diagram: Two large overlapping circles, one labeled "Forest" and one "Grassland." The overlapping region is labeled "Ecotone (Edge Effect)". List sample species for each zone.

4. Species Diversity

Species diversity is a measure of the variety of species in a community. It has two components:

Species Richness: The total number of different species.
Species Evenness (or Relative Abundance): The proportion of individuals that belong to each species. High evenness means all species have similar population sizes.

A community is considered more diverse if it has high richness AND high evenness. Ecologists often use indices like the Shannon-Wiener Index to calculate diversity.

5. Species Interactions (Positive & Negative)

Species interactions are classified by their effect (+, -, or 0) on the species involved.

Mnemonic: This table is essential to memorize.

Interaction	Species 1	Species 2	Description
Competition	- (Negative)	- (Negative)	Both are harmed by the struggle for a limited resource.
Predation	+ (Positive)	- (Negative)	One species (predator) kills and eats another (prey).
Herbivory	+ (Positive)	- (Negative)	One species (herbivore) eats parts of a plant.
Parasitism	+ (Positive)	- (Negative)	One species (parasite) lives on or in another (host), harming it.
Mutualism	+ (Positive)	+ (Positive)	Both species benefit. (e.g., bees and flowers).
Commensalism	+ (Positive)	0 (Neutral)	One species benefits, the other is unaffected. (e.g., barnacles on a whale).
Amensalism	- (Negative)	0 (Neutral)	One species is harmed, the other is unaffected. (e.g., an elephant stepping on an ant).

6. Ecological Succession and Climax

Ecological Succession: The predictable and orderly process of change in a community's species structure over time, following a disturbance.

Types of Succession

Primary Succession

This occurs in a lifeless environment where no soil exists.

Starting Point: Bare rock (volcanic eruption), sand dunes.
Pioneer Species: The first to colonize, such as lichens and mosses, which create soil.
Process: Very slow. Lichens → Mosses → Grasses → Shrubs → Trees.

Secondary Succession

This occurs in an area where a previous community was destroyed, but the soil remains intact.

Starting Point: Forest after a fire, abandoned farm field.
Pioneer Species: Often annual plants or "weeds" from the soil seed bank.
Process: Much faster. Annuals → Grasses/Perennials → Shrubs → Pioneer Trees → Mature Trees.

Seral Stages and Climax Community

Seral Stage: Each intermediate community stage in succession.
Climax Community: The final, stable, and mature community that is self-perpetuating and in equilibrium with the environment (e.g., a mature oak-hickory forest).