AP Biology / Unit 8

AP Biology Unit 8: Ecology

Ecology is the most underestimated AP Biology unit. It's 10–15% of the exam, includes quantitative problems, and got a 2025–26 update — biogeochemical cycles are now explicitly in Topic 8.2. Don't skip it.

Aligned to the 2025–26 College Board CED (Topics 8.1–8.7).

Energy flow through ecosystems

Energy flows one direction — producers → consumers → heat — and is not recycled. Matter is recycled (biogeochemical cycles). Mixing these up is a common FRQ point-loser.

The 10% rule

About 10% of energy at one trophic level transfers to the next; ~90% is lost as heat through respiration. That's why pyramids widen at the base and food chains rarely exceed 4–5 levels.

Productivity

GPP: total energy producers capture.
NPP = GPP − Respiration: what's actually available to consumers.

Biogeochemical cycles (new emphasis in 2025–26)

Cycle	Key steps and organisms
Carbon	Respiration / combustion / decomposition release CO₂; photosynthesis and ocean dissolution remove it. Fossil fuel burning is the major human input.
Nitrogen	N₂ → NH₄⁺ (nitrogen-fixing bacteria, Rhizobium) → NO₂⁻ → NO₃⁻ (nitrifiers) → plants assimilate; ammonification recycles; denitrifiers return N₂.
Phosphorus	No atmospheric reservoir. Weathering of rocks → PO₄³⁻ → plants → animals → decomposition. Limiting nutrient in many aquatic systems → eutrophication risk.
Water	Transpiration/evaporation → atmosphere → precipitation → roots, runoff, or groundwater.

Population ecology

Model	Conditions	Curve	Equation
Exponential	Unlimited resources, no predation	J-curve	dN/dt = rN
Logistic	Limited resources; carrying capacity K	S-curve	dN/dt = rN(K−N)/K

Density-dependent factors: competition, predation, disease — drive logistic growth.
Density-independent factors: floods, fires, drought — can crash populations regardless of size.
K is not fixed. Drought lowers K; reforestation raises it.

Community ecology

Interaction	Species A	Species B	Example
Predation	+	–	Wolf–deer
Competition	–	–	Two plants for sunlight
Mutualism	+	+	Clownfish–anemone; legumes–Rhizobium
Commensalism	+	0	Cattle egret following grazers
Parasitism	+	–	Tick–host

Trophic cascade: wolf reintroduction in Yellowstone reduced elk grazing → riparian vegetation recovered → habitat returned for beavers and fish.

Competitive exclusion vs. niche partitioning: two species can't share a limited resource indefinitely (one outcompetes the other) — unless they partition the resource (different times, prey sizes, microhabitats).

Biodiversity and resilience

Simpson's Diversity Index: D = 1 − Σ(n/N)². Higher D → more diverse community → more functional redundancy → faster recovery from disturbance.

Keystone species: disproportionately large effect for their abundance. Sea otters control sea urchins → kelp forests persist.

Why students lose points here

Confusing energy flow (one-way) with nutrient cycling (recycled).
Applying the 10% rule backwards — 100 kcal at top means ~1,000 kcal one level down.
Saying a species “evolves in response to” an environmental change in one lifetime — populations evolve, individuals respond.
Forgetting biogeochemical cycles entirely (2025–26 addition).
Treating carrying capacity as a permanent number — it shifts with conditions.

FRQ patterns

Growth curve analysis: identify exponential vs. logistic; if plateau, identify K and explain.
Food web disruption: trace effects through at least two trophic levels.
Energy calc: apply the 10% rule with shown work.
Cycle explanation: name the bacteria — fixers, nitrifiers, denitrifiers.
Simpson's D: calculate and interpret differences between ecosystems.

How Unit 8 connects

Unit 3: photosynthesis and respiration are the molecular machinery behind ecosystem energy flow.
Unit 7: ecology provides the selective pressures that drive natural selection.
Unit 1: C, N, and P cycle through ecosystems as parts of the macromolecules from Unit 1.

FAQ

What percentage of the AP Biology exam is Unit 8?

Unit 8 (Ecology) is 10–15% of the exam — one of the heavier units. Because it's taught last in many courses, students underestimate it and consistently lose points they didn't have to lose.

What is a keystone species?

A species with a disproportionately large effect on its ecosystem relative to its abundance. Removal causes dramatic cascading changes. Examples: sea otters maintaining kelp forests by eating sea urchins, wolves regulating elk behavior in Yellowstone.

What is eutrophication?

Excess nutrients (often nitrogen and phosphorus from runoff) trigger algal blooms. When the algae die, decomposer bacteria multiply and consume dissolved oxygen through respiration, creating hypoxic dead zones that kill fish and other aerobic organisms.

What's the difference between GPP and NPP?

Gross Primary Productivity (GPP) is the total energy producers capture via photosynthesis. Net Primary Productivity (NPP) is what's left after producers respire: NPP = GPP − Respiration. Only NPP is available to primary consumers.

Why are there usually fewer than 5 trophic levels in a food chain?

The 10% rule. Only ~10% of energy passes to each successive level. By the 5th level, less than 0.001% of the original producer energy remains — not enough to support a population.

What changed in Unit 8 for the 2025–26 school year?

Biogeochemical cycles were explicitly added to Topic 8.2 (Energy Flow Through Ecosystems). Students should know the carbon, nitrogen, phosphorus, and water cycles — including the organisms involved (e.g., nitrogen-fixing, nitrifying, and denitrifying bacteria).

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