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Credit Sprint's Guide to Biology Unit 8: Ecology

AP® weight 10–15%CLEP® weight 14–18%

Ecology studies how organisms interact with each other and their environment. Energy flows through food webs and is lost as heat, matter cycles, populations grow until resources limit them, and communities change after disturbances.

Exponential and logistic growth K TimePopulation (N)J-curve (exponential) S-curve (logistic)

What you need to know

  • Organisms respond to the environment through behavior (taxis, kinesis, communication) and physiology. Endotherms use metabolic heat; ectotherms rely on the surroundings.
  • Only about 10% of energy passes from one trophic level to the next. The rest is lost mostly as heat from respiration.
  • Gross primary productivity minus respiration = net primary productivity.
  • Carbon, nitrogen, phosphorus and water cycle through ecosystems. Nitrogen-fixing bacteria turn N₂ into usable ammonia; phosphorus has no atmospheric phase.
  • Exponential growth (J-curve) happens when resources are unlimited. Logistic growth (S-curve) levels off at carrying capacity, K.
  • Density-dependent factors (competition, disease, predation) intensify as a population grows; density-independent factors (fire, floods, weather) don't depend on density.
  • Community interactions: competition (−/−), predation and parasitism (+/−), mutualism (+/+), commensalism (+/0).
  • Keystone species have outsized effects on community structure; removing one can collapse a food web.
  • Diversity makes ecosystems more resilient. Invasive species, habitat loss and climate change reduce biodiversity.
  • Ecological succession: primary (bare rock, starting with pioneer species like lichens) and secondary (soil remains after a disturbance).

Equations and rules

Energy at the next level ≈ 10% of the level below
Exponential growth: dN/dt = rN
Logistic growth: dN/dt = rN × (K − N)/K
Simpson's diversity index: D = 1 − Σ(n/N)²

Exam traps

Watch forEnergy does NOT cycle. Matter cycles; energy flows one way and leaves as heat.
Watch forPopulation growth is fastest at K/2 in logistic growth, not at K.
Watch forRemoving a predator can lower diversity if its prey then outcompetes other species.

Key terms

  • Trophic level: A step in a food chain: producers, primary consumers and so on.
  • 10% rule: About 10% of energy moves up each trophic level.
  • Net primary productivity: Gross primary productivity minus the energy producers use in respiration.
  • Nitrogen fixation: Bacteria convert N₂ gas into ammonia that plants can use.
  • Carrying capacity (K): The largest population an environment can support long term.
  • Logistic growth: Growth that slows as the population nears carrying capacity (S-curve).
  • Density-dependent factor: A limit whose effect grows with population density, such as disease.
  • Mutualism: An interaction where both species benefit.
  • Keystone species: A species with a large effect on its community relative to its numbers.
  • Primary succession: Community development on bare rock with no soil.
  • Endotherm: An animal that keeps its body warm with metabolic heat.
  • Invasive species: A non-native species that spreads and harms its new ecosystem.

Practice questions

Five of the 27 Unit 8 questions in Credit Sprint. Every choice is explained in the app.

1. A plant that grows toward light and a moth that flies toward light both show
  1. learned behavior
  2. natural selection in one generation
  3. responses to an environmental stimulus
  4. mutualism
Show answer

responses to an environmental stimulus. Phototropism and phototaxis.

2. Bacteria in legume root nodules convert N₂ gas into ammonia. This is
  1. nitrification
  2. decomposition
  3. denitrification
  4. nitrogen fixation
Show answer

nitrogen fixation. It makes nitrogen usable by plants.

3. Which is a density-dependent limiting factor?
  1. A disease that spreads faster in crowded populations
  2. A volcanic eruption
  3. A sudden freeze
  4. A hurricane
Show answer

A disease that spreads faster in crowded populations. Its effect grows with density.

4. Lichens colonize bare rock after a glacier retreats, slowly forming soil. This begins
  1. eutrophication
  2. secondary succession
  3. a climax community
  4. primary succession
Show answer

primary succession. No soil existed.

5. Honeybees perform a waggle dance that tells other bees where food is. This is an example of
  1. a learned courtship display
  2. competition
  3. communication that increases the colony's fitness
  4. kinesis
Show answer

communication that increases the colony's fitness. Information helps the group find food.