Biology  by Bradford
IB Biology 2025 · Theme D · Continuity and change

D4.2 Stability and change. Practice questions with markscheme.

45 original IB-style questions on D4.2, written from the 2025 guide: 20 multiple-choice, 16 short-answer, 5 data-based, 3 extended-response part, 1 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

11 statements at SL and HL, 4 additional higher level.

  1. D4.2.1SL / HL Stability as a property of natural ecosystems
  2. D4.2.2SL / HL Requirements for stability in ecosystems
  3. D4.2.3SL / HL Deforestation of Amazon rainforest as an example of a possible tipping point in ecosystem stability
  4. D4.2.4SL / HL Use of a model to investigate the effect of variables on ecosystem stability
  5. D4.2.5SL / HL Role of keystone species in the stability of ecosystems
  6. D4.2.6SL / HL Assessing sustainability of resource harvesting from natural ecosystems
  7. D4.2.7SL / HL Factors affecting the sustainability of agriculture
  8. D4.2.8SL / HL Eutrophication of aquatic and marine ecosystems due to leaching
  9. D4.2.9SL / HL Biomagnification of pollutants in natural ecosystems
  10. D4.2.10SL / HL Effects of microplastic and macroplastic pollution of the oceans
  11. D4.2.11SL / HL Restoration of natural processes in ecosystems by rewilding
  12. D4.2.12HL Ecological succession and its causes
  13. D4.2.13HL Changes occurring during primary succession
  14. D4.2.14HL Cyclical succession in ecosystems
  15. D4.2.15HL Climax communities and arrested succession

In the bank for D4.2

  • 20 multiple-choice
  • 16 short-answer
  • 5 data-based
  • 3 extended-response part
  • 1 drawing
  • 9 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 D4.2 Stability and change
Name:
1.

What is a mesocosm?

[1]
  1. A small experimental model of an ecosystem
  2. A natural ecosystem of intermediate size
  3. A computer simulation of nutrient cycling
  4. A graph comparing two ecosystems' stability
2.

Microplastics are accumulating in ocean food webs. Which properties make plastic pollution so persistent?

[1]
  1. Plastics dissolve quickly in seawater
  2. Plastics evaporate into the atmosphere
  3. Plastics resist biological decomposition
  4. Plastics are converted into oxygen
3.

Why can very small amounts of the insecticide DDT in water become lethal to fish-eating birds?

[1]
  1. Birds drink far more water than fish
  2. Birds are more sensitive to all chemicals
  3. DDT evaporates into the birds' lungs
  4. DDT is magnified along the food chain
4.

Which requirements must be met for an ecosystem to remain stable over long periods?
I. A continuous supply of energy
II. Recycling of nutrients
III. High genetic diversity within populations

[1]
  1. I, II and III
  2. I and III only
  3. II and III only
  4. I and II only
5.

The food web shows feeding relationships on a rocky shore. Sea stars prefer to eat mussels, which are the strongest competitors for space on the rock.

producersprimary consumerssecondary consumerstertiary consumerseaweedphytoplanktonlimpetmusselbarnaclesea stardog whelkgull
(a)Define keystone species.[1]
(b)Predict, with reasons, the effect on the community of removing all of the sea stars.[3]
6.

Untreated sewage was discharged into a river at a single point. Scientists measured the dissolved oxygen concentration and counted mayfly larvae (which need well-oxygenated water) at stations upstream and downstream of the outfall.

Distance from outfall / kmDissolved oxygen / mg dm⁻³Mayfly larvae per m²
−1 (upstream)9.6120
0.54.18
22.80
54.912
107.464
209.2105
(a)Describe the change in dissolved oxygen along the river.[2]
(b)Explain the fall in dissolved oxygen downstream of the outfall.[3]
(c)Suggest why mayfly larvae numbers follow the oxygen curve.[1]
(d)Outline why the river recovers with distance from the outfall.[2]
7.

Deforestation of the Amazon may push the forest past a tipping point. Discuss what this means and the possible consequences.

[4]

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-EAAAAAAAQBQAO9CC

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. D4.2.4 [1]
  • A: mesocosms (tanks, enclosures, sealed jars/ecospheres) are physical models of ecosystems used to investigate how variables affect stability;
2. D4.2.10 [1]
  • C: synthetic polymers are (largely) non-biodegradable; they fragment into micro/nanoplastics that organisms ingest;
3. D4.2.9 [1]
  • D: bioaccumulation within organisms plus step-by-step magnification along trophic levels concentrates persistent pollutants in top predators (thin eggshells etc.);
4. D4.2.1 [1]
  • A: stability requires ongoing energy supply, nutrient cycling, and (genetic) diversity allowing populations to adapt, plus climatic variables staying within tolerance limits;
5. D4.2.5, C4.1.17
  • (a) [1]
    • a species that has a disproportionately large effect on the structure/stability of a community relative to its abundance;
  • (b) [3 max]
    • mussel population increases (released from predation);
    • mussels outcompete barnacles, limpets and seaweed for space, so those species decline;
    • species diversity falls / the community becomes dominated by mussels;
    • dog whelks (and gulls) may increase at first as mussels become more abundant;
6. D4.2.8
  • (a) [2]
    • falls sharply below the outfall, reaching a minimum (2.8 mg dm⁻³) at about 2 km;
    • then recovers progressively, approaching the upstream value by 20 km, OWTTE;
  • (b) [3]
    • sewage adds organic matter/nutrients;
    • (saprotrophic) bacteria/decomposers multiply and respire aerobically as they break it down;
    • their respiration consumes dissolved oxygen / high biochemical oxygen demand (BOD), OWTTE;
  • (c) [1 max]
    • mayfly larvae need high oxygen (they are indicator species), so they die out where oxygen is low and recolonize as it recovers, OWTTE;
  • (d) [2 max]
    • the organic matter is (progressively) decomposed / diluted;
    • (so) bacterial respiration/BOD declines;
    • oxygen re-enters from the air / photosynthesis restores oxygen, allowing communities to return, OWTTE;

    Content pivot.

7. D4.2.3 [4 max]
  • a tipping point is a threshold beyond which change becomes self-reinforcing/irreversible (positive feedback);
  • much of the Amazon's rainfall is recycled by the forest itself (transpiration → clouds/rain);
  • clearing forest reduces transpiration, so rainfall falls, stressing/killing more forest, further reducing rainfall;
  • (beyond the threshold) forest could shift to (drier) savannah/grassland even if clearing stops;
  • consequences: loss of biodiversity/habitat, release of stored carbon (accelerating climate change), altered regional/global rainfall;
  • uncertainty: the threshold's position (estimated ~20–25 % loss) is not known precisely, so precaution argues for halting deforestation early, OWTTE;

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