AP Biology Unit 8 Ecology: How Communities Work, Why Diversity Matters, and What Happens When Ecosystems Are Disturbed

Community ecology

The study of how species living in the same place and time interact and how those interactions affect which species are present and how abundant they are.

Habitat

The physical place where an organism lives (its “address”).

Ecological niche

How a species lives—its role and resource use (e.g., what it eats, when/where it is active, how it avoids predators, where it reproduces).

Fundamental niche

The full range of conditions and resources a species could use in the absence of limiting biotic interactions (like competition or predation).

Realized niche

The range of conditions and resources a species actually uses in nature after biotic interactions restrict it.

Competition

An interaction where organisms use the same limited resource (e.g., food, space, light); can occur within a species (intraspecific) or between species (interspecific).

Competitive exclusion

The principle that two species with identical niches cannot stably coexist in the same environment; one will outcompete the other over time.

Resource (niche) partitioning

Coexistence mechanism where species reduce competition by using resources differently (different times, places, or forms of a resource).

Predation

An interaction in which one organism kills and eats another, potentially reshaping population sizes and community composition.

Trophic cascade

A chain of indirect effects across trophic levels caused by a change at one level (often predator changes affecting herbivores, then plant biomass/species composition).

Mutualism

A symbiotic relationship in which both species benefit (e.g., pollinators and flowering plants; mycorrhizal fungi and plant roots).

Commensalism

A symbiotic relationship in which one species benefits and the other is not significantly helped or harmed (e.g., epiphytes using trees for support).

Parasitism

A symbiotic relationship in which one species benefits while the host is harmed (e.g., pathogens, ticks).

Keystone species

A species with a disproportionately large effect on community structure relative to its abundance; removing it can cause major shifts in species composition and trophic structure.

Disturbance regime

The typical pattern of disturbances in an ecosystem (frequency, intensity, and timing); changes to the regime can strongly alter community structure and recovery.

Ecological succession

Gradual change in community composition after a disturbance, driven by changing conditions and species interactions over time.

Primary succession

Succession beginning where no soil exists (e.g., new lava, glacial retreat); early colonizers help create/build soil.

Secondary succession

Succession after a disturbance that leaves soil intact (e.g., fire, farming); recovery is often faster due to remaining soil/seed banks.

Biodiversity

The variety of life at multiple levels (genetic, species, ecosystem); often linked to ecosystem functioning, productivity, and stability.

Species richness

The number of different species present in a community.

Species evenness

How evenly individuals are distributed among species in a community (high evenness = similar abundances; low evenness = one species dominates).

Theory of island biogeography

Explains species richness on islands (or habitat patches) as a balance between immigration and extinction; predicts more species on larger islands and on islands closer to a source.

Invasive species

A typically non-native species that spreads rapidly and causes ecological or economic harm (not all introduced species are invasive).

Eutrophication

Nutrient enrichment (often N and P) in aquatic systems that triggers algal blooms; decomposition of algae can deplete dissolved oxygen (hypoxia), causing die-offs and reduced biodiversity.

Biomagnification

The increase in concentration of a persistent toxin at higher trophic levels; distinct from bioaccumulation (buildup within an organism over time).