Inside This Unit: The Full Breakdown
Ecology examines how organisms interact with each other and their environments, how energy flows through ecosystems, how matter cycles, and how populations grow and are regulated. This unit connects individual biology to the global scale.
Why it matters
Ecology questions on the AP exam often require you to interpret data on population growth, energy pyramids, or nutrient cycles. Free-response questions frequently combine ecology with evolution or cell biology, so be prepared to make cross-unit connections.
Key concepts
- Energy flows through ecosystems in one direction: producers → primary consumers → secondary consumers → tertiary consumers. Only about 10% of energy transfers between trophic levels.
- Biogeochemical cycles (carbon, nitrogen, water, phosphorus) recycle matter through biotic and abiotic components of ecosystems.
- Population growth follows exponential or logistic models; carrying capacity (K) limits growth through density-dependent factors like competition and disease.
- Community interactions (competition, predation, mutualism, parasitism) and disturbances shape biodiversity and ecosystem stability.
Energy Flow and Trophic Levels
Ecosystems are powered by energy from the sun, captured by autotrophs (producers) through photosynthesis. This energy moves through trophic levels: primary consumers eat producers, secondary consumers eat primary consumers, and so on. At each transfer, roughly 90% of energy is lost as heat through cellular respiration, which is why food chains rarely exceed four or five levels. Ecological pyramids illustrate this loss — energy pyramids always narrow toward the top. Gross primary productivity (GPP) is the total energy captured by producers; net primary productivity (NPP) is what remains after producers use some for their own respiration. NPP determines how much energy is available to support consumers in an ecosystem.
Biogeochemical Cycles
Unlike energy, matter is recycled within ecosystems through biogeochemical cycles. The carbon cycle moves carbon through the atmosphere (CO₂), living organisms (organic molecules), fossil fuels, and the ocean. Photosynthesis removes CO₂ from the atmosphere; respiration and combustion return it. The nitrogen cycle converts atmospheric N₂ into usable forms through nitrogen fixation (by bacteria), then cycles it through nitrification, assimilation, ammonification, and denitrification. The water cycle moves water through evaporation, transpiration, condensation, and precipitation. The phosphorus cycle has no atmospheric component — phosphorus moves from rocks through weathering into soil, water, and organisms. Human activities (burning fossil fuels, using fertilizers) are disrupting these cycles at unprecedented rates.
Population Ecology and Community Interactions
Population ecologists study how populations grow, what limits them, and how they change over time. Exponential growth (dN/dt = rN) occurs when resources are unlimited, producing a J-shaped curve. Logistic growth (dN/dt = rN[(K−N)/K]) incorporates carrying capacity (K), producing an S-shaped curve as growth slows near K. Density-dependent factors (competition, predation, disease) intensify as population size increases, while density-independent factors (natural disasters, weather) affect populations regardless of size. Within communities, species interact through competition (both lose resources), predation (one gains, one loses), mutualism (both benefit), and parasitism (one gains, one is harmed). Keystone species have disproportionate effects on community structure relative to their abundance.
AP exam tip
When interpreting population growth graphs on the AP exam, always identify whether the population is in exponential or logistic growth phase. For logistic growth, note that the fastest growth rate occurs at N = K/2 (the inflection point of the S-curve), not at the beginning.
Connections to other units
- Unit 3 (Cellular Energetics): Photosynthesis and respiration are the biochemical basis of energy flow through ecosystems.
- Unit 7 (Natural Selection): Ecological interactions like predation and competition are selective pressures driving evolution.
- Unit 1 (Chemistry of Life): Biogeochemical cycles involve the same molecules (carbon compounds, water, nitrogen) studied at the molecular level.
