Ecology
Unit 8 of AP Biology, worth 10–15% of the exam. 15 questions below, each with the working. Every answer was checked by a second pass before it was published.
Responses to the environment, energy flow, population ecology, community ecology, biodiversity, disruptions to ecosystems.
How this unit is tested
Ecology questions on the AP exam almost always ask you to either (1) do a calculation — population growth rate, energy transfer through trophic levels, or a diversity index — or (2) reason about cause and effect in a system — what happens to a community or population when one variable changes. Start every problem by identifying which of these two modes it is. For calculations, write down the formula first and label every quantity before you plug in numbers; energy-flow problems in particular punish students who multiply and divide in the wrong order.
For conceptual questions, build the causal chain explicitly: environmental cue → organism response → population-level consequence → community-level consequence. AP graders reward answers that name the mechanism (e.g., 'resource partitioning reduces niche overlap, allowing coexistence') rather than just the vocabulary word. When a question gives you a graph (population size over time, a survivorship curve, a species-area curve), read the axes first and describe the shape in words before you interpret it — many wrong answers come from misreading which axis is which.
For community and biodiversity questions, always classify the interaction by its effect on each species (+, -, or 0) before naming it; this prevents the common error of confusing commensalism with mutualism or parasitism with predation. For disruption questions, think in terms of resistance (ability to withstand a disturbance) versus resilience (ability to recover), and always connect the disruption back to energy flow or population dynamics — the AP exam rewards answers that trace an effect through the whole system rather than stopping at the first link.
What you have to know
Exponential growth model
$\frac{dN}{dt} = rN$, where N is population size and r is the per capita growth rate (r = birth rate − death rate). Growth is unlimited and accelerates as N increases; describes populations with abundant resources.
Logistic growth model
$\frac{dN}{dt} = rN\frac{(K-N)}{K}$, where K is carrying capacity. Growth rate slows as N approaches K and reaches zero at N = K.
Net primary productivity
$NPP = GPP - R$: net primary productivity equals gross primary productivity (total energy captured by producers) minus the energy producers use for their own respiration.
Rule of ten (energy transfer efficiency)
On average, only about 10% of the energy available at one trophic level is incorporated into the biomass of the next trophic level; the rest is lost as heat, used in respiration, or not consumed/digested.
Simpson's Diversity Index
$D = 1 - \sum\left(\frac{n_i}{N}\right)^2$, where $n_i$ is the number of individuals of species i and N is total individuals in the community. D ranges from 0 (no diversity) to nearly 1 (high diversity), reflecting both species richness and evenness.
Competitive exclusion principle
Two species competing for exactly the same limiting resource cannot stably coexist; one will outcompete and locally eliminate the other, unless resource partitioning or niche differentiation reduces the overlap.
15 practice questions
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A population has a per capita growth rate r = 0.04/year and current size N = 800. Using the exponential growth model, what is dN/dt?
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Answer. 32 individuals per year
Using $\frac{dN}{dt}=rN$, substitute r = 0.04 and N = 800: 0.04 × 800 = 32. This is the instantaneous rate of population increase assuming unlimited resources. -
Explain why logistic growth produces an S-shaped curve while exponential growth produces a J-shaped curve.
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Answer. Logistic growth includes the term (K−N)/K, which approaches zero as N nears K, slowing growth until it levels off at carrying capacity; exponential growth has no limiting term, so growth accelerates continuously.
In $\frac{dN}{dt}=rN\frac{(K-N)}{K}$, as N approaches K, (K−N)/K shrinks toward 0, driving dN/dt toward 0 and flattening the curve (S-shape). Without this term, as in $\frac{dN}{dt}=rN$, growth rate keeps increasing with N, producing an unchecked J-shape. -
A forest's producers have a gross primary productivity of 30,000 kcal/m²/yr and use 10,000 kcal/m²/yr for respiration. What is the net primary productivity?
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Answer. 20,000 kcal/m²/yr
NPP = GPP − R = 30,000 − 10,000 = 20,000 kcal/m²/yr. NPP represents the energy actually available to be consumed by primary consumers. -
If producers store 40,000 kcal/m²/yr of net productivity, roughly how much energy is available to tertiary consumers (producer → primary → secondary → tertiary), using the standard 10% transfer rule?
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Answer. About 40 kcal/m²/yr
Apply the 10% rule three times: 40,000 × 0.10 = 4,000 (primary consumers) × 0.10 = 400 (secondary consumers) × 0.10 = 40 (tertiary consumers). This shows why food chains rarely exceed four or five trophic levels — too little energy remains. -
A lichen species grows on a tree's bark, gaining a stable surface to live on while causing no measurable harm or benefit to the tree. What type of symbiotic relationship is this?
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Answer. Commensalism
Assign effects to each species: lichen (+), tree (0). A +/0 relationship, where one species benefits and the other is unaffected, defines commensalism, distinguishing it from mutualism (+/+) or parasitism (+/−). -
Two species of warbler feed in the same tree but forage at different heights and on different insect species. What ecological principle explains their coexistence despite occupying the same tree?
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Answer. Resource partitioning (niche differentiation)
By foraging at different heights and on different prey, the warblers reduce niche overlap, avoiding direct competition for the identical resource. This resolves what would otherwise lead to competitive exclusion of one species. -
Compare the life history traits typically seen in r-selected versus K-selected species, and give one example organism for each.
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Answer. r-selected species (e.g., insects, dandelions) have many small offspring, early reproduction, and little parental care, thriving in unstable environments. K-selected species (e.g., elephants, humans) have few large offspring, late reproduction, and high parental investment, thriving in stable environments near carrying capacity.
r-selection favors rapid population growth to exploit unpredictable or temporary resources, sacrificing offspring quality for quantity. K-selection favors offspring quality and competitive ability in stable, resource-limited environments close to K. -
Sea otters prey on sea urchins, which graze on kelp forests. When sea otter populations were hunted to near extinction along the Pacific coast, kelp forests collapsed. What ecological concept does this illustrate, and why?
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Answer. Trophic cascade caused by loss of a keystone species
Sea otters are a keystone species: removing them released urchin populations from predation control, urchins overgrazed the kelp, and the entire kelp forest community (which many other species depend on) collapsed. This indirect, multi-level effect is a classic trophic cascade. -
A drought reduces a rabbit population regardless of how many rabbits were present beforehand, while an outbreak of disease spreads faster and kills a larger proportion when rabbit density is high. Classify each limiting factor.
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Answer. The drought is density-independent; the disease outbreak is density-dependent.
Density-independent factors (like drought) affect a population regardless of its size or crowding. Density-dependent factors (like disease transmission) have a stronger proportional effect as density increases, because contact rates between individuals rise. -
An area of bare volcanic rock is gradually colonized first by lichen, then mosses, then grasses, shrubs, and eventually trees. What kind of succession is this, and what is the first colonizing group called?
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Answer. Primary succession; the first colonizers are called pioneer species
Because the area starts with no pre-existing soil, this must be primary succession. Pioneer species like lichens begin the process by breaking down rock and beginning soil formation, enabling later, more complex species to establish. -
A species with a Type I survivorship curve has high survival through most of life but a sharp die-off in old age. Which organism best fits this pattern, and why?
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Answer. Humans (or other large mammals with few offspring and high parental care)
Type I curves are typical of K-selected species that invest heavily in few offspring, giving most individuals a high chance of surviving to old age, after which mortality rises sharply. This contrasts with Type III curves (e.g., fish, insects), where most offspring die young. -
Two communities each contain 100 individuals across 4 species. Community A has 25 individuals of each species; Community B has 91, 3, 3, and 3 individuals. Which community has higher species evenness, and how does this affect its Simpson's Diversity Index?
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Answer. Community A has higher evenness and therefore a higher Simpson's Diversity Index than Community B.
Simpson's index $D = 1-\sum(n_i/N)^2$ rewards both richness and evenness. Community A's even distribution (25 each) gives a lower sum of squared proportions than Community B's skewed distribution (dominated by one species), so 1 minus that sum is larger for A, meaning A is more diverse. -
Zebra mussels, an invasive species, were introduced into the Great Lakes and rapidly outcompeted native mussel species for food and space. What term describes this ecological outcome, and what underlying interaction caused it?
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Answer. Competitive exclusion of native mussels, driven by interspecific competition for the same limiting resources
Because zebra mussels and native mussels share the same niche (filtering the same food from the same substrate), the introduced species' competitive advantage led to the native species' local decline or elimination — a textbook case of competitive exclusion triggered by an invasive species. -
DDT, a persistent pesticide, was found at much higher concentrations in fish-eating birds of prey than in the water or algae at the base of the food chain. What process explains this pattern, and why does it worsen at higher trophic levels?
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Answer. Biomagnification, because the toxin is not broken down or excreted and becomes more concentrated as it is passed up the food chain in ever-smaller total biomass
Since ~90% of energy (and biomass) is lost at each trophic transfer but the toxin persists in tissue, predators must eat many prey to get enough energy, concentrating the toxin further with each step up the food chain, which is why apex predators suffer the worst effects. -
A grassland ecosystem experiences a wildfire. Afterward, grasses and shrubs quickly return within a few years because the soil and some root systems survived. What term describes both this process and the property of the ecosystem that allowed such quick recovery?
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Answer. This is secondary succession, made possible by the ecosystem's high resilience
Secondary succession occurs where soil and some organisms remain after a disturbance, allowing much faster recovery than primary succession. The ecosystem's ability to recover quickly and return to its prior state after disturbance is called resilience, distinct from resistance (the ability to avoid change in the first place).
What people get wrong
- Confusing GPP with NPP: students plug GPP straight into the 10% rule and forget to subtract the producers' own respiration first. Always compute NPP = GPP − R before doing any trophic transfer calculation.
- Applying the 10% rule by subtraction instead of multiplication (e.g., subtracting 10% instead of keeping only 10%). Remember that only about 10% is *retained*, so you multiply by 0.10 at each step, not subtract 10%.
- Mislabeling symbiotic relationships by only considering one species' benefit. Always assign a +, −, or 0 to *each* species before naming the interaction — this is how you correctly distinguish commensalism (+/0) from mutualism (+/+) or parasitism (+/−).
- Treating exponential growth as the normal, expected pattern in nature. In reality most populations are limited by resources and follow logistic growth; exponential growth only appears briefly, e.g., after colonizing a new habitat or during a population crash's absence of predators.
- Confusing density-dependent and density-independent limiting factors. A factor is density-dependent only if its impact intensifies as population density increases (e.g., disease, competition); factors like a wildfire or flash flood affect a fixed proportion regardless of density and are density-independent.
- Assuming primary and secondary succession start from the same conditions. Primary succession begins with no soil (e.g., bare rock after a glacier retreats) and starts with pioneer species like lichen; secondary succession begins where soil already exists (e.g., after a fire) and proceeds much faster.
Drill this unit until it sticks
These questions come back on a schedule built from what you get wrong, alongside the rest of AP Biology. Free, and no account needed to start.