Community Ecology: Interactions, Diversity, and Structure

Community Ecology

Definition of Community

A community in ecology refers to an assemblage of different species living together in a particular area, interacting with each other in various ways. Communities are characterized by the species present, their abundance, and the nature of their interactions.

Interspecific Interactions

Species within a community interact in multiple ways, which can be classified based on their effects on the individuals involved:

• Mutualism: Both species benefit (+/+).

• Commensalism: One species benefits, the other is unaffected (+/0).

• Parasitism: One species benefits at the expense of the other (+/−).

• Predation: One organism (predator) kills and eats another (prey) (+/−).

• Herbivory: An animal eats part of a plant or alga (+/−).

• Competition: Both species are harmed by the interaction (−/−).

Each interaction affects the population densities and evolutionary trajectories of the species involved.

Competition and the Competitive Exclusion Principle

• Competition occurs when two or more species use the same limited resource.

• The competitive exclusion principle states that two species competing for the same limiting resource cannot coexist at constant population values; one will outcompete the other.

• Ecological niche: The sum of a species' use of biotic and abiotic resources in its environment.

• Resource partitioning: The differentiation of niches that enables similar species to coexist in a community.

Types of Interspecific Interactions

• Predation: Adaptations such as camouflage, mimicry, and defensive mechanisms evolve in prey species.

• Herbivory: Plants may develop chemical toxins or physical defenses against herbivores.

• Parasitism: Parasites derive nourishment from their host, often harming but not immediately killing it.

• Mutualism: Both partners benefit, such as pollinators and flowering plants.

• Commensalism: One species benefits, the other is unaffected, e.g., barnacles on whales.

Community Structure: Richness, Evenness, and Trophic Structure

Communities are often described by their species richness (number of different species) and relative abundance (proportion of each species). Trophic structure refers to the feeding relationships among organisms, which determine the flow of energy and nutrients.

• Species richness: The total number of species in a community.

• Relative abundance: The proportion each species represents of all individuals in the community.

• Trophic levels: Positions in the food chain (producers, primary consumers, secondary consumers, etc.).

Measuring Diversity: Simpson's Diversity Index

The Simpson's Diversity Index is a quantitative measure of diversity in a community, taking into account both richness and evenness. The formula is:

$D = 1 - \sum \left( \frac{n_i}{N} \right)^2$

• $n_i$ = number of individuals of species i

• $N$ = total number of individuals of all species

The value of D ranges from 0 (no diversity) to 1 (infinite diversity). Higher values indicate greater diversity.

Example: If a community has 3 species with 10, 20, and 30 individuals, respectively, calculate $D$ as follows:

• Total individuals, $N = 10 + 20 + 30 = 60$

• Proportions: $10/60$, $20/60$, $30/60$

• Sum of squares: $(10/60)^2 + (20/60)^2 + (30/60)^2$

• Calculate $D$ using the formula above.

Diversity and Stability

There is often a positive correlation between diversity and stability in communities. More diverse communities tend to be more resilient to disturbances and better able to recover from environmental changes.

Foundation and Keystone Species

• Foundation species: Species that have a strong role in structuring a community by creating or maintaining habitat (e.g., corals, trees).

• Keystone species: Species that have a disproportionately large effect on community structure relative to their abundance (e.g., sea otters, wolves).

Removal of keystone species can lead to dramatic changes in community composition.

Models of Community Organization

• Individualistic hypothesis: Communities are collections of species with similar environmental requirements.

• Interactive hypothesis: Communities are integrated units with stable relationships among species.

Disturbance and Succession

• Disturbance: An event that changes a community by removing organisms or altering resource availability (e.g., fire, storm).

• Succession: The sequence of community and ecosystem changes after a disturbance.

• Primary succession: Occurs in lifeless areas (e.g., after a volcanic eruption).

• Secondary succession: Occurs where a disturbance has cleared an existing community but left the soil intact.

Biogeography and Community Diversity

• Biogeography: The study of the distribution of species and ecosystems in geographic space and through geological time.

• Factors such as area size, isolation, and habitat diversity affect community diversity.

• Island biogeography: Explains how species richness is affected by island size and distance from the mainland.

Pathogens and Community Structure

• Pathogens can alter community structure by affecting the abundance and distribution of species.

• Examples include disease outbreaks that reduce populations of dominant species, allowing others to increase.

Summary Table: Types of Interspecific Interactions

Interaction Type	Effect on Species 1	Effect on Species 2	Example
Mutualism	+	+	Bees and flowering plants
Commensalism	+	0	Barnacles on whales
Parasitism	+	−	Tapeworms in mammals
Predation	+	−	Lions hunting zebras
Herbivory	+	−	Caterpillars eating leaves
Competition	−	−	Plants competing for sunlight

Key Equations

• Simpson's Diversity Index:

$D = 1 - \sum \left( \frac{n_i}{N} \right)^2$

Additional info: Academic context and examples have been added to expand on the brief points in the original file, ensuring the notes are self-contained and suitable for exam preparation.