Community Ecology

Learning Objectives

Community ecology explores the interactions among species within a defined area and how these interactions shape the structure and dynamics of biological communities. Key objectives include understanding species richness, abundance, distribution, and the outcomes of species interactions.

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

• Species Abundance: The number of individuals of each species in a community.

• Species Evenness: The relative abundance of different species, indicating how evenly individuals are distributed among species.

• Biodiversity: A measure that incorporates both species richness and evenness.

• Community Structure: The organization and physical arrangement of species within a community.

Characteristics of Biological Communities

Definition and Structure

A biological community is an assemblage of populations of different species living close enough together to form short-term and long-term interactions. Communities are defined by their species composition and the interactions among those species.

• Species Richness: The total number of species present.

• Species Evenness: The relative abundance of each species.

• Complex Interactions: Communities are dynamic systems with self-regulating interactions shaped by ongoing evolution.

Example: Coral reefs, savannas, rainforests, and human microbiomes all represent biological communities with distinct species compositions and interactions.

Comparing Communities

Metrics for Comparison

Ecologists compare communities using several metrics:

• Species Richness – Number of species present.

• Species Evenness – How evenly individuals are distributed among species.

• Species Diversity – Weighted measurement incorporating both richness and evenness (e.g., Shannon index).

• Species Distribution – Geographic range and abundance patterns.

Example Table: Community Comparison

Species Interactions

Types of Interactions

Species interactions are central to community ecology and can be classified by their effects on fitness:

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

• Competition (-/-): Both species are harmed by the interaction as they compete for the same resources.

• Consumption (+/-): Includes predation, herbivory, and parasitism; one species benefits, the other is harmed.

• Mutualism (+/+): Both species benefit from the interaction.

Example: Barnacles on whales (commensalism), lions and hyenas competing for prey (competition), bees pollinating flowers (mutualism).

Competition and the Niche Concept

Competition occurs when individuals use the same resources, limiting fitness for both. The niche is the range of resources and conditions a species can use or tolerate.

• Fundamental Niche: The full range of environmental conditions and resources a species could theoretically use.

• Realized Niche: The portion of the fundamental niche actually occupied, limited by competition and other biotic factors.

Competitive Exclusion Principle: Two species with completely overlapping niches cannot coexist indefinitely; one will outcompete the other.

Resource Partitioning: Evolutionary change leads to differentiation of niches, allowing similar species to coexist.

Character Displacement: Evolution of differences in traits that reduce niche overlap and competition.

Consumption Interactions

Types and Effects

Consumption interactions include predation, herbivory, and parasitism. These interactions can drive population dynamics and evolutionary change.

• Predation: One organism kills and eats another.

• Herbivory: An organism eats parts of a plant or algae.

• Parasitism: One organism lives on or in a host, harming it.

Example: Lynx and hare population cycles, biocontrol agents in agriculture.

Coevolution: Reciprocal evolutionary adaptations between interacting species, such as predator-prey arms races.

Defensive Adaptations

• Physical Defenses: Shells, spines, thorns.

• Chemical Defenses: Toxins, secondary metabolites (e.g., nicotine, caffeine).

• Behavioral Defenses: Flocking, mimicry, warning coloration.

Batesian Mimicry: Edible species mimics a poisonous species.

Müllerian Mimicry: Two or more unpalatable species resemble each other.

Mutualism

Types and Examples

Mutualistic interactions provide net fitness benefits to both species involved. These can be obligate (essential for survival) or facultative (beneficial but not essential).

• Pollination: Bees and flowers.

• Mycorrhizal Fungi: Fungi and plant roots.

• Microbiomes: Gut bacteria and humans.

• Coral Reefs: Sea anemones and clownfish.

Generalists vs. Specialists: Generalists interact with many species; specialists interact with few. Specialists may be more efficient but are more vulnerable to extinction.

Community Structure

Key Attributes

Community structure is shaped by the number and relative abundance of species, the sum of interactions, and physical attributes (abiotic and biotic factors).

• Dominant Species: Species with the highest abundance or biomass.

• Keystone Species: Species with a disproportionately large effect on community structure relative to their abundance.

Example: Sea otters in kelp forests, wolves in Yellowstone.

Food Webs and Trophic Structure

Food webs illustrate consumption interactions and energy flow through trophic levels:

• Autotrophs: Primary producers at the base.

• Primary Consumers: Herbivores.

• Secondary/Tertiary Consumers: Carnivores and omnivores.

Trophic Cascade: Changes in abundance at one trophic level can cause effects two or more levels away.

Community Dynamics: Disturbance and Succession

Disturbance

A disturbance is a short-lived event that disrupts community structure, altering the distribution of living and nonliving resources. Examples include fires, storms, floods, and disease outbreaks.

• Resistance: Ability of a community to withstand disturbance.

• Resilience: Ability to recover after disturbance.

Communities with higher species richness tend to have greater resistance and resilience.

Succession

Succession is the predictable series of changes in community composition following disturbance.

• Primary Succession: Occurs when disturbance removes soil and organisms (e.g., after a glacier retreat or volcanic eruption).

• Secondary Succession: Occurs when disturbance removes organisms but leaves soil intact (e.g., after fire or logging).

Pioneering Species: First to colonize disturbed areas; typically r-selected, fast-growing, and short-lived.

• Facilitation: Early species make conditions favorable for later arrivals.

• Tolerance: Species tolerate existing conditions and establish independently.

• Inhibition: Early species hinder the establishment of others.

Example: Successional stages in Glacier Bay, Alaska.

Key Equations and Indices

Shannon Diversity Index

The Shannon index is a common measure of species diversity:

Where is the proportion of individuals in the th species, and is the total number of species.

Summary Table: Types of Species Interactions

Additional info: Academic context and examples have been expanded for clarity and completeness. All key terms are defined, and tables are reconstructed for study purposes.