AP Biology Ecology — Worked Answer Explanations

Unit 8 · 12 questions explained

Below is a complete answer key for our AP Biology Ecology practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Ecology practice test and come back here to review, or head back to the Ecology unit overview.

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Question 1 · Easy
Which of the following correctly describes the role of decomposers in a terrestrial ecosystem?
- A
  They convert sunlight energy into chemical energy stored in organic molecules
  Why not A: Converting light to chemical energy describes producers (autotrophs), not decomposers.
- B
  They break down dead organic matter and return nutrients to the abiotic environmentCorrect
- C
  They consume primary producers and transfer energy to higher trophic levels
  Why not C: Consuming primary producers describes primary consumers (herbivores), not decomposers.
- D
  They fix atmospheric nitrogen into ammonium for use by plants
  Why not D: Nitrogen fixation is performed by specialized bacteria (e.g., Rhizobium), not by decomposers generally.
Explanation
Decomposers (bacteria and fungi) break down complex organic molecules in dead organisms and waste products into simpler inorganic compounds (e.g., CO₂, water, mineral nutrients like ammonium and phosphate), returning them to the abiotic environment. This makes nutrients available again for producers, completing biogeochemical cycles. Without decomposers, nutrients would remain locked in organic matter and ecosystems would collapse.

Key takeaway
Decomposers recycle nutrients from dead organic matter back into the abiotic environment, sustaining biogeochemical cycles.
Question 2 · Easy
Which of the following describes a density-independent factor that limits population growth?
- A
  Predation rate increases as prey population density rises
  Why not A: A predation effect that intensifies with density is a density-dependent limiting factor.
- B
  A severe drought that kills a fixed proportion of organisms regardless of population sizeCorrect
- C
  Disease transmission rises as individuals are crowded together
  Why not C: Disease transmission that increases with crowding is a density-dependent factor.
- D
  Intraspecific competition for food intensifies as population density increases
  Why not D: Competition that worsens with density is a classic density-dependent limiting factor.
Explanation
Density-independent factors affect population growth regardless of population size or density. Natural disturbances such as drought, fires, floods, or extreme temperature events kill or drive away organisms at a rate that does not depend on how many organisms are present. In contrast, density-dependent factors (predation, disease, competition) have greater effects at higher population densities, regulating populations toward a carrying capacity.

Key takeaway
Density-independent factors (weather, natural disasters) affect populations regardless of size; density-dependent factors intensify with population density.
Question 3 · Easy
A food chain is: grass → rabbit → fox → eagle. If energy transfer efficiency between each trophic level is approximately 10%, and the grass (producers) fix 10,000 kcal of energy, how much energy is available to eagles?
- A
  1,000 kcal
  Why not A: 1,000 kcal is the energy available to rabbits (one trophic transfer from 10,000 kcal); eagles are three transfers removed from grass.
- B
  10 kcalCorrect
- C
  100 kcal
  Why not C: 100 kcal is the energy available to foxes (two trophic transfers), not eagles.
- D
  0.1 kcal
  Why not D: 0.1 kcal would be the energy for a fifth trophic level; eagles are the fourth.
Explanation
The 10% rule states that approximately 10% of energy at one trophic level is transferred to the next. Starting from 10,000 kcal: Grass (1st) → Rabbits (2nd): 10,000 × 0.10 = 1,000 kcal; Rabbits → Foxes (3rd): 1,000 × 0.10 = 100 kcal; Foxes → Eagles (4th): 100 × 0.10 = 10 kcal. Eagles have access to only 10 kcal, representing 0.1% of the original energy. This explains why food chains are short and why top predators require large territories.

Key takeaway
Only ~10% of energy transfers between trophic levels; eagles (4th level) receive 10,000 × 0.10³ = 10 kcal.
Question 4 · Easy
In the nitrogen cycle, which group of organisms is responsible for converting ammonium ( $N H_{4}^{+}$ ) into nitrite ( $N O_{2}^{-}$ ) and then nitrate ( $N O_{3}^{-}$ )?
- A
  Denitrifying bacteria
  Why not A: Denitrifying bacteria convert nitrate into nitrogen gas (N₂), not the other way around.
- B
  Nitrifying bacteriaCorrect
- C
  Nitrogen-fixing bacteria
  Why not C: Nitrogen-fixing bacteria convert atmospheric N₂ into ammonium, the first step in biological nitrogen fixation.
- D
  Decomposers
  Why not D: Decomposers perform ammonification (breaking organic N into NH₄⁺), not nitrification.
Explanation
Nitrification is a two-step aerobic process performed by chemoautotrophic (nitrifying) bacteria: (1) Ammonia-oxidizing bacteria (e.g., Nitrosomonas) oxidize NH₄⁺ to NO₂⁻ (nitrite); (2) Nitrite-oxidizing bacteria (e.g., Nitrobacter) oxidize NO₂⁻ to NO₃⁻ (nitrate). Nitrate is the form of nitrogen most readily taken up by plant roots. This process is an important part of the nitrogen cycle, making nitrogen available from decomposition products back to plants.

Key takeaway
Nitrifying bacteria convert ammonium to nitrite to nitrate, making nitrogen available in a plant-usable form.
Question 5 · Medium
A group of ecologists introduces a new predator (wolves) into an ecosystem that previously had none. Over the following years, the deer population declines and vegetation in the riparian zone recovers. Beaver populations also increase as vegetation returns. The wolves did not directly interact with beavers. This is an example of which ecological phenomenon?
- A
  Bottom-up regulation, where primary producers drive changes in higher trophic levels
  Why not A: Bottom-up regulation is driven by changes in producer abundance; this scenario begins with a predator addition at the top.
- B
  A trophic cascade, where predator addition triggers indirect changes through multiple trophic levelsCorrect
- C
  Competitive exclusion between wolves and beavers for shared resources
  Why not C: Wolves and beavers occupy very different niches and trophic levels; this is not a competition scenario.
- D
  Character displacement between deer and beavers due to increased competition
  Why not D: Character displacement involves evolutionary divergence in sympatric competing species; this is an ecological (population-level) change, not evolutionary.
Explanation
A trophic cascade occurs when a predator at the top of the food chain indirectly affects organisms further down by altering the abundance or behavior of intermediate consumers. In the classic Yellowstone wolf reintroduction example: wolves → reduce deer → vegetation recovers → beavers (who depend on riparian vegetation) increase. This is a top-down trophic cascade. The wolves never interacted directly with beavers, yet beaver populations changed indirectly through the herbivore layer. This illustrates how keystone predators can restructure entire ecosystems.

Key takeaway
A trophic cascade occurs when a top predator indirectly affects multiple lower trophic levels through a chain of consumer-prey interactions.
Question 6 · Medium
Ecologists studying a forest community notice that one species of tree (species A) grows quickly and shades other species. Over 50 years, species A is replaced by longer-lived tree species that are shade-tolerant. This process is best described as which of the following?
- A
  Primary succession, because the change begins with pioneer species colonizing bare substrate
  Why not A: Primary succession starts on bare, lifeless substrate (e.g., after glacial retreat); this scenario already has established forest, making it secondary succession or directional succession.
- B
  Secondary succession, where one community replaces another following a disturbance
  Why not B: Secondary succession follows a disturbance that removes a community (e.g., fire, clear-cut); the question describes directional change in an undisturbed community.
- C
  Ecological succession toward a climax community, as early-successional species are replaced by late-successional speciesCorrect
- D
  Competitive exclusion, where species A eliminates all other tree species from the community
  Why not D: Competitive exclusion results in one species eliminating another; here species A is replaced, not the eliminator.
Explanation
Ecological succession describes the directional change in species composition of a community over time. In this scenario, early-successional species (fast-growing, shade-intolerant species A) modify the environment by creating shade, making conditions less favorable for themselves and more favorable for shade-tolerant, late-successional species. This process of facilitation and replacement continues until a relatively stable climax community is established. The scenario does not describe a disturbance-initiated secondary succession but rather the progression toward climax within a continuous successional trajectory.

Key takeaway
Ecological succession involves directional replacement of species as early colonizers modify the environment in ways that favor later-successional species.
Question 7 · Medium
A population of rabbits in an isolated meadow is growing according to the logistic growth model. The carrying capacity (K) is 500 rabbits, and the current population size (N) is 250. Which of the following statements correctly predicts the population growth rate relative to when N = 50?
- A
  Growth rate is lower at N=250 than at N=50 because density-dependent factors increase as N increases
  Why not A: Growth rate (dN/dt) is NOT simply lower at N=250; it depends on N × (K-N)/K, which is maximized at N = K/2 = 250.
- B
  Growth rate is at its maximum when N=250 because N equals K/2Correct
- C
  Growth rate is higher at N=50 than at N=250 because fewer density-dependent constraints exist
  Why not C: Although per-capita growth rate (r) is higher at N=50, total population growth rate (dN/dt) is lower because fewer individuals are reproducing.
- D
  Growth rate is equal at N=50 and N=250 because the intrinsic rate of increase (r) is constant
  Why not D: r (intrinsic growth rate) is constant in the logistic model, but dN/dt = rN(K-N)/K varies with N.
Explanation
In the logistic growth model, $\frac{d N}{d t} = r N (\frac{K - N}{K})$ . The term N(K-N) is maximized when N = K/2 (because this is the vertex of a downward parabola). With K=500: at N=50, dN/dt = r(50)(450/500) = 45r; at N=250, dN/dt = r(250)(250/500) = 125r. The population growth rate is highest at N = K/2 = 250. At N=50 (far from K/2), the relatively small population size limits total growth despite a high per-capita rate.

Key takeaway
Logistic population growth rate (dN/dt) is maximized at N = K/2; both very small and near-K populations grow more slowly.
Question 8 · Medium
Mycorrhizal fungi form associations with plant roots, providing phosphorus to the plant in exchange for carbohydrates. Which type of species interaction does this represent?
- A
  Commensalism (+/0): the fungus benefits and the plant is unaffected
  Why not A: The plant also benefits by gaining phosphorus; both species benefit, which is mutualism, not commensalism.
- B
  Mutualism (+/+): both the fungus and the plant benefit from the associationCorrect
- C
  Parasitism (+/−): the fungus benefits and the plant is harmed
  Why not C: In parasitism the host is harmed; mycorrhizal associations benefit the plant by improving phosphorus uptake.
- D
  Amensalism (0/−): the fungus is unaffected and the plant is harmed
  Why not D: The fungus clearly benefits by obtaining carbohydrates; amensalism involves no benefit to either party.
Explanation
Mutualism is a species interaction in which both partners benefit (+/+). In mycorrhizal associations, the fungus colonizes plant roots and extends hyphae into the soil, dramatically increasing the plant's effective surface area for phosphorus (and other nutrient) absorption. In return, the plant provides the fungus with photosynthetically derived carbohydrates, which the fungus cannot produce itself. Both organisms increase fitness through this association. Mycorrhizae are among the most widespread and ecologically important mutualistic associations on Earth.

Key takeaway
Mycorrhizal fungi-plant associations are mutualisms: the fungus gains carbohydrates and the plant gains phosphorus access.
Question 9 · Medium
An invasive grass species is introduced to a prairie ecosystem and rapidly spreads, outcompeting native grasses. Native insect species that depend on native grasses for food and reproduction begin to decline. Which of the following best describes the mechanism by which the invasive grass is harming native insects?
- A
  The invasive grass directly predates on native insects
  Why not A: Grasses are plants; they do not predate on insects.
- B
  Habitat and food resource loss from competitive displacement of native plants causes indirect harm to dependent insectsCorrect
- C
  The invasive grass produces toxins that directly kill native insects
  Why not C: While allelopathy (chemical inhibition) occurs in some plants, the primary mechanism described here is resource and habitat loss, not direct chemical toxicity to insects.
- D
  Native insects voluntarily migrate away because they prefer habitats with lower plant diversity
  Why not D: Insects do not preferentially seek lower diversity habitats; their decline reflects loss of required resources, not voluntary emigration.
Explanation
Invasive species disrupt ecosystems through multiple pathways. When an invasive grass outcompetes native grasses, it reduces the abundance and diversity of native plant species. Insects that are specialist feeders or that depend on native plants for oviposition (egg-laying) lose their food source and breeding habitat. This indirect harm — where the invasive species impacts non-plant organisms by transforming the vegetation — is an example of ecosystem disruption through habitat modification and trophic cascade effects. It illustrates how species interactions are interconnected across trophic levels.

Key takeaway
Invasive species can indirectly harm organisms at other trophic levels by competitively displacing the native species those organisms depend on.
Question 10 · Hard
The phosphorus cycle, unlike the carbon and nitrogen cycles, lacks a significant atmospheric reservoir. What are the primary environmental consequences of this difference?
- A
  Phosphorus cycles very rapidly between organisms and the atmosphere, making it rarely limiting
  Why not A: The absence of an atmospheric phase makes phosphorus cycle slowly (via geological weathering), making it frequently limiting.
- B
  Phosphorus cycles slowly through geological weathering and is often the limiting nutrient in aquatic ecosystemsCorrect
- C
  Phosphorus volatilizes from soil and is replenished regularly by precipitation, similar to nitrogen
  Why not C: Phosphorus does not form stable gaseous compounds under normal conditions; it cannot volatilize and return via precipitation.
- D
  Phosphorus is more abundant than nitrogen in most ecosystems because it does not escape to the atmosphere
  Why not D: Phosphorus is typically less available than nitrogen in ecosystems precisely because it has no atmospheric reservoir and is released slowly from rock weathering.
Explanation
The phosphorus cycle is a sedimentary cycle: phosphorus moves from rocks (via weathering) to soil and water, through food webs, back to sediments, and eventually (over geological time) back to rock. There is no significant atmospheric phase because phosphorus does not form stable, abundant gases at Earth's surface (contrast with C as CO₂ or N as N₂). This means: (1) the cycle operates on geological timescales; (2) biologically available phosphate depends on weathering and decomposition rates; (3) phosphorus is often the limiting nutrient in freshwater and some marine ecosystems. Anthropogenic addition of phosphorus (from fertilizers) causes eutrophication in lakes.

Key takeaway
Phosphorus lacks an atmospheric phase; it cycles slowly through geological weathering, making it the most common limiting nutrient in freshwater ecosystems.
Question 11 · Hard
A species of bird exclusively eats large seeds. Another species of bird exclusively eats small seeds. When they are allopatric (living separately), each species occupies a range of seed sizes. When the two species are sympatric (living together), each species shifts its diet to the extremes (one eats only very large seeds; the other eats only very small seeds). This pattern is best explained by which of the following?
- A
  Competitive exclusion, where one species is eliminated from the shared habitat
  Why not A: Competitive exclusion results in one species disappearing; here both species coexist but shift their niches (character displacement).
- B
  Character displacement, where competition drives evolutionary divergence in resource useCorrect
- C
  Resource partitioning without evolutionary change, where the species simply exploit different microhabitats
  Why not C: Resource partitioning is behavioral/ecological; character displacement is evolutionary — it involves heritable changes in morphology or behavior over generations, not just immediate behavioral switching.
- D
  Mutualism between the two species, where each benefits by avoiding competition over seeds
  Why not D: Mutualism requires both species to benefit from a direct interaction; avoiding competition by niche divergence is not a mutualistic interaction.
Explanation
Character displacement is the evolutionary divergence of two competing species in traits that reduce niche overlap when they are sympatric. The pattern described — broader niche in allopatry, narrower and divergent niche in sympatry — is the signature of character displacement. Competition in sympatry selects against individuals with overlapping diets (intermediate seed sizes), favoring individuals that specialize on the extremes. Over generations, this drives morphological and behavioral divergence (e.g., beak size). The classic example is Darwin's finches in the Galápagos. Note: this must be distinguished from behavioral plasticity (resource partitioning), which is reversible and non-heritable.

Key takeaway
Character displacement is evolutionary niche divergence in sympatric species driven by interspecific competition, reducing niche overlap.
Question 12 · Hard
A researcher measures the carbon flux in a forest ecosystem. She finds that gross primary productivity (GPP) is 8,000 g C/m²/year and plant respiration is 3,500 g C/m²/year. She also measures that herbivores consume 1,000 g C/m²/year of plant biomass, and decomposers respire 2,500 g C/m²/year. Which of the following correctly calculates the net primary productivity (NPP) and identifies whether this ecosystem is a net carbon sink or source?
- A
  NPP = 4,500 g C/m²/year; the ecosystem is a carbon sink because NPP exceeds decomposer respirationCorrect
- B
  NPP = 4,500 g C/m²/year; the ecosystem is a carbon source because total respiration exceeds GPP
  Why not B: Total ecosystem respiration (3,500 + 2,500 = 6,000) is less than GPP (8,000), so the ecosystem is a net carbon sink.
- C
  NPP = 3,000 g C/m²/year; the ecosystem is carbon neutral because NPP equals herbivore consumption plus decomposer respiration
  Why not C: NPP = GPP − plant respiration = 8,000 − 3,500 = 4,500, not 3,000; subtracting herbivore consumption gives NEP, not NPP.
- D
  NPP = 8,000 g C/m²/year; plant respiration is not subtracted because it is already included in GPP
  Why not D: GPP includes all carbon fixed; NPP = GPP − plant (autotroph) respiration; not subtracting plant respiration gives GPP, not NPP.
Explanation
Net primary productivity (NPP) = Gross primary productivity (GPP) − Autotroph (plant) respiration. NPP = 8,000 − 3,500 = 4,500 g C/m²/year. NPP represents carbon available to heterotrophs (consumers and decomposers). Net ecosystem productivity (NEP) = NPP − Heterotroph respiration = 4,500 − (herbivore consumption partially goes to herbivore respiration/decomposer; but we approximate total heterotrophic respiration as decomposer respiration 2,500) = 4,500 − 2,500 = 2,000 g C/m²/year > 0. Since NEP > 0, the ecosystem sequesters more carbon than it releases — it is a net carbon sink. Total ecosystem respiration = 3,500 + 2,500 = 6,000 < GPP 8,000, confirming the sink status.

Key takeaway
NPP = GPP − plant respiration; when NPP exceeds heterotrophic respiration, the ecosystem is a net carbon sink.