Community Ecology: Structure, Interactions, and Dynamics

Community Structure and Interactions

Factors Influencing Community Structure

Community structure is shaped by the number, composition, and relative abundance of different species, as well as the interactions among them. These interactions can be mutualistic, competitive, predatory, or neutral, and they collectively determine the organization and function of the community.

• Ecological interactions include mutualism, competition, predation, herbivory, parasitism, and commensalism.

• Example: The cleaner wrasse feeds on parasites in the mouth of a moray eel, benefiting both species (mutualism).

Interspecific Interactions

Types of Interspecific Interactions

Interspecific interactions are relationships between individuals of different species. These interactions can be classified based on their effects on the survival and reproduction of the species involved:

• Competition (–/–): Both species are harmed.

• Exploitation (+/–): One species benefits, the other is harmed (includes predation, herbivory, parasitism).

• Positive interactions (+/+ or +/0): At least one species benefits, and neither is harmed (includes mutualism and commensalism).

Competition and Competitive Exclusion

Competition occurs when species vie for the same limited resources, reducing the fitness of both. The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist permanently in the same place.

• Example: Paramecium aurelia outcompetes P. caudatum when grown together.

Ecological Niches and Resource Partitioning

An organism’s ecological niche is the sum of its use of biotic and abiotic resources. If two species have identical niches, they cannot coexist. However, resource partitioning allows similar species to coexist by differentiating their niches in space or time.

• Fundamental niche: The full range of conditions a species can potentially use.

• Realized niche: The actual range occupied due to competition.

• Example: The common spiny mouse and golden spiny mouse partition their activity times to reduce competition.

Exploitation: Predation, Herbivory, and Parasitism

Predation

Predation is an interaction where one species (predator) kills and eats another (prey). Predators have adaptations for capturing prey, while prey have evolved defenses to avoid predation.

• Behavioral defenses: Hiding, fleeing, forming groups.

• Mechanical defenses: Physical structures like quills.

• Chemical defenses: Production of toxins or repellents.

• Aposematic coloration: Bright warning colors signal toxicity.

• Cryptic coloration: Camouflage to avoid detection.

• Batesian mimicry: Harmless species mimics a harmful one.

• Müllerian mimicry: Two or more unpalatable species resemble each other.

• Predator mimicry: Some predators mimic harmless species to approach prey.

Herbivory

Herbivory is an interaction where an herbivore eats parts of a plant or alga. Herbivores have adaptations for locating and consuming plants, while plants have evolved mechanical and chemical defenses.

Parasitism

Parasitism is a +/– interaction where the parasite derives nourishment from its host, harming it in the process. Parasites can be internal (endoparasites) or external (ectoparasites) and can significantly affect host populations.

Positive Interactions: Mutualism and Commensalism

Mutualism

Mutualism is an interaction that benefits both species. Some mutualisms are obligate, while others are facultative.

• Example: Acacia trees and ants—ants protect the tree, and the tree provides food and shelter.

Commensalism

Commensalism benefits one species without affecting the other. Some commensal relationships can shift to mutualism under certain conditions.

• Example: Cattle egrets feed on insects disturbed by grazing herbivores; sometimes, they also remove parasites from the herbivores.

Facilitation

Some species can positively influence community structure by making the environment more suitable for other species.

• Example: The black rush (Juncus) increases plant diversity in salt marshes by improving soil conditions.

Community Diversity and Trophic Structure

Species Diversity

Species diversity is a measure of the variety of organisms in a community, incorporating both species richness (number of species) and relative abundance (proportion of each species).

• The Shannon diversity index (H) is commonly used to quantify diversity:

$H = -\sum (p_i \cdot \ln p_i)$ where $p_i$ is the relative abundance of species $i$.

Diversity and Community Stability

Higher-diversity communities tend to be more productive, stable, and resistant to invasive species. Experimental studies show that plant communities with greater species richness produce more biomass and recover better from disturbances.

Trophic Structure and Food Webs

Trophic structure describes the feeding relationships among species in a community. Energy flows from primary producers to various levels of consumers and decomposers, forming food chains and complex food webs.

• Trophic level: The position an organism occupies in a food chain.

• Food web: A network of interconnected food chains.

• Most food chains are short, typically with five or fewer links, due to energy loss at each trophic level (about 10% transfer efficiency).

$\text{Energy transfer efficiency} \approx 10\%$

Species with Large Impacts

Some species have a disproportionate effect on community structure:

• Foundation species: High abundance or biomass; provide habitat or resources (e.g., trees).

• Keystone species: Low abundance but pivotal ecological roles (e.g., sea stars controlling mussel populations).

• Ecosystem engineers: Modify the physical environment (e.g., beavers building dams).

Disturbance and Community Dynamics

Disturbance and Its Effects

A disturbance is an event that changes a community by removing organisms or altering resource availability. Disturbances vary in frequency and intensity, and their effects can shape community structure and diversity.

• Intermediate disturbance hypothesis: Moderate levels of disturbance foster the highest diversity by preventing dominance by a few species and allowing coexistence.

• Example: Lodgepole pine forests recover rapidly after large-scale fires.

Ecological Succession

Ecological succession is the sequence of community changes following a disturbance:

• Primary succession: Begins in lifeless areas (e.g., new volcanic islands); initial colonizers are prokaryotes and protists, followed by lichens, mosses, and eventually plants.

• Secondary succession: Occurs where a disturbance removes most organisms but leaves soil intact (e.g., abandoned farmland).

Human Disturbance

Human activities are the most significant source of disturbance in modern ecosystems, often reducing species diversity and altering community structure in both terrestrial and marine environments.