Unit 3: Ecology of Communities

Community Concept

Community (Definition): A community (or biocenosis) is an assemblage of all the different populations of species (plants, animals, fungi, microbes) living and interacting in a particular geographic area at a particular time.

Community ecology studies the interactions between these species and how they influence the community's structure and organization.

Keystone Species

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

They are "engineers" of their community. Their removal can lead to a "trophic cascade" and the collapse or dramatic restructuring of the entire community. The term was coined by Robert Paine in 1969.

Classic Example: The Sea Otter (Enhydra lutris)

1. Keystone Species: Sea Otter.
2. Prey: Sea Urchin.
3. Primary Producer: Kelp (a large seaweed).
4. Interaction: Sea otters eat sea urchins. Sea urchins eat kelp.
5. Effect:
   • With Otters: Otters keep the urchin population in check. This allows large, healthy "kelp forests" to grow, which in turn provide a critical habitat for hundreds of other species (fish, crabs, etc.).
   • Without Otters: Urchin populations explode. They graze down all the kelp, creating "urchin barrens" (empty seabeds). The entire kelp forest ecosystem collapses.

Other Examples:

• Beaver: A "ecosystem engineer" that builds dams, creating wetlands that are used by many other species.
• Wolf (in Yellowstone): Reintroduction of wolves controlled elk populations, which allowed willows and aspens to regrow, which in turn stabilized riverbanks and brought back beavers and songbirds.

Ecotone and Edge Effect

These concepts were introduced in Unit 1 but are central to community ecology.

• Ecotone: The transition zone where two or more different communities meet and integrate (e.g., the boundary between a forest and a grassland).
• Edge Effect: The tendency for ecotones to have a greater variety and density of organisms than either of the adjoining communities.

Why does the Edge Effect happen?

• The edge provides habitats and resources from both adjacent communities.
• The unique microclimate and structure of the edge itself may create habitats for "edge specialists" that don't exist in either core community.

Example: The edge of a forest may have light-loving grassland plants, shade-tolerant forest plants, and edge-specialist shrubs. This variety of plants supports a high variety of insects, birds, and mammals.

Note: While edges often increase diversity, "hard" or artificial edges (like a clear-cut forest next to a highway) can be harmful, increasing exposure to wind, predators, and invasive species.

Species Interactions (Positive and Negative)

Interactions between species (interspecific) are the "glue" that holds a community together. They can be categorized by their effect (+ for benefit, - for harm, 0 for no effect) on the two species involved.

Interaction	Species 1	Species 2	Description & Example
Negative Interactions
Competition	-	-	Both species are harmed as they compete for the same limited resource (e.g., lions and hyenas competing for zebra).
Predation	+	-	One species (predator) kills and consumes the other (prey) (e.g., fox eating a rabbit).
Parasitism	+	-	One species (parasite) lives on or in another (host), deriving nourishment at the host's expense. The host is usually weakened, not killed (e.g., a tick on a deer).
Herbivory	+	-	A "predation" on plants. One species (herbivore) consumes parts of a plant (e.g., a deer eating leaves).
Amensalism	-	0	One species is harmed, and the other is unaffected. (e.g., allelopathy, where a Black Walnut plant releases chemicals that inhibit the growth of nearby plants).
Positive and Neutral Interactions
Mutualism	+	+	Both species benefit from the interaction. Obligate: Species cannot survive without each other (e.g., lichens = fungus + alga). Facultative: Species benefit but can survive alone (e.g., bees pollinating flowers).
Commensalism	+	0	One species benefits, and the other is completely unaffected (e.g., barnacles on a whale; the barnacle gets a home and free transport, the whale is unharmed).
Neutralism	0	0	Two species interact, but neither is affected. This is very rare or impossible to prove in nature, as all species are linked in the web.

Community Structure

Community structure refers to the physical and biological organization of a community. Key components include species diversity and stratification.

Species Diversity

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

1. Species Richness: The total number of different species in the community.
   • Community A (10 oaks, 10 maples) and Community B (100 oaks, 1 maple) have the same richness (2 species).
2. Species Evenness (or Relative Abundance): The proportion of individuals that belong to each species. A community is "even" if all species are present in similar numbers.
   • Community A is very even. Community B is very uneven (dominated by oaks).

High species diversity (both high richness and high evenness) is generally a sign of a healthy, stable, and resilient ecosystem (like a tropical rainforest). Low diversity is common in harsh environments (like the arctic tundra) or polluted sites.

Stratification

Definition: Stratification is the vertical layering of a community, especially in forests and aquatic ecosystems.

Different layers provide different microhabitats (e.g., different levels of light, temperature, and humidity), which allows more species to coexist by partitioning (dividing) resources.

Example: Stratification in a Temperate Forest

• Canopy Layer: The uppermost layer of tall, mature trees (e.g., oaks, maples). Receives full sunlight. Home to eagles, squirrels.
• Understory Layer: Shorter trees and young trees (e.g., dogwood, saplings). Receives filtered sunlight. Home to many songbirds.
• Shrub Layer: Woody shrubs (e.g., berries, rhododendrons). Low light. Home to rabbits, deer.
• Herb/Ground Layer: Non-woody plants, ferns, and mosses on the forest floor. Very low light. Home to insects, mice, salamanders.
• Forest Floor (Litter/Soil): Decomposing organic matter (leaves, logs). Home to fungi, bacteria, worms, and millipedes.

Aquatic Stratification (in a Lake):

• Epilimnion: Top layer, warm, well-lit (photosynthesis).
• Metalimnion (Thermocline): Middle layer, rapid temperature drop.
• Hypolimnion: Bottom layer, cold, dark, low oxygen.

Ecological Succession

Definition and Types

Definition: Ecological succession is the predictable and orderly process of change in a community's structure over time, following a disturbance or the creation of a new habitat.

One set of species colonizes an area, changes the environment, and is then replaced by another set of species better suited to the new conditions.

There are two main types:

1. Primary Succession:
   • Starts on: A brand new, lifeless substrate where no soil exists.
   • Examples: A bare rock exposed by a landslide, a new volcanic island (lava flow), or land exposed by a retreating glacier.
   • Process: Very slow. Starts with pioneer species (like lichens and mosses) that can break down rock to create the very first layer of soil. Over centuries, small herbs, then shrubs, and finally trees can establish.
2. Secondary Succession:
   • Starts on: An area where a previous community was disturbed or destroyed, but the soil remains intact.
   • Examples: An abandoned farm field, a clear-cut forest, or an area after a forest fire.
   • Process: Much faster than primary succession. Pioneer species are usually fast-growing "weeds" and grasses from the existing seed bank in the soil. These are followed by shrubs, then fast-growing "pioneer trees" (like pine), and finally shade-tolerant "hardwood trees" (like oak and hickory).

Climax Community

Definition: A climax community is the stable, mature, and self-sustaining community that is the final stage of ecological succession.

This community is in equilibrium with its environment (especially the climate) and will remain relatively unchanged unless a major disturbance occurs. This traditional view was proposed by Frederic Clements.

Characteristics of a Climax Community:

• High species diversity.
• Complex food webs.
• High biomass.
• Nutrient cycling is efficient and "closed" (nutrients are recycled within the system rather than lost).
• Resistant to small disturbances.