Ecological Communities

Introduction to Community Ecology

Community ecology is the study of interactions between species and the factors that affect the number, type, and relative abundance of species in a community. These interactions can be mutually beneficial, harmful, or neutral, and they play a critical role in shaping community structure.

• Community structure is influenced by competition, exploitation, positive interactions, foundation species, and disturbances.

• Foundation species provide habitat and food for other organisms, affecting community structure.

• Disturbances such as climate events and human activities can remove organisms or alter resource availability.

Factors Influencing Community Structure

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

• Disturbances can change community composition by removing organisms or altering resources.

• Foundation species (e.g., corals in reefs) provide essential structure for other species.

Interspecific Interactions

Types of Interspecific Interactions

Ecologists classify relationships between species in a community as interspecific interactions, grouped by their effects on survival and reproduction:

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

• Predation ( +/− ): One species benefits, the other is harmed.

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

• Parasitism ( +/− ): Parasite derives nourishment from a host, harming it.

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

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

Competition

Interspecific competition occurs when individuals of different species compete for a resource that limits their survival and reproduction. Species do not compete for resources that are abundant.

• Example: Weeds compete with garden plants for nutrients and water.

• Terrestrial species do not compete for oxygen, as it is plentiful.

Competitive Exclusion and Coexistence

The competitive exclusion principle states that two species competing for the same limiting resources cannot permanently coexist in one place. The inferior competitor will be locally eliminated.

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

Ecological Niches and Resource Partitioning

An ecological niche is the specific set of biotic and abiotic resources used by an organism. Ecologically similar species can coexist if their niches differentiate over time, a process called resource partitioning.

• Fundamental niche: The potential niche a species could occupy.

• Realized niche: The actual niche occupied in the presence of competitors.

Character Displacement

Character displacement occurs when two competing species with overlapping geographic ranges evolve differences in traits, reducing competition. Sympatric populations (living together) show more divergence than allopatric populations (living apart).

• Example: Darwin's finches show different beak sizes on the same island.

Exploitation: Predation, Herbivory, and Parasitism

Predation

Predation is an interaction where one species, the predator, kills and eats another, the prey. Predators have adaptations for capturing prey, while prey have evolved defenses.

• Predator adaptations: acute senses, claws, fangs, poison.

• Prey defenses: hiding, fleeing, herding, mechanical defenses (spines), chemical defenses (toxins), and camouflage (cryptic coloration).

Defensive Adaptations

• Cryptic coloration: Camouflage that makes prey difficult to see.

• Aposematic coloration: Bright warning colors in animals with chemical defenses.

• Batesian mimicry: A harmless species mimics a harmful one.

Herbivory

Herbivory is an exploitative interaction where an herbivore eats parts of a plant or alga, usually not killing it. Herbivores have adaptations for feeding on plants, and plants have evolved defenses such as toxins and spines.

• Examples: Cattle, sheep, sea urchins, and manatees are herbivores.

Parasitism

Parasitism is an interaction where one organism (the parasite) derives nourishment from another (the host), harming it. Parasites may live inside (endoparasites) or on the surface (ectoparasites) of the host.

• Parasites can have complex life cycles involving multiple hosts.

• They may alter host behavior to increase transmission.

Positive Interactions: Mutualism and Commensalism

Mutualism

Mutualism is an interaction that benefits both species. In some cases, one or both species cannot survive without the other; in others, both can survive independently but benefit from the interaction.

• Example: Acacia trees and ants depend on each other for survival.

• Both partners incur costs and benefits, but the benefits must outweigh the costs.

Commensalism

Commensalism benefits one species without affecting the other. For example, cattle egrets feed on insects flushed out by grazing herbivores, which are usually unaffected.

• Positive interactions can have large effects on community diversity and structure.

Community Structure: Diversity and Trophic Structure

Species Diversity

Species diversity is a key feature of community structure, consisting of species richness (number of species) and relative abundance (proportion of each species).

• Diversity can be measured using the Shannon diversity index:

• High diversity communities are more productive, stable, and resistant to invasive species.

Trophic Structure

Trophic structure describes the feeding relationships between organisms in a community. Food chains link trophic levels from producers to top carnivores, and food webs show complex feeding interactions.

• The position an organism occupies in a food chain is its trophic level.

Species with Large Impact

• Foundation species: Large or abundant species that provide habitat or food (e.g., trees, corals).

• Keystone species: Not abundant but have a pivotal role in maintaining community structure (e.g., sea stars).

• Ecosystem engineers: Species that cause physical changes in the environment (e.g., beavers building dams).

Bottom-Up and Top-Down Controls

• Bottom-up control: Community structure is controlled by nutrient availability and primary producers.

• Top-down control: Predators control the abundance of herbivores, which in turn control producers.

Disturbance and Succession

Disturbance and Community Dynamics

Disturbances such as storms, fires, and human activities can alter community structure by removing organisms or changing resource availability. The nonequilibrium model describes communities as constantly changing after disturbances.

• The intermediate disturbance hypothesis states that moderate levels of disturbance promote greater diversity than high or low levels.

Ecological Succession

Ecological succession is the sequential change in community composition following a disturbance.

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

• Secondary succession: Occurs where a disturbance removes most but not all organisms.

• Succession involves pioneer species, followed by intermediate and climax communities.

Biogeographic Factors Affecting Community Diversity

Latitudinal Gradients

• Species richness is greatest in the tropics and declines toward the poles.

• Key factors: evolutionary history and climate (sunlight and precipitation).

• Evapotranspiration (evaporation + transpiration) is correlated with diversity in terrestrial communities.

Area Effects and Island Biogeography

• Larger geographic areas support more species due to greater habitat diversity.

• The island equilibrium model predicts species richness based on island size and distance from the mainland.

• Species richness reaches a dynamic equilibrium where immigration balances extinction.

Pathogens and Community Structure

Effects of Pathogens

Pathogens, including disease-causing microorganisms and viruses, can dramatically alter community structure, especially when introduced to new habitats.

• Examples: White-band disease in coral reefs, sudden oak death in forests.

Zoonotic Diseases

• Many emerging human diseases are caused by zoonotic pathogens (transferred from animals to humans).

• Vectors (e.g., ticks, mosquitoes) often transmit these pathogens.

• Understanding host and vector communities helps prevent disease spread (e.g., Lyme disease, avian flu, COVID-19).