Population Ecology

Population Ecology is topic 8.3 of AP Biology, inside Ecology. This page works through three real practice questions on it, with the full reasoning behind each credited answer.

Practise this topic freeThe whole unit
39 questionsRepresenting and Describing Data100% with a figure
Worked examples

Three real population ecology questions

From the practice pool, not the mock papers — each with the reasoning that produces the answer.

100% of them come with a figure. Reading the graph or diagram correctly is most of the work here before any content knowledge applies.

Concept Explanation · with figure

A food web shows foxes eating rabbits and rabbits eating grasses. If the fox population decreases sharply, which short-term effect is most likely?

  1. AReduced predation can allow rabbit abundance to risecorrect
  2. BGrass biomass must increase because rabbits vanish.
  3. CThe food web stops obeying conservation of matter.
  4. DRabbit predation by foxes increases immediately.
Why A is correct

If a predator decreases, its prey may increase in the short term because predation pressure is reduced.

The evidence in the prompt matters before the topic label: "A food web shows foxes eating rabbits and rabbits eating grasses. If the fox population decreases sharply, which...". The supported answer is "Rabbit abundance may increase because predation pressure decreases" because If a predator decreases, its prey may increase in the short term because predation pressure is reduced. Distractors such as Grass biomass must increase because rabbits vanish or food web stops obeying conservation of matter either ignore the stated pattern or name a related concept without matching the evidence.

Statistical Tests and Data Analysis · with figure

A population begins the year with 200 individuals. During the year there are 50 births, 20 deaths, 10 immigrants, and 5 emigrants. What is the percent growth relative to the starting population?

  1. A25%, because births are 50 and 50/200 x 100 equals 25%.
  2. BNet change is 35 individuals, which is 17.5% of 200correct
  3. C42.5%, because all four event counts should be added before dividing by 200.
  4. D15%, because births minus deaths equals 30.
Why B is correct

Net change equals births minus deaths plus immigrants minus emigrants: 50 - 20 + 10 - 5 = 35. Percent growth relative to the starting population is 35/200 x 100 = 17.5%.

The stem asks for a numerical or quantitative judgment: "A population begins the year with 200 individuals. During the year there are 50 births, 20 deaths, 10 immigrants,...". The safe route is to identify the counted event, use the full denominator, and then interpret the number biologically. Here the worked reasoning is Net change equals births minus deaths plus immigrants minus emigrants: 50 - 20 + 10 - 5 = 35, so choices based on 25%, because births are 50 and 50/200 x 100 equals 25% or 42.5%, because all four event counts should be added before dividing... lose the required setup.

Statistical Tests and Data Analysis · with figure

A mark-recapture study marks 40 fish in a pond. In a later sample of 50 fish, 10 are marked. Using the estimate N = marked first sample x total second sample / marked recaptured, what is the estimated population size?

  1. A100 fish, because 40 plus 50 plus 10 equals 100.
  2. B80 fish, because 40 plus 50 minus 10 equals 80.
  3. C20 fish, because 40 x 50 / 100 equals 20.
  4. D200 fish, because 40 x 50 / 10 equals 200.correct
Why D is correct

Using the stated mark-recapture estimate, N = 40 x 50 / 10 = 200 fish.

The stem asks for a numerical or quantitative judgment: "A mark-recapture study marks 40 fish in a pond. In a later sample of 50 fish, 10 are marked. Using the estimate N...". The safe route is to identify the counted event, use the full denominator, and then interpret the number biologically. Here the worked reasoning is Using the stated mark-recapture estimate, N = 40 x 50 / 10 = 200 fish, so choices based on 100 fish, because 40 plus 50 plus 10 equals 100 or 80 fish, because 40 plus 50 minus 10 equals 80 lose the required setup.

39 questions on this topic, free to start.

Ten a day at no cost. One account covers SAT, ACT and AP.

Start free
Last reviewed 2026-08-28. Topic and unit names follow the College Board course framework. Question counts describe the PrepScore practice bank, not the exam.
Same unit
Community Ecology33 questionsEnergy Flow Through Ecosystems24 questionsBiodiversity21 questions

Work population ecology until the reasoning is automatic.

Real AP questions with a full explanation on every answer, and a mistake bank that only clears when you get it right.

Start freeBack to the unit