AP Biology 8.5-8.6: Community Ecology

Ecological community

A group of populations of multiple species that occupy the same geographic area at the same time. Community ecology examines their distributions, abundances, demographic patterns, and interactions, along with relevant abiotic factors.

Community ecology

The study of how populations of different species interact and how biotic and abiotic factors shape community composition, structure, abundance, and diversity.

How can abiotic factors influence the composition of an ecological community?

Variables such as temperature, precipitation, soil pH, and nutrient availability affect which species can survive and how strongly they interact. Consequently, communities in deserts may differ greatly from those in tropical rainforests.

Community structure

The composition and organization of species in a community, including their abundances and ecological interactions. Food webs and other biological networks can be used to represent it.

Niche

The role of a species in its community, including how it uses resources, interacts with other organisms, and responds to environmental conditions.

Niche partitioning

The division of resources or environmental conditions among species, such as feeding at different times or using different prey. It reduces interspecific competition and promotes coexistence.

Why can species with different niches coexist more readily than species with identical niches?

Different niches reduce overlap in resource use, so species limit their own populations more than they limit one another. Intraspecific competition is therefore greater than interspecific competition.

How does the number of filled niches generally relate to community biodiversity?

A community with more distinct, occupied niches can support more species, provided that suitable resources and environmental conditions are available.

Trophic level

An organism's position in a food chain or food web, based on how it obtains energy. Producers occupy the first trophic level, followed by consumers at progressively higher levels.

Primary producers

Autotrophs that make organic compounds using photosynthesis or chemosynthesis. They form the energetic base of a community's food web.

Decomposers

Organisms such as fungi and bacteria that obtain energy by breaking down dead organisms and wastes. They return usable nutrients to the environment and support primary producers.

How do energy and matter move through trophic levels?

Energy flows from producers to consumers and decreases at each transfer because organisms use energy for metabolism and release heat. Decomposers break down dead material and recycle nutrients back to producers.

Food chain versus food web

A food chain shows one sequence of energy transfer, such as grass → rabbit → fox. A food web connects many overlapping food chains and more accurately represents most communities.

Apex predator

A predator at the top of a community's food web that is not normally eaten by another species in that community. Its trophic position can change if a larger predator is introduced.

Why can an organism's trophic level change between communities?

Its trophic level depends on the species it consumes and the predators present. For example, tuna may be an apex predator in one community but a prey species when sharks are present.

Guild

A group of species that uses the same resources in similar ways, regardless of whether the species are closely related. Examples include herbivores, carnivores, or birds and mammals that forage for ground-dwelling arthropods.

How does membership in the same ecological guild affect species interactions?

Guild members have overlapping resource-use patterns, so they are likely to compete. Closely related species may belong to the same guild, but shared function does not require close evolutionary relatedness.

Influential species

Species whose direct or indirect interactions have unusually large effects on community structure, stability, or functioning. Their loss can trigger broad changes in other populations.

Foundation species

Species that strongly shape community dynamics by creating or modifying physical habitat. They may occur at any trophic level but are often producers, such as mangroves that provide nursery habitat.

Keystone species

A species whose effect on community structure is disproportionately large relative to its abundance. Removing it can cause a trophic cascade and reduce community stability or biodiversity.

Foundation species versus keystone species

Foundation species influence communities mainly by creating or modifying habitat, whereas keystone species have an unusually large ecological effect relative to their abundance, often through trophic interactions. A species can be important in either role.

How did the loss of wolves illustrate a top-down trophic cascade?

Without wolves, elk populations increased and overgrazed vegetation. Reduced vegetation altered food and habitat availability for other organisms, decreasing community biodiversity and changing ecosystem functioning.

Ecological engineer

A species that creates, maintains, or modifies physical features of a habitat, changing the resources and conditions available to other organisms.

How can beavers act as ecological engineers?

By cutting trees and building dams, beavers alter water flow, riparian vegetation, and habitat connections. These changes can increase biodiversity and facilitate movement of organisms such as frogs.

Living shield or biotic resistance

The resistance of an intact, mature community to invasion because native species occupy niches and interact strongly with potential invaders. Native predators and high species richness can suppress invasive populations.

Interspecific interaction

An ecological interaction between individuals or populations of different species. Its effects are commonly represented as positive (+), negative (-), or neutral (0) for each participant.

Competition

A negative-negative interaction $(-/-)$ in which species use the same limited resource. Competition can restrict population size, biomass, species richness, and community diversity.

Interference competition

Competition in which one organism directly prevents another from accessing a resource, territory, or opportunity. Examples include a lion driving a hyena from a carcass or allelopathic chemicals inhibiting another plant.

Exploitative competition

Indirect competition that occurs when one species consumes a limited resource, leaving less available for another species. Herbivores eating the same meadow grass are an example.

Apparent competition

A negative interaction between two prey species that share a predator. The prey do not compete directly, but an increase in one may increase predator abundance and thereby increase predation on the other.

How can size asymmetry affect competition within a community?

If larger individuals obtain disproportionately more resources, competition becomes size-asymmetric. Strong size asymmetry can alter community structure and reduce the diversity of species or individuals that can persist.

Predation

A positive-negative interaction $(+/-)$ in which one organism gains energy by consuming another. It includes kill-and-consume interactions, herbivory, and the resource-taking aspect of parasitism.

Why can predator and prey populations undergo cycles?

Predator abundance often rises when prey are abundant, increasing predation and causing prey numbers to fall. Reduced prey availability then lowers predator abundance, allowing prey populations to recover.

Specialist versus generalist predation

A specialist predator relies primarily on one prey species, whereas a generalist predator can use multiple prey types and switch when its preferred prey becomes scarce.

Mutualism

A positive-positive interaction $(+/+)$ in which both species benefit. Examples include Rhizobium bacteria and legumes, and bees obtaining nectar while pollinating flowering plants.

How does the Rhizobium-legume relationship exemplify mutualism?

The legume supplies photosynthetic products to bacteria living in root nodules. The bacteria fix atmospheric nitrogen and provide the plant with nitrogen-containing compounds such as ammonium or amino acids.

Commensalism

A positive-neutral interaction $(+/0)$ in which one species benefits while the other experiences no detectable benefit or harm.

Inquilinism, phoresy, and metabiosis

Inquilinism occurs when one species uses another for permanent shelter or support, such as an orchid growing on a tree. Phoresy uses another organism for transport, while metabiosis uses an environment prepared by another organism.

Amensalism

A negative-neutral interaction $(-/0)$ in which one species is harmed while the other is not significantly affected. A large tree shading and outcompeting a sapling can illustrate this relationship.

Parasitism

A long-term positive-negative interaction $(+/-)$ in which a parasite obtains resources from a living host while harming it. Parasites may live inside the host or on its surface and do not necessarily kill the host immediately.

Vector in a parasitic life cycle

An organism that transmits a parasite between hosts. In malaria, an infected female Anopheles mosquito carries Plasmodium and passes it to a vertebrate during feeding.

How does brood parasitism benefit a cuckoo?

The cuckoo lays its egg in another bird's nest, shifting the costs of incubation and feeding to the host. The cuckoo chick may eliminate host offspring, increasing its access to parental resources.

How can coevolution arise from predation or parasitism?

Each species exerts selection on the other: prey or hosts evolve defenses, while predators or parasites evolve counteradaptations. This reciprocal process can produce an evolutionary arms race.

Neutralism

A neutral-neutral interaction $(0/0)$ in which neither species shows a noticeable effect on the other. True neutralism is difficult to establish because indirect interactions often connect species in a community.

How can human activities alter community structure?

Habitat disturbance, invasive species, climate change, and chemical pollution can change species abundances, interactions, and niche availability. These effects may propagate through food webs and alter entire ecosystems.

Biodiversity

The variety of life in a region. At the community level, biodiversity is commonly described using species richness and the relative abundance or evenness of the species present.

Species richness

The number of different species in a community or sample. Richness does not account for how evenly individuals are distributed among those species.

Relative abundance and species evenness

Relative abundance is the proportion of individuals belonging to each species. Evenness describes how similar those proportions are; a community dominated by one species has lower evenness than one with similarly abundant species.

How can two communities have the same species richness but different biodiversity?

They may contain the same number of species but differ in relative abundance. The community with more even abundances generally has greater species diversity.

Simpson's diversity index

A measure that incorporates both species richness and relative abundance. It is commonly expressed as $D=1-\sum(n_i/N)^2$, where $n_i$ is the number of individuals of species $i$ and $N$ is the total number of individuals. Larger values indicate greater diversity.