Chapter 54: Community Ecology – Study Notes

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Chapter 54: Community Ecology

Introduction to Community Ecology

Community ecology is the study of interactions between species that coexist in the same geographic area. These interactions shape the structure and dynamics of biological communities.

Biological community: An assemblage of populations of different species living close enough for potential interaction.
Communities are dynamic, with species composition and abundance changing over time.
Example: The Salmonella disease event demonstrates how community interactions can affect species and ecosystem health.

Community Interactions

Types of Species Interactions

Species interactions are classified by whether they help, harm, or have no effect on the species involved.

Competition (-/-): Both species are harmed by the interaction.
Predation (+/-): One species benefits (predator), the other is harmed (prey).
Herbivory (+/-): An animal eats part of a plant or alga.
Parasitism (+/-): Parasite derives nourishment from host, harming it.
Mutualism (+/+): Both species benefit.
Commensalism (+/0): One species benefits, the other is unaffected.

Competitive Exclusion Principle

The competitive exclusion principle states that two species competing for the same limiting resource cannot coexist permanently in the same place.

One species will outcompete the other, leading to local elimination of the inferior competitor.
Example: Paramecium species grown together in the same environment.

Ecological Niches and Resource Partitioning

An ecological niche is the sum of a species' use of biotic and abiotic resources in its environment.

Resource partitioning: Differentiation of niches that enables similar species to coexist in a community.
Example: Anole lizards occupying different parts of a tree to reduce competition.

Fundamental vs. Realized Niche

The fundamental niche is the niche a species could potentially occupy, while the realized niche is the niche it actually occupies due to competition and other biotic factors.

Example: Barnacle species occupying different zones on a rocky shore.

Character Displacement

Character displacement is the tendency for characteristics to diverge more in sympatric populations of two species than in allopatric populations of the same species.

Example: Variation in beak size among Galápagos finches.

Exploitation

Predation

Predation is an interaction in which one species, the predator, kills and eats the other, the prey.

Predators have adaptations such as claws, fangs, and poison.
Prey have evolved defenses, including behavioral, morphological, and physiological adaptations.
Examples of prey defenses: camouflage, mimicry, chemical defenses, and warning coloration.

Herbivory

Herbivory is an interaction in which an herbivore eats parts of a plant or alga.

Plants have evolved mechanical and chemical defenses against herbivores.

Parasitism

Parasitism is a relationship in which one organism (the parasite) derives nourishment from another organism (the host), which is harmed in the process.

Parasites may significantly affect community structure by influencing the abundance and distribution of their hosts.

Positive Interactions

Mutualism

Mutualism is an interspecific interaction that benefits both species.

Obligate mutualism: At least one species cannot survive without the other.
Facultative mutualism: Both species can survive alone.
Example: Acacia trees and ants, pollinators and flowering plants.

Commensalism

Commensalism is an interaction in which one species benefits and the other is neither helped nor harmed.

Example: Cattle egrets and African buffalo.

Community Structure

Species Diversity

Species diversity is the variety of different kinds of organisms that make up the community.

Species richness: The number of different species in the community.
Relative abundance: The proportion each species represents of all individuals in the community.

Measuring Diversity

Shannon Diversity Index (): where is the relative abundance of species and is the number of species.

Diversity and Community Stability

Communities with higher diversity are generally more productive and more stable in their productivity. They are better able to withstand and recover from environmental stresses and are more resistant to invasive species.

Trophic Structure

Food Chains and Food Webs

Trophic structure is the feeding relationship between organisms in a community.

Food chain: The transfer of food energy up the trophic levels from primary producers to top carnivores.
Food web: A network of interconnecting food chains.

Limits on Food Chain Length

Most food chains are short, typically containing five or fewer links.
Energetic hypothesis: Length of a food chain is limited by the inefficiency of energy transfer along the chain.

Species with a Large Impact

Dominant species: Species that are most abundant or have the highest biomass.
Keystone species: Species that exert strong control on community structure by their ecological roles, not necessarily by abundance.
Foundation species (ecosystem engineers): Species that dramatically alter their environment.

Bottom-Up and Top-Down Controls

Bottom-up model: Unidirectional influence from lower to higher trophic levels (N → V → H → P).
Top-down model (trophic cascade): Predation controls community organization (N ← V ← H ← P).

Disturbance and Community Structure

Disturbance

Disturbance is an event that changes a community by removing organisms or altering resource availability.

Examples: Fires, storms, floods, human activity.

The Intermediate Disturbance Hypothesis

This hypothesis suggests that moderate levels of disturbance foster greater species diversity than do high or low levels of disturbance.

Low disturbance: Dominance by a few species.
High disturbance: Exclusion of many species.

Ecological Succession

Ecological succession is the sequence of community and ecosystem changes after a disturbance.

Primary succession: Occurs in lifeless areas where soil is not yet formed.
Secondary succession: Occurs where a disturbance has destroyed a community but left the soil intact.
Example: Succession on the moraines of Glacier Bay, Alaska.

Biogeographic Factors Affecting Community Diversity

Latitudinal Gradients

Species diversity is generally higher in the tropics and declines toward the poles.
Possible causes: Evolutionary history, climate, and productivity.

Area Effects

Larger areas tend to have more species due to greater habitat diversity and opportunities for colonization.

Island Equilibrium Model

This model describes the balance between immigration and extinction rates on islands, predicting the number of species that an island can support.

Equilibrium is reached when immigration equals extinction.
Model can be applied to "islands" of habitat in terrestrial environments.

Pathogens and Community Structure

Effects of Pathogens

Pathogens can alter community structure locally and globally by affecting dominant species, keystone species, and ecosystem engineers.
Example: Introduction of chestnut blight fungus in North America.

Community Ecology and Zoonotic Diseases

Zoonotic pathogens: Diseases transferred from animals to humans, often via vectors.
Example: Lyme disease, West Nile virus.

Summary Table: Types of Species Interactions

Interaction	Effect on Species 1	Effect on Species 2	Example
Competition	-	-	Paramecium species
Predation	+	-	Lion and zebra
Herbivory	+	-	Caterpillar and leaf
Parasitism	+	-	Tape worm and human
Mutualism	+	+	Bees and flowers
Commensalism	+	0	Cattle egret and buffalo

Key Equations

Shannon Diversity Index:

Summary

Community ecology explores the interactions among species and their effects on community structure and dynamics.
Species interactions include competition, predation, herbivory, parasitism, mutualism, and commensalism.
Community structure is shaped by species diversity, trophic structure, disturbance, and biogeographic factors.
Pathogens and human activities can significantly alter community composition and stability.