Community Ecology

Characterizing Biological Communities

Biological communities are groups of interacting species living in the same area. Communities are characterized by their species diversity, trophic structure, and the presence of species that have a disproportionate impact on the community.

• Species Diversity: Refers to the variety and abundance of different species in a community.

• Trophic Structure: Describes the feeding relationships among organisms, which determine the flow of energy and matter.

• Disproportionate Impact Species: Certain species play critical roles in maintaining community structure and function.

Species Diversity

Species diversity is a measure of the number of different species (species richness) and their relative abundance (evenness) in a given area. It is an important indicator of ecosystem health and stability.

• Species Richness: The total number of different species present in a community.

• Species Evenness: The relative abundance of each species in the community.

• Shannon Diversity Index: A commonly used mathematical formula to quantify species diversity, taking into account both richness and evenness.

Formula:

Where is the Shannon Diversity Index, is the total number of species, and is the proportion of individuals belonging to species .

• Importance of Diversity: Diverse communities are more resilient to disturbances such as disease, predation, and environmental changes. High diversity prevents the community from becoming vulnerable to the loss of any single species.

• Example: A tropical rainforest with hundreds of plant and animal species is more stable than a monoculture crop field.

Trophic Structure of Communities

The trophic structure of a community describes how energy and nutrients flow through different levels, from producers to consumers.

• Producers (Autotrophs): Organisms that produce their own food through photosynthesis (e.g., plants).

• Primary Consumers (Herbivores): Organisms that eat producers (e.g., deer).

• Secondary Consumers (Carnivores): Organisms that eat primary consumers (e.g., wolves).

• Tertiary Consumers: Organisms that eat secondary consumers (e.g., hawks).

• Decomposers: Organisms that break down dead organic matter (e.g., fungi, bacteria).

Energy Flow: Energy moves from producers up through the trophic levels via consumption. Arrows in diagrams typically show the direction of energy and matter flow.

• Example: Grass (producer) → Grasshopper (primary consumer) → Frog (secondary consumer) → Snake (tertiary consumer)

Additional info: Energy transfer between trophic levels is inefficient; typically only about 10% of energy is passed to the next level.

Species with Disproportionate Impact

Some species have a much greater effect on community structure than would be expected based on their abundance or biomass. These include dominant species, keystone species, and ecosystem engineers.

• Dominant Species: Species that are most abundant or have the highest biomass in a community. They exert significant control over the distribution and abundance of other species. Example: Oak trees in a temperate forest.

• Keystone Species: Species that have a disproportionately large effect on community structure relative to their abundance. Their removal can cause dramatic changes in the ecosystem. Example: Sea otters in kelp forest ecosystems; their predation on sea urchins maintains kelp populations.

• Ecosystem Engineers: Species that create, modify, or maintain physical habitat for other organisms. Example: Beavers build dams, creating wetlands that support diverse communities.

Summary Table: Types of Impactful Species

Key Takeaways

• Species diversity and trophic structure are fundamental aspects of community ecology.

• High diversity increases community resilience.

• Energy flows from producers to consumers, with decomposers recycling nutrients.

• Dominant, keystone, and ecosystem engineer species play critical roles in maintaining ecosystem structure and function.