Biology  by Bradford
IB Biology 2025 · Theme C · Interaction and interdependence

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

46 original IB-style questions on C4.1, written from the 2025 guide: 23 multiple-choice, 13 short-answer, 6 data-based, 2 extended-response part, 2 drawing. Below is a 20-mark standard-level practice paper built from them, ready to hand out as a class quiz or homework, or to sit 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

  • 23 multiple-choice
  • 13 short-answer
  • 6 data-based
  • 2 extended-response part
  • 2 drawing
  • 0 higher level only

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

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The practice paper

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Biology · topic quiz
Standard level · topic practice, not an exam format
30 minutes20 marks

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Covers C4.1 Populations and communities
Name:
1.

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
2.

Which sampling method suits which organism?

[1]
RowSessile organisms (e.g. limpets, plants)Motile organisms (e.g. beetles, fish)
A.capture–mark–release–recapturerandom quadrats
B.random quadrats (count within known areas)capture–mark–release–recapture (Lincoln index)
C.neither can be sampledboth need nets only
D.transects onlyquadrats only
  1. Row A
  2. Row B
  3. Row C
  4. Row D
3.

Some plants release chemicals from their roots or leaves that inhibit the germination or growth of neighbouring plants. What is this called?

[1]
  1. Commensalism
  2. Pollination
  3. Allelopathy
  4. Parasitism
4.

Which is an example of intraspecific cooperation?

[1]
  1. Meerkats taking turns to watch for predators
  2. Two plant species competing for the same light
  3. A parasitic worm feeding on its host
  4. Lions competing with hyenas for a carcass
5.

Which factor limiting a population of field mice acts in a density-dependent way?

[1]
  1. An unusually cold winter killing part of the population
  2. A flood covering the field
  3. Increased spread of a parasite as the mice become more crowded
  4. Application of a pesticide by a farmer
6.

An invasive species often outcompetes native species in its new range. Which factor commonly contributes to this?

[1]
  1. Invasive species always have larger bodies
  2. The invader arrives without its natural enemies
  3. Invaders cannot reproduce in the new range
  4. Native species always cooperate with invaders
7.

Explain why a population cannot grow exponentially for long, with reference to carrying capacity.

[3]
8.

Duckweed (Lemna sp.) is a small floating plant that reproduces asexually. A student places five fronds in a container of pond water on a sunny windowsill to test whether the population grows according to the sigmoid growth model.

(a)Describe how the student could collect data to test the model.[2]
(b)Predict, with a reason, the appearance of the graph towards the end of the investigation if the sigmoid model applies.[2]
9.

Ants of one tropical species live inside the hollow thorns of an acacia tree and feed on sugary secretions produced by the tree. Trees from which the ants were experimentally removed grew more slowly, and lost far more leaf tissue to large herbivores, than trees on which the ants were left. Justify the classification of this relationship as mutualism rather than parasitism.

[3]
10.

A student estimated the population of a sessile 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.

(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]

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. C4.1.1 [1]
  • D: same species, same area, potentially interbreeding; all populations together form a community;
2. C4.1.2 [1]
  • B: stationary organisms are counted in quadrats; mobile animals are estimated by mark–recapture;
3. C4.1.18 [1]
  • C: allelopathic chemicals suppress competitors (e.g. walnut trees' juglone); antibiotic secretion by microbes is the microbial equivalent;
4. C4.1.9 [1]
  • A: cooperation within a species (sentinel behaviour); the others are interspecific interactions or competition;
5. C4.1.6 [1]
  • C: density-dependent factors (competition, predation, disease/parasitism) intensify as population density rises; weather, floods and pesticide application act regardless of density;
6. C4.1.13 [1]
  • B: release from natural enemies plus (often) generalist habits gives invaders a competitive edge over natives (e.g. cane toads, zebra mussels);
7. C4.1.5 [3 max]
  • exponential growth occurs (only) while resources are abundant / limiting factors are weak;
  • as numbers rise, resources (food/space/water) per individual fall / wastes and enemies increase;
  • mortality rises and/or natality falls (density-dependent);
  • numbers level off at the carrying capacity, the maximum population the environment can sustain, OWTTE;
8. C4.1.8
  • (a) [2 max]
    • count the number of fronds/plants at regular intervals (e.g. every two days) over several weeks;
    • keep other conditions (light, temperature, volume of water/nutrients) constant;
    • plot population size against time and compare the shape of the curve with the (sigmoid) model;
  • (b) [2]
    • the curve levels off / reaches a plateau (at the carrying capacity);
    • because resources (space on the water surface / nutrients / light) become limiting, so natality falls / mortality rises until they balance, OWTTE;
9. C4.1.11 [3 max]
  • in mutualism both species gain a benefit, whereas in parasitism one species benefits at the expense of the other;
  • the ants clearly benefit, since the tree provides them with shelter in the thorns and with food in the form of sugary secretions;
  • the tree also benefits: trees with ants grew faster and lost less tissue to herbivores, so the ants must be defending the tree;
  • the removal experiment shows that the difference is caused by the ants rather than by some other factor, so the benefit to the tree is real;

A definition of mutualism alone, without reference to the data, gains a maximum of one mark.

10. 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);

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