IB DP Biology: Stability and change in ecosystems, keystone species and succession (DP Biology) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions cover IB Diploma Programme Biology D4.2 Stability and change (D4.2.1 to D4.2.15), from the IB Biology guide, first assessment 2025. Questions 1 to 11 are SL and HL. Question 12 is HL only. Study first with the study guide and the revision notes. Course links: IB DP Biology hub and printable checklist.

Questions

1. State two requirements for the stability of a natural ecosystem. [2]

2. A tropical forest region originally covered 480 thousand km² and now covers 396 thousand km².

(a) Calculate the percentage change in forest area. [2] (b) Suggest why scientists cannot say exactly how much forest can be lost before a tipping point is reached. [1]

3. A teacher sets up sealed glass jars of pond water containing algae, small crustaceans and microbes, and places them at three different temperatures.

(a) Outline why sealed glass jars are preferred to open tanks for this investigation. [2] (b) Identify one variable that should be kept constant and one limitation of using the jars as a model of a natural pond. [2]

4. On an island, a fruit bat eats fruit and disperses the seeds of several tree species. After the bat disappeared, seedlings of bat-dispersed trees fell from 220 to 64 per hectare.

(a) Calculate the percentage change in seedlings per hectare. [2] (b) Explain why the bat could be called a keystone species. [3]

5. A marine fishery has an estimated annual replacement of 12 000 t. Annual catch is 14 500 t. An earlier survey recorded 520 t landed with 800 boat-days of effort; the latest recorded 410 t with 1025 boat-days.

(a) Compare the catch with the replacement rate and state the effect on the stock. [2] (b) Calculate the percentage change in catch per unit effort between the surveys. [3] (c) Suggest one other measurement that would show whether the stock is declining. [1]

6. A wild tree is harvested for its bark. A forest holds 120 mature trees per hectare. Each year 9 per hectare are harvested and 6 per hectare grow to maturity.

(a) Calculate the net annual change in mature trees per hectare. [1] (b) Predict the number after ten years if rates stay constant. [1] (c) Suggest how the harvest could be made sustainable. [1] (d) State one way the sustainability of this harvest could be monitored. [1]

7. Explain two ways in which intensive arable farming can reduce the long-term sustainability of agriculture. [4]

8. Water was sampled upstream and downstream of a farm. Dissolved oxygen was measured on day 0 and after five days sealed in the dark at constant temperature. Sample A (downstream): 8.6 and 1.9 mg dm⁻³. Sample B (upstream): 8.8 and 7.9 mg dm⁻³.

(a) Calculate the biochemical oxygen demand (BOD) of each sample. [2] (b) Explain the difference, referring to fertilizer use. [3] (c) Explain why the samples were kept in the dark. [1]

9. Mercury concentrations (mg kg⁻¹ of tissue) in a marine food chain are: zooplankton 0.05, sardine 0.2, tuna 1.6.

(a) Calculate how many times greater the concentration is in tuna than in zooplankton. [2] (b) Explain this pattern. [3]

10. A survey found plastic in the stomachs of 21 of 84 seabirds examined.

(a) Calculate the percentage containing plastic. [1] (b) Explain why plastics persist in the ocean. [2] (c) Describe two effects of plastic pollution on marine organisms. [2]

11. Discuss how rewilding can restore natural processes in a large degraded upland area. Refer to the Hinewai Reserve in your answer. [8]

12. (HL only) A glacier has retreated, exposing bare ground. Data from sites of different ages are shown.

Years since ice retreat	5	30	90	250
Plant species	2	8	15	12
Primary production (g m⁻² year⁻¹)	15	140	520	610

(a) Calculate the percentage increase in plant species between 5 and 90 years. [2] (b) Describe two trends in the data and suggest a reason for the fall in species between 90 and 250 years. [3] (c) Explain how grazing by farm livestock could arrest succession at a similar site. [3]

Answers

1. A supply of energy [1]; recycling of nutrients [1]. Equivalent credit: genetic diversity; climatic variables within tolerance levels. [2] Examiner insight: Name the requirement itself; “good conditions” is too vague to earn a mark.

2. (a) Change = 396 − 480 = −84 [1]. (−84 ÷ 480) × 100 = −17.5% [1]. (b) The minimum area needed to keep transpiration, cooling and rainfall running is uncertain, so the threshold position is unknown [1]. [3] Examiner insight: Divide by the original value, and show the sign or the word “decrease”, or the answer can lose credit.

3. (a) Matter cannot enter or leave a sealed vessel [1]. Light energy can still pass through the glass [1]. (b) Constant: light intensity, volume of water or species added (any one) [1]. Limitation: far simpler than a natural pond, so results may not apply directly [1]. [4] Examiner insight: Name the specific variable. Temperature itself cannot be the answer because it is the independent variable.

4. (a) (64 − 220) ÷ 220 × 100 [1] = −70.9% [1]. (b) The bat’s removal has a large effect on community structure [1] relative to its numbers, because many tree species depend on it for seed dispersal [1]. Fewer seedlings mean the tree community may change or collapse [1]. [5] Examiner insight: Link the effect to community structure. Saying only that the bat is important does not distinguish keystone from common.

5. (a) Catch exceeds replacement by 2500 t [1], so the stock will decline [1]. (b) Earlier CPUE = 520 ÷ 800 = 0.65 t per boat-day; latest = 410 ÷ 1025 = 0.40 [1]. Change = (0.40 − 0.65) ÷ 0.65 [1] = −38.5% [1]. (c) Any one: size or age of fish caught; survey estimate of stock biomass [1]. [6] Examiner insight: Show both CPUE values before the percentage change so an arithmetic slip does not hide correct method.

6. (a) 6 − 9 = −3 trees per hectare per year [1]. (b) 120 − 30 = 90 per hectare [1]. (c) Reduce harvest to no more than 6 per hectare per year, or plant seedlings [1]. (d) Count seedlings, or survey age structure [1]. [4] Examiner insight: Sustainable means harvest below replacement, so (c) must refer to the rate.

7. Any two factors, each named with its consequence. Soil erosion: bare ploughed soil is lost by wind or rain [1], reducing fertility and future yield [1]. Leaching: nutrients wash out of the soil [1], so more fertilizer is needed and waterways are polluted [1]. Equivalent credit: agrochemicals harm non-target organisms and reduce biodiversity; machinery and fertilizer manufacture release carbon dioxide, adding to climate change. [4] Examiner insight: Each factor needs its consequence. A list of five factors with no effect earns little.

8. (a) A: 8.6 − 1.9 = 6.7 mg dm⁻³ [1]. B: 8.8 − 7.9 = 0.9 mg dm⁻³ [1]. (b) Fertilizer nitrate and phosphate leach into the water downstream [1], causing algal growth, and dead algae supply organic matter [1]. Aerobic decomposers respire this matter and use oxygen, so BOD is higher [1]. (c) Darkness stops photosynthesis, which would add oxygen [1]. [6] Examiner insight: Name decomposers as the oxygen users. Saying “the algae use up the oxygen” is a frequent error.

9. (a) 1.6 ÷ 0.05 [1] = 32 times [1]. (b) Mercury is persistent and is not excreted [1]. A consumer takes in many prey organisms from the trophic level beneath it [1], so the toxin accumulates and concentration rises at each trophic level, highest in top consumers [1]. [5] Examiner insight: Include the step about taking in many prey, which explains why concentration rises.

10. (a) 21 ÷ 84 × 100 = 25% [1]. (b) Plastics are not biodegradable [1], so decomposers cannot break the polymers down [1]. (c) Any two: entanglement restricting movement [1]; gut blockage or feeling full so less feeding [1]; also creditworthy: microplastics ingested and passed to consumers. [5] Examiner insight: A named effect on an organism is needed. “Harms wildlife” is not specific enough.

11. Credit any of the following, up to 8. Rewilding restores natural processes with minimal control [1]. Methods: reintroduce apex predators or keystone species [1]; re-establish connectivity of habitats over large areas [1]; minimize human impact and use ecological management [1]. Predators reduce herbivore numbers [1], so vegetation recovers [1] and the food web becomes more stable [1]. Hinewai Reserve, New Zealand: degraded, gorse-covered farmland regenerating into native forest with little intervention [1]. Judgement: restoration may be slow, uncertain and need long-term management. Also creditworthy: corridors allow movement and gene flow, supporting genetic diversity. [8] Examiner insight: “Discuss” rewards a balanced answer, so include a limitation, not only benefits.

12. (a) (15 − 2) ÷ 2 × 100 [1] = 650% [1]. (b) Species increase to 90 years and then fall [1]; primary production keeps rising [1]. Species fall because taller, dominant plants shade or outcompete pioneers [1]. (c) Livestock graze seedlings and shrubs [1]. Trees cannot establish [1], so grassland persists instead of the woodland climax community [1]. [8] Examiner insight: Describe the trend with values from the table, and name the human influence in (c).

Where marks are usually lost

• Calculating percentage change against the new value.
• Giving a bare number for percentage change without sign or units.
• Calling a keystone species merely abundant.
• Forgetting that decomposers, not algae, remove the oxygen in eutrophication.
• Skipping the mention of leaching.
• Writing “toxins get bigger” instead of concentration rising at each trophic level.
• Giving a rewilding method but no effect.
• Not showing intermediate values in multi-step calculations.
• Putting succession content into an SL answer.

Next steps

• Revision notes
• Study guide
• IB DP Biology hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization, subtopic D4.2.