Community Ecology

Community Ecology

Communities in Motion

• A biological community is made up of different species living close enough to interact with each other.

Community Interactions

• Interactions between species in a community can include competition, predation, herbivory, symbiosis, and facilitation.

• These interactions can be positive (+), negative (−), or neutral (0) for each species.

Competition

• Interspecific competition (-/-) happens when different species compete for the same limited resource.

• Competitive exclusion principle: If two species need the same limited resources, they can't both live in the same place.

Ecological Niches and Natural Selection

• Ecological niche: How a species uses all the resources (biotic and abiotic) available to it.

• Resource partitioning: When species evolve to use resources differently, allowing them to live together.

• Fundamental niche: The full range of resources a species could potentially use.

• Realized niche: The actual resources a species uses, often less than it could due to competition.

• Character displacement: Differences in characteristics are more noticeable when species live together compared to when they live apart.

Predation

• Predation (+/-) is when one species (the predator) kills and eats another (the prey).

• Prey have defenses like hiding, running away, making alarm calls, and physical or chemical defenses.

Herbivory

• Herbivory (+/-) is when an herbivore eats parts of a plant or alga.

• Plants have defenses like thorns and toxic chemicals.

Symbiosis

• Symbiosis: When two or more species live closely together.

Parasitism

• Parasitism (+/-): One organism (the parasite) benefits by feeding on another (the host), which is harmed.

• Endoparasites live inside the host; ectoparasites live on the outside.

Mutualism

• Mutualism (+/+) benefits both species involved.

• Obligate mutualism: Species that cannot survive without each other.

• Facultative mutualism: Species that can survive on their own.

Commensalism

• Commensalism (+/0): One species benefits, while the other is not affected.

Facilitation

• Facilitation (+/+ or 0/+): One species helps another without direct contact.

Diversity and Trophic Structure

• Species diversity: The variety of species in a community.

• Species richness: The number of different species.

• Relative abundance: How common each species is.

• Shannon diversity index (H): H = − (pA ln pA + pB ln pB + pC ln pC + …)

Trophic Structure

• Trophic structure: The feeding relationships in a community.

• Food chains connect who eats whom, from producers to top predators.

• Food web: A complex network of interconnected food chains.

• Energetic hypothesis: Food chains are limited by how much energy is lost at each level (about 90%).

Species Impact

• Dominant species: The most common species by number or biomass.

• Keystone species: Species that have a big impact on the community.

• Bottom-up model: N \to V \to H \to P

• Top-down model: N \leftarrow V \leftarrow H \leftarrow P

Disturbance and Equilibrium

• Disturbance: An event that changes a community and the resources available.

• Nonequilibrium model: Communities are always changing due to disturbances.

• Intermediate disturbance hypothesis: Moderate disturbance can lead to more diversity.

Ecological Succession

• Ecological succession: How a community changes over time after a disturbance.

• Primary succession: Happens where there is no soil.

• Secondary succession: Happens where soil is still present.

Biogeographic Factors

• Latitude and area affect how diverse a community is.

• Species richness is highest in the tropics.

• Evapotranspiration: Water evaporating from the soil and plants.

• Species-area curve: S = cA^z

Island Equilibrium Model

• Species richness on islands depends on size, distance from the mainland, immigration, and extinction.

• Equilibrium model: The number of species will stabilize when immigration equals extinction.

Pathogens

• Pathogens can change community structure.

• Zoonotic pathogens: Pathogens that can transfer from animals to humans.

• Avian flu: A very contagious bird virus.