The final unit — zooming out from molecules and cells to populations, communities, and ecosystems. How organisms respond to their environment, how populations grow (and what limits them), how energy flows through food webs, and how disruptions and biodiversity shape ecosystems.
Seven topics from the College Board CED, in order.
Topic 8.1
Responses to the Environment
Behavioral and physiological responses organisms use to survive changing or challenging environmental conditions.
Topic 8.2
Energy Flow Through Ecosystems
Trophic levels, food chains and food webs, energy pyramids, and the roughly 10% energy transfer rule between levels.
Topic 8.3
Population Ecology
Population size, density, dispersion, and the exponential vs. logistic growth models.
Topic 8.4
Effect of Density on Populations
Density-dependent factors (competition, disease) vs. density-independent factors (weather, natural disasters) limiting population growth.
Topic 8.5
Community Ecology
Species interactions — competition, predation, mutualism, commensalism, parasitism — and their effects on community structure.
Topic 8.6
Biodiversity
Why biodiversity matters for ecosystem stability and resilience, and how human activity affects it.
Topic 8.7
Disruptions in Ecosystems
How natural and human-caused disturbances (fires, invasive species, climate change) affect ecosystem structure and function.
About Unit 8
Unit 8 zooms out from the molecular and cellular scale of earlier units to the level of populations, communities, and ecosystems. You'll learn how organisms respond to and survive environmental challenges, how energy flows (and is lost) as it moves up trophic levels in a food web, and what factors determine how big a population can grow — including density-dependent factors like competition and disease, and density-independent factors like natural disasters.
This unit is 10–15% of the AP Bio exam and takes about 13–15 class periods. The most-tested topics are reading and interpreting energy pyramids and food webs, distinguishing exponential from logistic population growth, identifying types of species interactions (competition, predation, mutualism, etc.), and explaining how disruptions and biodiversity loss affect ecosystem stability.
The College Board ties Unit 8 to three of the four Big Ideas:
Big Idea 1
Evolution — environmental responses and species interactions create selective pressures
Big Idea 2
Energetics — energy flows through trophic levels and is lost as heat at each transfer
Big Idea 4
Systems — populations, communities, and ecosystems are interacting systems with feedback