Practice a College Board-style free response question on Ecology. Write your response, then reveal the model answer to see exactly what earns each point.
Free Response Question · Unit 8 · Population Growth and Carrying Capacity
A population of deer is introduced to an island with abundant food and no natural predators. Researchers track the population size over 10 years:
Year
Population size
0
20
2
45
4
100
6
180
8
210
10
215
A
Describe the overall shape of this population's growth curve over the 10 years, and identify which growth model (exponential or logistic) best fits the data. Justify your answer using specific data from the table.
✓ Model answer (earns the point)
The population follows an S-shaped (sigmoidal) curve, consistent with the logistic growth model. From year 0 to year 6, the population grows rapidly (20 → 180), similar to exponential growth. However, growth then slows dramatically between years 6 and 10 (180 → 215, a much smaller increase than in earlier years), and the population appears to be leveling off near 215 by year 10. This slowing and plateauing — rather than continued rapid growth — is the signature of logistic growth as the population approaches its carrying capacity.
Why it scores: Correctly identifies the logistic model (not exponential) AND describes the S-shape AND cites specific numbers from the table showing the slowdown (e.g., comparing the year 6→8 change to the year 0→2 change) to justify the answer.
B
Based on the data, estimate the carrying capacity (K) of this island for the deer population. Explain what is biologically happening to cause the population's growth rate to slow as it approaches this value.
✓ Model answer (earns the point)
The carrying capacity is approximately 215-220 deer, based on where the population size levels off in the data (years 8-10). As the population approaches this size, density-dependent factors intensify: with more deer competing for the same fixed food supply, space, and other resources, each individual has access to fewer resources. This increased competition (and potentially increased disease transmission at higher density) reduces the birth rate and/or increases the death rate, slowing population growth until it stabilizes near the environment's carrying capacity.
Why it scores: Gives a reasonable numeric estimate of K based on the leveling-off point in the data, AND explains the mechanism (density-dependent factors like competition for limited resources) that causes growth to slow as the population nears K.
C
Suppose wolves are introduced to the island in year 11. Predict how this would affect the deer population's growth pattern going forward, and explain why, in terms of limiting factors.
✓ Model answer (earns the point)
Introducing wolves would add predation as a new density-dependent limiting factor on the deer population. The deer population would likely decline from its current size (or its effective carrying capacity would be lowered), since wolves would now be removing deer in addition to existing resource competition. Predation is density-dependent because the rate at which wolves encounter and capture deer tends to increase as deer density increases, making this an additional pressure layered on top of (rather than replacing) the resource-based limits already shown in the data.
Why it scores: Correctly predicts a decrease in deer population size/lowered effective carrying capacity, AND identifies predation as the new limiting factor, AND explains that predation is density-dependent (its effect scales with deer population density).
How to score points on AP Biology FRQs
Use the data when it's provided. If there's a table or graph in the stimulus, reference specific values — cite actual numbers, not just "the population increased."
Name specific biology, not vague concepts. "Density-dependent competition for resources" beats "things got crowded." "Carrying capacity" beats "the max amount."
Answer the verb in the prompt. "Describe" needs an observation. "Explain" needs a mechanism. "Predict and justify" needs a prediction AND the biology behind it.
Trace cause to effect. Don't just state outcomes — connect them. "Population nears K → resources become limited → competition increases → growth rate slows" is a strong causal chain.
For ecology questions, name the specific type of limiting factor or interaction. AP graders want to see WHETHER something is density-dependent or independent, and WHICH interaction type (predation, competition, mutualism, etc.) is at play — not just "the population was affected."