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AP Biology Unit 8 Practice Questions: Ecology

10 original exam-style questions on Ecology. Answer each one to see the explanation — no account needed.

Question 1 of 10 · Foundation Species and Habitat Loss

A marine ecologist studying a coral reef notes that a severe bleaching event (caused by elevated water temperature) kills 60% of coral colonies. Which of the following is the most likely long-term consequence for the reef community if water temperatures remain elevated?
  1. A. The reef will recover rapidly because coral larvae are highly abundant and settle on bare substrate within one season.
  2. B. Loss of coral structural complexity reduces habitat for fish and invertebrates, decreasing overall reef biodiversity.
  3. C. Algae will colonize bare substrate briefly but coral will outcompete algae and restore the reef within 2-3 years.
  4. D. Fish populations will increase because reduced coral cover increases open water for foraging.
Show answer and explanation

Correct answer: B

Coral reefs are foundation species — their three-dimensional structure provides habitat for thousands of species; when coral cover is severely reduced, habitat complexity collapses, causing cascading loss of reef-associated species. At chronically elevated temperatures, algae competitively exclude recovering coral, preventing restoration — choice C incorrectly assumes coral recovers easily under continued thermal stress.

Question 2 of 10 · Exponential Growth Rate Calculation

A population ecologist models a species with overlapping generations. At time tt, the population has 500 individuals. The per-capita birth rate is 0.4 and the per-capita death rate is 0.15. What is the instantaneous rate of population growth (rr), and what will the population be at time t+1t+1 (using ΔN=rN\Delta N = rN)?
  1. A. r=0.25r = 0.25; ΔN=125\Delta N = 125; population at t+1=625t+1 = 625
  2. B. r=0.55r = 0.55; ΔN=275\Delta N = 275; population at t+1=775t+1 = 775
  3. C. r=0.25r = 0.25; ΔN=62.5\Delta N = 62.5; population at t+1=562t+1 = 562
  4. D. r=0.40r = 0.40; ΔN=200\Delta N = 200; population at t+1=700t+1 = 700
Show answer and explanation

Correct answer: A

r=b−d=0.4−0.15=0.25r = b - d = 0.4 - 0.15 = 0.25. Then ΔN=rN=0.25×500=125\Delta N = rN = 0.25 \times 500 = 125, so population at t+1=500+125=625t+1 = 500 + 125 = 625. Choice D incorrectly uses only the birth rate as rr, ignoring mortality.

Question 3 of 10 · Trophic Cascades and Community Interactions

The table below shows energy transfer data collected from a freshwater lake ecosystem. Researchers measured the energy available at each trophic level over one year.

Energy Flow in a Freshwater Lake Ecosystem
Trophic LevelOrganism TypeEnergy Available (kcal/m²/yr)Energy Transferred to Next Level (kcal/m²/yr)
1st (Producers)Phytoplankton20,0001,800
2nd (Primary Consumers)Zooplankton1,800180
3rd (Secondary Consumers)Small fish18016
4th (Tertiary Consumers)Large fish16—
A researcher proposes removing all large fish (4th trophic level) from this lake. Which of the following best predicts the most immediate ecological consequence of this removal?
  1. A. Phytoplankton biomass would immediately decline because large fish recycle nutrients that support primary production
  2. B. Small fish populations would likely increase, intensifying predation pressure on zooplankton populations
  3. C. Zooplankton populations would rapidly increase as the removal of large fish directly reduces zooplankton predation
  4. D. Total ecosystem energy input would decrease as the loss of a trophic level reduces overall photosynthetic efficiency
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Correct answer: B

Removing the apex predator (large fish) releases small fish from top-down predation pressure, causing their populations to grow; the trophic cascade then increases predation on zooplankton. Choice C is incorrect because large fish do not directly predate zooplankton — they prey on small fish — so zooplankton are affected indirectly, not immediately.

Question 4 of 10 · Decomposers and Detritivores

Detritivores and decomposers play essential roles in nutrient cycling. Which of the following correctly distinguishes these two groups?
  1. A. Detritivores are autotrophs; decomposers are heterotrophs.
  2. B. Detritivores physically fragment dead organic matter; decomposers chemically break down organic molecules via extracellular enzymes and absorption.
  3. C. Detritivores break down only plant material; decomposers break down only animal carcasses.
  4. D. Detritivores release nutrients directly into the atmosphere; decomposers release nutrients only into the soil.
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Correct answer: B

Detritivores (e.g., earthworms, millipedes) are animals that ingest and mechanically fragment dead organic matter, increasing surface area for microbial decomposition; decomposers (fungi and bacteria) secrete extracellular enzymes that chemically break down organic macromolecules and absorb the resulting monomers. Both are heterotrophs — choice A is incorrect.

Question 5 of 10 · Energy Flow Through Ecosystems

A student constructs a food web for a freshwater pond. She finds that removing cattails (emergent macrophytes) decreases the abundance of invertebrates, small fish, and large fish. Which of the following BEST explains why removing one plant species has cascading effects on multiple trophic levels?
  1. A. Cattails are a foundational primary producer whose removal reduces energy input at the base of the food web, propagating through all trophic levels.
  2. B. Cattails are a keystone predator that regulate the populations of invertebrates and fish.
  3. C. Cattails compete with invertebrates for the same nutrient resources, so their removal releases invertebrates from competition.
  4. D. Cattails fix atmospheric nitrogen, and their removal causes a nitrogen limitation that prevents all other organisms from growing.
Show answer and explanation

Correct answer: A

Primary producers form the energetic base of food webs; removing them reduces the total energy available to primary consumers (invertebrates), which in turn reduces energy for secondary consumers (small fish) and tertiary consumers (large fish) — a trophic cascade. Cattails are a primary producer, not a predator, so choice B is incorrect.

Question 6 of 10 · Species Diversity Indices

A student measures the number of species of plants in 1 m² quadrats across a meadow and calculates a Simpson's diversity index for each quadrat. Which of the following best describes what a higher index value indicates?
  1. A. Greater total number of individual organisms, regardless of how many species are present.
  2. B. Greater species richness and evenness, indicating a more diverse community.
  3. C. A greater proportion of the community dominated by a single species.
  4. D. Higher rates of primary productivity in that quadrat compared to surrounding areas.
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Correct answer: B

Simpson's diversity index increases when a community has more species (richness) and when individuals are distributed more evenly among species (evenness); a high index value reflects a community where no single species dominates. A community dominated by one species would have a low index value, not a high one.

Question 7 of 10 · Fundamental vs. Realized Niche

Which of the following best describes the difference between an organism's fundamental niche and its realized niche?
  1. A. The fundamental niche is where the organism actually lives; the realized niche is where it could live without competition.
  2. B. The fundamental niche is the full range of conditions an organism can tolerate; the realized niche is the narrower range it actually occupies due to biotic interactions.
  3. C. The fundamental niche includes all biotic interactions; the realized niche excludes the effects of predation and competition.
  4. D. The fundamental niche changes with evolution; the realized niche is fixed by physiology and does not change.
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Correct answer: B

The fundamental niche is the theoretical range of conditions (abiotic and resources) where a species can survive and reproduce in the absence of competitors and predators; the realized niche is the actual, narrower subset it occupies in nature, reduced by competition, predation, and other biotic interactions. Choice A reverses the definitions — a common student misconception.

Question 8 of 10 · Competitive Exclusion

Gause's competitive exclusion principle states that two species competing for identical resources cannot coexist indefinitely. Which of the following outcomes is predicted if two species occupy completely overlapping niches?
  1. A. Both species will coexist by partitioning available resources equally.
  2. B. The competitively superior species will eventually eliminate the inferior competitor.
  3. C. Both species will evolve to share the same niche to maximize resource use efficiency.
  4. D. Mutualistic interactions will develop to allow both species to coexist on the same resource.
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Correct answer: B

Competitive exclusion predicts that when two species have identical resource requirements (completely overlapping niches), the species with even a slight competitive advantage will eventually drive the other to local extinction; stable coexistence requires niche differentiation. Resource partitioning (choice A) is the evolutionary outcome that prevents exclusion — but it requires niche divergence, not identical niches.

Question 9 of 10 · r- and K-Selection

Which of the following comparisons most accurately distinguishes r-selected species from K-selected species?
  1. A. r-selected species have long lifespans and high parental investment; K-selected species have short lifespans and many offspring.
  2. B. r-selected species produce many offspring with little parental care; K-selected species produce few offspring with high parental investment.
  3. C. r-selected species maintain populations near K; K-selected species experience boom-and-bust population dynamics.
  4. D. r-selected species are top predators; K-selected species are primary producers.
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Correct answer: B

r-selected species maximize intrinsic rate of increase by producing many offspring quickly with minimal care (e.g., insects, annual plants); K-selected species invest heavily in few offspring and maintain populations near carrying capacity (e.g., elephants, humans). Choice A reverses these life history traits, a common student misconception.

Question 10 of 10 · Ecological Succession

A newly formed volcanic island undergoes ecological succession over 200 years. Which of the following sequences BEST represents the expected order of colonization from earliest to latest?
  1. A. Lichens and mosses → grasses and shrubs → pioneer trees → climax forest
  2. B. Climax forest → pioneer trees → grasses and shrubs → lichens and mosses
  3. C. Grasses and shrubs → lichens and mosses → pioneer trees → climax forest
  4. D. Pioneer trees → lichens and mosses → grasses and shrubs → climax forest
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Correct answer: A

Primary succession on bare rock begins with pioneer species (lichens, mosses) that can tolerate harsh, nutrient-poor conditions and initiate soil formation, followed by grasses and shrubs, then early-succession trees, and finally the climax community. Each stage modifies the environment to facilitate the next — a process called facilitation.

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