Ecology

What this unit covers

The topics below follow the published Biology course framework for Unit 8. This unit is worth 10–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Energy Flow & Trophic Levels13 min · 3 objectivesTrace energy from producers through consumers and decomposers along a food chain · Apply the 10% rule to calculate energy available at each trophic level · Explain why energy pyramids taper and why food chains are short
• Population Growth & Regulation14 min · 3 objectivesContrast exponential and logistic growth and compute the growth rate from each model · Relate carrying capacity, r/K selection, and survivorship curves to a species’ life strategy · Distinguish density-dependent from density-independent limiting factors
• Community Interactions & Succession13 min · 3 objectivesClassify symbiotic relationships by their effect on each partner · Explain how competition, predation, and keystone species shape community structure · Distinguish primary from secondary succession and predict the sequence of change
• Biodiversity & Ecosystem Disruption12 min · 3 objectivesExplain why biodiversity strengthens ecosystem stability and resilience · Describe how invasive species and human activity disrupt ecosystems · Predict how populations and communities respond to environmental change
• Population Growth Models — Quantitative15 min · 3 objectivesCompute dN/dt and the per-capita growth rate for both the exponential and logistic models at any N · Show why logistic growth is fastest at N = K/2 and interpret the S-curve inflection point · Connect the model parameters r and K to r- vs K-selected life histories and survivorship curves
• Community Ecology, Succession & Biodiversity14 min · 3 objectivesApply the competitive exclusion principle and niche partitioning to predict whether species coexist · Distinguish keystone from foundation species and predict the consequences of removing each · Calculate and interpret Simpson’s Diversity Index for a community

Formulas in Unit 8

Every term in Unit 8

All 42 terms we publish for Ecology, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Population density and dispersion

Density is individuals per unit area; dispersion is their pattern — clumped, uniform or random. Clumped is by far the most common in nature.

Exponential growth

dN/dt = rN. J-shaped curve, only sustained when resources are effectively unlimited — typically a new or recovering population.

Logistic growth

dN/dt = rN(K−N)/K. S-shaped; growth slows as N approaches carrying capacity because per-capita resources fall.

Carrying capacity (K)

The maximum population an environment can sustain indefinitely. Not fixed — it changes with resources and conditions.

Density-dependent vs independent factors

Density-dependent effects intensify as population grows (disease, competition, predation); density-independent ones do not (fire, flood, cold snap).

r-selected vs K-selected

r-selected species produce many low-investment offspring in unstable environments; K-selected produce few, high-investment offspring in stable ones.

Trophic levels

Producers, primary consumers, secondary consumers, decomposers. Energy enters as sunlight and leaves as heat at every step.

Ten percent rule

Roughly 10% of energy passes to the next trophic level, the rest being lost to respiration and heat. This is why food chains rarely exceed four or five levels.

Gross vs net primary productivity

GPP is total energy fixed by producers; NPP is what remains after their own respiration, and NPP is what is available to consumers.

Biogeochemical cycling

Matter cycles while energy flows. Carbon, nitrogen, phosphorus and water move between organisms and reservoirs and are never used up.

Nitrogen fixation

Bacteria convert atmospheric N₂ into ammonia usable by plants. Nitrogen is abundant in air but the triple bond makes it inaccessible without them.

Competitive exclusion principle

Two species cannot occupy the identical niche indefinitely; one outcompetes the other, or they diverge through resource partitioning.

Resource partitioning

Coexisting species divide a resource by time, space or type, reducing direct competition — Anolis lizards using different perch heights.

Fundamental vs realized niche

The fundamental niche is where a species could live; the realized niche is where it actually does, once competition and predation are accounted for.

Symbiosis types

Mutualism benefits both; commensalism benefits one with no effect on the other; parasitism benefits one at the other's expense.

Keystone species

A species with an effect on community structure far out of proportion to its abundance — sea otters holding urchin numbers down and preserving kelp forests.

Ecological succession

Predictable community change over time. Primary succession starts on bare rock with no soil; secondary follows a disturbance that left soil intact.

Biodiversity and stability

More diverse communities recover from disturbance better, because functional redundancy means the loss of one species need not end a process.

Invasive species

Introduced organisms that spread unchecked because their new range lacks their predators, parasites and competitors.

Eutrophication

Nutrient runoff triggers algal blooms; when the algae die, decomposers consume the oxygen and fish suffocate — a dead zone.

Greenhouse effect and climate change

CO₂ and methane absorb outgoing infrared radiation and re-emit it downward. Rising concentrations shift ranges, phenology and ocean chemistry.

Behavioral responses to environment

Taxis is directed movement toward or away from a stimulus; kinesis is undirected change in activity level. Both alter where an organism ends up.

Levels of ecological organization

Individual, population, community, ecosystem, biome, biosphere. Each level asks different questions about the same organisms.

Survivorship curves

Type I has high survival until old age (large mammals), Type II constant mortality (many birds), Type III heavy early mortality (fish, most invertebrates).

Age structure diagram

Population by age and sex. A broad base predicts growth, a column predicts stability, and a narrow base predicts decline.

Population growth rate

r = (births − deaths)/N. Positive r means growth, negative means decline, regardless of how large the population currently is.

Doubling time

Roughly 70 divided by the percentage growth rate — a quick estimate of how fast a population expands.

Predator-prey cycles

Prey numbers rise, predators follow, prey crash, predators crash. The predator peak lags the prey peak, which is the diagnostic feature.

Mimicry

Batesian mimicry: a harmless species resembles a harmful one. Müllerian: two harmful species converge on one warning pattern, so predators learn faster.

Aposematic coloration

Bright warning colors advertising toxicity. Effective because predators learn to avoid the pattern after one experience.

Primary vs secondary consumers

Primary consumers eat producers; secondary eat primary. Position determines how much of the original energy remains available.

Decomposers and detritivores

Return nutrients from dead material to the soil and water. Without them, matter would stay locked in dead tissue and cycling would stop.

Carbon cycle

Photosynthesis removes CO₂ and respiration, decomposition and combustion return it. Fossil fuel burning moves carbon from a long-term reservoir to the atmosphere.

Nitrogen cycle steps

Fixation, nitrification, assimilation, ammonification and denitrification — each mediated by a different group of bacteria.

Phosphorus cycle

Has no significant atmospheric phase, so it moves slowly through rock weathering, soil and organisms. Often the limiting nutrient in fresh water.

Limiting nutrient

The nutrient in shortest supply relative to need, which caps productivity no matter how abundant everything else is.

Trophic cascade

Change at one trophic level propagates down through the food web — removing wolves increases elk, which reduces willow.

Species richness vs evenness

Richness counts species present; evenness measures how equally individuals are distributed among them. Both contribute to diversity.

Simpson's diversity index

D = 1 − Σ(n/N)². Values near 1 indicate high diversity; the index accounts for both richness and evenness.

Ecosystem services

Benefits ecosystems provide — pollination, water filtration, carbon storage, flood control — that are costly or impossible to replace artificially.

Lab: Energy Dynamics

Measuring biomass gained by cabbage white larvae against biomass consumed, quantifying how little ingested energy becomes new tissue.

Lab: Fruit Fly Behavior

A choice chamber tests whether flies move toward or away from a stimulus; a chi-square test determines whether the distribution differs from random.

What examiners penalize here

• For any energy-flow free-response, lead with the one-way principle: **energy enters as sunlight, flows up one direction, and is lost as heat at every step — it is never recycled.** Then apply the 10% rule quantitatively. Naming that ~90% loss (respiration, heat, waste) is what earns the explanation point.
• On the exam, tie the pieces together: **exponential = J-curve, no limits (dN/dt = rN); logistic = S-curve leveling at K (dN/dt = rN((K − N)/K)).** The (K − N)/K term *is* the density-dependent brake in equation form — link the graph, the equation, and the limiting factors in one answer.
• The whole primary-vs-secondary distinction hinges on one word: **soil**. No soil to start (bare rock, lava) → **primary**, slow, begins with pioneer lichens/mosses. Soil already present (after fire or farming) → **secondary**, faster. State the soil condition explicitly and the classification follows.
• The unifying thread of this unit: **diversity buys stability.** High species and genetic diversity lets ecosystems absorb disturbance and lets populations adapt to change; losing it — through invasion, habitat destruction, or pollution — makes systems fragile. On free-response, connect a specific disruption to reduced biodiversity to reduced resilience.
• A quantitative FRQ often hands you r, K, and N and asks for dN/dt. Write the logistic equation, substitute, and **show the (K − N)/K step explicitly** — that is where points live. Then interpret: is N below, at, or above K/2? Below K/2, growth is still accelerating; above it, growth is decelerating toward the S-curve plateau at K.
• Nail the two "big impact" categories: a **keystone species** shapes the community through *interactions* despite *low* abundance (remove it → diversity collapses); a **foundation species** shapes it by *building habitat* and is *highly abundant*. And on any diversity calculation, show the full path — proportions → squared → summed → subtracted from 1 — because partial credit tracks each step.

Practice Biology

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 8 units of AP Biology

1. Unit 1 · Chemistry of Life
2. Unit 2 · Cell Structure & Function
3. Unit 3 · Cellular Energetics
4. Unit 4 · Cell Communication & Cycle
5. Unit 5 · Heredity
6. Unit 6 · Gene Expression & Regulation
7. Unit 7 · Natural Selection
8. Unit 8 · Ecology