Ecology — AP Biology Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: Population dynamics, community interactions, ecosystem structure & energy flow, biogeochemical cycles, and human impact for AP Biology Unit 8.

You should already know: Cellular respiration & photosynthesis (Unit 3), basic genetics (Unit 5), natural selection (Unit 7).

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official College Board mark schemes for grading conventions.

1. What is Ecology in AP Biology?

Ecology is the study of how organisms interact with each other and with their physical environment. Unit 8 is the smallest unit by exam weight (about 10–15% of your AP Biology score) but ties together every prior unit — energy flow uses photosynthesis & respiration, population genetics uses Hardy-Weinberg, and natural selection drives community change.

The four levels of ecological organisation you must distinguish: organism → population → community → ecosystem. A population is one species in one area; a community is all populations interacting in that area; an ecosystem adds the abiotic environment.

2. Population growth — exponential vs logistic

Exponential growth assumes unlimited resources. Population grows at a constant per-capita rate $r$: $$\frac{dN}{dt}=rN$$ Plotted, it produces a J-curve. This applies short-term: a few rabbits introduced to a new island, or bacteria in fresh medium for the first hour.

Logistic growth adds a carrying capacity $K$ (the maximum population the environment can sustain): $$\frac{dN}{dt}=rN\cdot \frac{K-N}{K}$$ This produces an S-curve: rapid growth at low $N$, slowing as $N$ approaches $K$, plateauing at $K$. The fraction $(K-N)/K$ is the "environmental resistance" — when $N=K$ it equals 0 and growth stops.

Reproductive strategies: r-selected species (small body, many offspring, little parental care, e.g. fish, insects) thrive in unstable environments via exponential bursts. K-selected species (large body, few offspring, high parental care, e.g. elephants, humans) live near $K$ in stable environments.

3. Community interactions

Five interaction types tested on the exam, classified by effect on each participant (+ benefit, − harm, 0 neutral):

Competitive exclusion principle: two species with identical niches cannot coexist — the better-adapted competitor drives the other to local extinction. Resource partitioning is the alternative: closely related species evolve to use slightly different resources (warblers feeding on different parts of the same tree) and so coexist.

Keystone species have an outsized effect on community structure relative to their biomass — sea otters control sea urchin populations and so protect kelp forests; remove the otters and urchin populations explode, kelp disappears, and the entire community collapses.

4. Energy flow — the 10% rule

Energy enters an ecosystem at primary producers (plants, algae) via photosynthesis. As it moves up trophic levels (producer → primary consumer → secondary consumer → tertiary consumer), roughly 90% is lost as heat at each step (cellular respiration, undigested matter, motion). Only ~10% is incorporated into the next level's biomass.

Consequence: ecosystems can rarely support more than 4–5 trophic levels — by the 5th level there is too little energy to sustain a population. This is also why eating plants directly is more energy-efficient than eating meat that ate plants.

Ecological pyramids visualise this: a pyramid of energy is always upright (producers > consumers); a pyramid of biomass is usually upright but can invert in marine systems (rapid phytoplankton turnover sustains larger zooplankton biomass at any single moment); a pyramid of numbers can take any shape.

5. Biogeochemical cycles

Carbon, nitrogen, phosphorus, and water cycle through the biosphere. The two most exam-tested:

Carbon cycle: CO₂ from atmosphere → producers (photosynthesis) → consumers (eating producers) → decomposers (breakdown of dead material) → atmosphere again (respiration). Combustion of fossil fuels releases stored carbon as CO₂ much faster than natural processes can absorb it, driving climate change.

Nitrogen cycle: N₂ in atmosphere is largely unusable. Nitrogen-fixing bacteria (e.g. Rhizobium in legume root nodules) convert N₂ to NH₃; nitrifying bacteria convert NH₃ to NO₂⁻ then NO₃⁻; plants absorb NO₃⁻; consumers eat plants; decomposers return N to soil; denitrifying bacteria convert NO₃⁻ back to N₂. Synthetic fertiliser short-circuits this and causes eutrophication when runoff reaches lakes.

6. Human impact and disturbance

Three exam-frequent human impacts:

1. Climate change: rising CO₂ and methane shift ecosystems poleward and upward in elevation; species that cannot migrate go extinct.
2. Habitat fragmentation: roads and farms split continuous habitat into patches too small to support viable populations of large animals.
3. Invasive species: organisms moved outside their native range encounter no natural predators and outcompete native species — Burmese pythons in the Florida Everglades, zebra mussels in the Great Lakes.

Ecological succession describes how disturbed communities recover: primary succession starts on bare rock (lichen → moss → grass → shrub → forest); secondary succession starts on disturbed soil (faster, because soil and seed bank persist).

7. Worked Example

A logistic-growth population of deer has $K=400$ and intrinsic rate $r=0.5$ per year. Calculate the rate of growth $dN/dt$ when $N=100$ and when $N=350$.

Solution. At $N=100$: $$\frac{dN}{dt}=(0.5)(100)\cdot \frac{400-100}{400}=50\cdot 0.75=37.5\text{ deer/year}$$

At $N=350$: $$\frac{dN}{dt}=(0.5)(350)\cdot \frac{400-350}{400}=175\cdot 0.125=21.875\text{ deer/year}$$

Interpretation: even though $N=350$ is much closer to $K$, growth has slowed because the environmental resistance term has shrunk. Maximum growth rate occurs at $N=K/2=200$ — verify by computing $dN/dt$ at $N=200$ and confirming it exceeds both numbers above.

8. Common Pitfalls

• Confusing biomass with energy: biomass is the dry weight of organic matter at one level; energy is the total ATP-equivalent flowing through. The 10% rule is about energy, not biomass directly.
• Assuming carrying capacity is fixed: $K$ depends on resource availability and changes with droughts, fires, climate shifts. AP free-response questions reward students who note this.
• Mixing up parasitism vs predation: parasites usually do not kill the host (a tapeworm benefits from a long-lived host); predators do. A "parasitoid" wasp is a hybrid case.
• Forgetting decomposers: bacteria and fungi recycle nutrients back to producers — without them, ecosystems would run out of usable C, N, and P.

9. Practice Questions (CED Style)

1. A grassland ecosystem has primary producers fixing $10000$ kJ/m²/year. Estimate the maximum energy available to a tertiary consumer (3rd-level carnivore).
2. A new bird species is introduced to an island and grows logistically with $r=0.4$/year and $K=2000$. Sketch the population over 25 years starting from $N_0=50$, and identify the inflection point.
3. Explain why the nitrogen cycle, but not the phosphorus cycle, has a major atmospheric reservoir, and how this difference affects the limiting nutrient in most aquatic ecosystems.

10. Quick Reference Cheatsheet

• Exponential: $dN/dt=rN$, J-curve, no $K$.
• Logistic: $dN/dt=rN(K-N)/K$, S-curve, plateau at $K$, max growth at $K/2$.
• r-selected: many small offspring, unstable habitat. K-selected: few large offspring, stable habitat.
• 10% rule: ~10% of energy passes to next trophic level; ~90% lost as heat.
• 5 community interactions: competition (−,−), predation (+,−), parasitism (+,−), mutualism (+,+), commensalism (+,0).
• Keystone species: outsized effect on community vs. biomass.
• N cycle key bacteria: nitrogen-fixing (N₂ → NH₃), nitrifying (NH₃ → NO₃⁻), denitrifying (NO₃⁻ → N₂).
• Succession: primary (bare rock, lichen first) vs secondary (disturbed soil, faster).

11. What's Next

After Ecology, you've covered all 8 AP Biology units. Strengthen weak areas using free-response practice — Unit 8 typically appears as part of a multi-unit FRQ tying ecology to natural selection (Unit 7) or cellular respiration (Unit 3). Use Ollie to ask any specific case: "Why might invasive species succeed in a community where native species saturated the niches?" or "Calculate trophic efficiency given energy at each level".