Unit 8: Ecology
AP Biology: 52 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Unit 8 scales biology up: how organisms respond to environments, how energy and matter move through ecosystems, and how populations and communities change.
Responses to the environment
Organisms sense and respond: phototropism in plants (auxin), taxis in animals, circadian rhythms. Behavioral responses (migration, hibernation) match seasonal change. Communication includes visual, chemical, and auditory signals.
Energy flow
Energy enters as sunlight, is captured by producers, and flows one way — about 10% transfers between trophic levels (efficiency), so biomass pyramids narrow upward and food chains rarely exceed 4–5 levels. Matter cycles (carbon, nitrogen, water) through biogeochemical cycles with reservoirs.
Population ecology
Populations grow exponentially when resources are unlimited (r-strategists) but logistically with carrying capacity K (density-dependent limits: competition, disease, predation). Density-independent factors (weather, fire) hit regardless of density. Survivorship curves (Type I/II/III) track mortality patterns.
Community ecology
Interactions: competition (−/−), predation (+/−), mutualism (+/+), commensalism (+/0), parasitism (+/−). The competitive exclusion principle says two species cannot share one niche indefinitely. Keystone species have outsized effects. Ecological succession: primary (bare rock → lichens → soil) and secondary (after disturbance, soil remains).
Biodiversity and disruption
Biodiversity (species richness + evenness) boosts resilience and ecosystem services. Disturbances — invasive species, habitat loss, pollution, climate change — reduce diversity. Invasive species lack native predators; habitat fragmentation isolates populations. Conservation focuses on keystone species and habitat corridors.