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IB Biology 2025 · Theme C · Interaction and interdependence

C4.1 Populations and communities. Practice questions with markscheme.

67 original IB-style questions on C4.1, written from the 2025 guide: 31 multiple-choice, 24 short-answer, 7 data-based, 2 extended-response parts, 2 drawing, 1 labelling. This subtopic has no additional higher level content, so the 20-mark practice paper below serves SL and HL alike. Hand one out as a class quiz or homework, or sit it yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

18 statements at SL and HL.

  1. C4.1.1SL / HL Populations as interacting groups of organisms of the same species living in an area
  2. C4.1.2SL / HL Estimation of population size by random sampling
  3. C4.1.3SL / HL Random quadrat sampling to estimate population size for sessile organisms
  4. C4.1.4SL / HL Capture–mark–release–recapture and the Lincoln index to estimate population size for motile organisms
  5. C4.1.5SL / HL Carrying capacity and competition for limited resources
  6. C4.1.6SL / HL Negative feedback control of population size by density-dependent factors
  7. C4.1.7SL / HL Population growth curves
  8. C4.1.8SL / HL Modelling of the sigmoid population growth curve
  9. C4.1.9SL / HL Competition versus cooperation in intraspecific relationships
  10. C4.1.10SL / HL A community as all of the interacting organisms in an ecosystem
  11. C4.1.11SL / HL Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as categories of interspecific relationship within communities
  12. C4.1.12SL / HL Mutualism as an interspecific relationship that benefits both species
  13. C4.1.13SL / HL Resource competition between endemic and invasive species
  14. C4.1.14SL / HL Tests for interspecific competition
  15. C4.1.15SL / HL Use of the chi-squared test for association between two species
  16. C4.1.16SL / HL Predator–prey relationships as an example of density-dependent control of animal populations
  17. C4.1.17SL / HL Top-down and bottom-up control of populations in communities
  18. C4.1.18SL / HL Allelopathy and secretion of antibiotics

In the bank for C4.1

  • 31 multiple-choice
  • 24 short-answer
  • 7 data-based
  • 2 extended-response parts
  • 2 drawing
  • 1 labelling
  • 0 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

Make your own

The practice paper

No additional higher level content in this subtopic, so one paper serves both · 20 marks
Take it online → Build a fresh paper Paper code BbB-EAAAAAAEABQAO0Qg
Biology · topic quiz
Standard level · topic practice, not an exam format
30 minutes20 marks

Paper code: this paper was generated, so the code is its recipe. Enter it at biologybybradford.com/exam-maker to rebuild this exact paper and its markscheme.

Covers C4.1 Populations and communities · SL
Name:
1.

Oak trees, the insects that feed on them, insect-eating birds, fungi and soil bacteria live together in a woodland. Which term describes all of these interacting populations?

[1]
  1. A species
  2. A population
  3. An ecosystem
  4. A community
2.

What is a population?

[1]
  1. All the organisms living in an ecosystem
  2. All the interacting species in a habitat
  3. The number of offspring produced by a species each year
  4. A group of individuals of the same species living in an area
3.

Which plant family depends on a mutualism with mycorrhizal fungi that is especially critical, because its dust-like seeds carry (almost) no food reserves?

[1]
  1. Poaceae (grasses)
  2. Fabaceae (legumes)
  3. Orchidaceae (orchids)
  4. Asteraceae (daisies)
4.

A deer population on an island with no predators fluctuates around a stable level of about 400 individuals. Explain how negative feedback keeps the population close to this level.

[3]
0 5 10 15 20 25 30 0 200 400 600 time / years number of deer on the island
5.

A student estimated the population of a tube-worm on a mudflat with a total area of 500 m². Twenty quadrats, each of area 0.25 m², were placed at positions determined using random numbers. The mean count was 8.0 tube-worms per quadrat.

the mudflat, seen from above (quadrats not to scale) 25 m 20 m 20 quadrats of 0.25 m² at random-number positions; total area 500 m²
(a)Calculate the estimated population size of the tube-worm on the mudflat.[2]
(b)Outline why the quadrat positions were determined using random numbers.[1]
(c)State one reason why quadrat sampling is suitable for the tube-worms but not for the crabs living on the same mudflat.[1]
6.

Sketch and annotate a logistic (sigmoid) population growth curve. On the same axes, show how a population that overshoots its carrying capacity might behave.

[3]
7.

Students estimated the size of a woodlouse population in a walled garden using the capture–mark–release–recapture method. On day 1 they captured woodlice under boards, marked each with a small dot of non-toxic paint, and released them where found. On day 3 they made a second capture. Their results: first capture, marked and released: 84; second capture: 63, of which 12 were marked.

(a)Calculate the Lincoln-index estimate of the population size.[2]
(b)Outline two assumptions of the capture–mark–release–recapture method.[2]
(c)Suggest one reason why this estimate might be too high.[1]
(d)The woodlice were found in dense clusters under the boards, which reduces water loss from each individual. Outline two ways in which living at high density could also be a disadvantage to the woodlice.[2]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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Markscheme BbB-EAAAAAAEABQAO0Qg

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. C4.1.10 [1]
  • D: a community is all the populations of different species living and interacting in an area; including the abiotic environment as well would make it an ecosystem;
2. C4.1.1 [1]
  • D: same species, same area, potentially interbreeding; all populations together form a community;
3. C4.1.12 [1]
  • C: orchid seeds cannot germinate in nature without a fungal partner supplying carbon compounds and minerals; the adult may repay the fungus later, a classic obligate mutualism;
4. C4.1.6 [3 max]
  • as the population rises (above the carrying capacity), density-dependent factors intensify;
  • competition (for food/resources) increases / food per individual decreases OR disease/parasites spread more easily at high density;
  • mortality increases / natality (birth rate) decreases, so the population falls;
  • when the population falls (below this level), competition/disease pressure is relaxed, so survival and reproduction increase and the population rises again;
  • this negative feedback returns the population towards the carrying capacity, so it fluctuates around 400, OWTTE;
5. C4.1.3
  • (a) [2]
    • 8.0 ÷ 0.25 = 32 (tube-worms) per m²;
    • 32 × 500 = 16 000 (tube-worms);

    Award 2 for the correct final answer; ECF from an incorrect first step.

  • (b) [1]
    • to avoid bias (in choosing where to sample) so that the sample is representative / the estimate is valid;
  • (c) [1]
    • tube-worms are sessile / do not move, so they remain in the quadrat while being counted, whereas crabs are motile (and move between quadrats / escape);
6. C4.1.5 [3]
  • axes labelled: population size (y) against time (x), with an S-shaped curve showing an exponential phase, a transitional (slowing) phase and a plateau;
  • the plateau annotated as the carrying capacity (K), drawn as a dashed horizontal line;
  • a second trace that rises above K, then falls back / oscillates toward K, annotated (resources depleted, mortality rises), OWTTE;

Award correct sigmoid shape even if freehand; the overshoot line and annotations are required for full marks.

7. C4.1.4
  • (a) [2]
    • N = (84 × 63) ÷ 12;
    • = 441 (woodlice);

    Accept 440–441. Award ECF for correct method.

  • (b) [2]
    • no (significant) births/deaths/migration between captures / the population is closed;
    • marked and unmarked individuals mix randomly / marking does not affect survival or behaviour / marks are not lost;

    Mark the first two answers only.

  • (c) [1]
    • marks rubbed off / marked individuals died or hid (reducing recaptures) / marking made animals easier for predators to remove, so fewer marked were recaptured, inflating N, OWTTE;
  • (d) [2 max]
    • (intraspecific) competition for food / space increases, so each woodlouse obtains fewer resources;
    • pathogens / parasites are transmitted more easily between individuals that are close together;
    • a dense group may be found by predators more easily / predators may remove more individuals, OWTTE;

    Mark the first two answers only.

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