IB DP Environmental Systems and Societies Topic 2: Ecology -- Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

Related: Topic 2 Ecology revision notes and the Topic 1 Foundations revision notes.

Section A

1. Explain why food chains rarely extend beyond four or five trophic levels. [2]

2. State the key difference between how energy and nutrients move through an ecosystem. [2]

3. Define “trophic cascade.” [2]

Section B

4. A forest ecosystem loses its top predator (a wolf population) due to hunting.

(a) Using systems thinking, explain the direct effect this is likely to have on the deer population the wolves preyed on. [2] (b) Explain one indirect, cascading effect this could have further down the food chain. [3]

5. A lake ecosystem receives excess nutrients (nitrogen and phosphorus) from agricultural runoff.

(a) Explain how this disrupts the lake’s normal nutrient cycling. [2] (b) Explain, using the concept of sustainability from Topic 1, why this disruption matters beyond the immediate ecological effect. [3]

Section C

6. Using a named or hypothetical ecosystem, examine how removing a single species can propagate effects through the whole food web, and explain why this outcome is better understood through systems thinking than through describing only the direct effect. [8]

7. (HL) Two similar grassland ecosystems have different numbers of trophic levels. Using the concept of ecological efficiency, explain why one ecosystem might support more trophic levels than the other, and outline how this could be investigated quantitatively. [8]

Worked answers

1. Because energy transfer between trophic levels is inefficient, with most energy lost as heat at each stage of transfer; after four or five transfers, too little energy remains to support a further trophic level. [2]

2. Energy flows through an ecosystem in one direction and is ultimately lost as heat, so it must be continually resupplied (e.g. by the sun); nutrients cycle and are reused repeatedly within the system rather than being lost. [2]

3. A chain of indirect effects that spreads through an ecosystem’s trophic levels after a disturbance to one part of it, such as the removal of a top predator, rather than the disturbance’s effect staying confined to the directly affected species. [2]

4. (a) With the wolves removed, predation pressure on the deer population falls, so the deer population is likely to increase, since fewer deer are being killed by predators. [2] (b) The larger deer population may over-consume the vegetation (e.g. young trees or shrubs) that forms their food source, depleting plant biomass further down the food chain; this could then reduce habitat or food availability for other species dependent on that vegetation, propagating the original disturbance well beyond the wolf-deer relationship alone. [3]

5. (a) Excess nitrogen and phosphorus entering the lake disrupt the normal, balanced cycling of these nutrients by adding far more than the ecosystem’s natural processes (uptake by producers, decomposition) can process at the usual rate, often triggering excessive algal growth (eutrophication) that consumes the cycle’s balance. [2] (b) This disruption undermines the ecosystem’s long-term capacity to support both itself and the human communities that depend on it (e.g. for fishing or clean water), since eutrophication can deplete dissolved oxygen as algae die and decompose, harming fish populations and water quality – connecting an ecological disruption directly to the course’s sustainability concept rather than treating it as an isolated scientific fact. [3]

6. A strong answer names a specific or clearly described ecosystem and traces a disturbance (such as the removal of a keystone or top predator species) through more than one trophic level: the direct effect (the immediate prey population’s numbers changing) and at least one indirect, cascading effect (the prey’s own food source being over- or under-consumed as a result, affecting species even further removed from the original disturbance). The answer should explicitly invoke systems thinking – explaining that because the ecosystem is a system of interconnected storages and transfers, a disturbance to one part propagates through connected parts via feedback, rather than staying confined to the directly affected species – and should explain why describing only the direct effect would give an incomplete, non-systems-based account of what actually happens ecologically. [8] (Marks for a clear, specific ecosystem scenario; both a direct and a genuinely traced indirect/cascading effect; and explicit systems-thinking framing.)

7. A strong HL answer explains that ecological efficiency (the proportion of energy transferred from one trophic level to the next) varies between ecosystems depending on factors such as the metabolic efficiency of the organisms involved, environmental conditions, and the types of primary producers and consumers present; an ecosystem with higher ecological efficiency loses proportionally less energy at each transfer, meaning enough energy remains to support additional trophic levels compared with a less efficient ecosystem, where energy is exhausted after fewer transfers. To investigate this quantitatively, a student could measure biomass or energy content at each trophic level in both grasslands (e.g. via sampling and drying biomass to estimate energy content, or using published conversion values) and calculate the percentage of energy transferred between consecutive levels in each ecosystem, then compare the resulting ecological efficiency values directly between the two grasslands to test whether the more efficient one does indeed support more trophic levels, as the concept predicts. [8] (Marks for correctly explaining ecological efficiency’s role in limiting trophic levels, and a genuinely quantitative, HL-appropriate investigation method.)

Why this set keeps returning to Topic 1’s vocabulary

Every extended-response question in this set requires explicit use of Topic 1’s systems vocabulary (system, feedback, sustainability) applied to Topic 2’s ecological content, because the revision notes identify this connection – not ecological facts recalled in isolation – as what the specification actually expects. Question 6 in particular is written to penalise an answer that describes only the direct effect of a disturbance, mirroring the exam trap the revision notes flag as the most common way students under-perform on this topic’s extended-response questions.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief – Sciences: Environmental Systems and Societies, first assessment 2026 – the same source cited by the Topic 2 Ecology revision notes and the full syllabus guide.