IB Biology · Theme D Continuity and change · Ecosystems
D4.2 Stability and change
Ecosystems can persist for millions of years if energy flows, nutrients cycle and species stay diverse. Deforestation, over-harvesting, agrochemicals, eutrophication, biomagnification and plastics all threaten that stability. Rewilding restores natural processes; at HL, succession explains how communities change and settle.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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D4.2.1 Ecosystems can stay the same kind of place for millions of years
Ecosystem stability means the overall structure and community type persist over long periods.
Populations fluctuate and individuals are replaced; stability belongs to the whole.
Evidence comes from dated sediment cores, fossil pollen, fossil wood and fossil animals.
Amazon rainforest has persisted tens of millions of years; the Namib Desert about 55 million.
Students often think stable means nothing changes. In fact populations rise and fall; the type of community persists.
Students often think only lush ecosystems can be stable. In fact the Namib has been desert for tens of millions of years.
D4.2.2 What an ecosystem needs to stay stable
A continuous supply of energy, almost always sunlight captured by producers; energy cannot be recycled.
Recycling of nutrients by decomposers, because the store of elements is finite.
Genetic diversity, so some individuals survive a new disease, pest or change.
Climatic variables staying within the tolerance levels of the species present.
Students often think energy is recycled through decomposers. In fact only nutrients cycle; energy must keep coming in.
Students often think a large population is safe without diversity. In fact identical individuals can all fall to the same threat.
D4.2.3 The Amazon: a possible tipping point
A tipping point is a threshold where a small change causes large, rapid, often irreversible change.
Transpiration from a huge forest area makes water vapour, cooling air, driving flows and giving rain.
Below some minimum forest area, this moisture recycling may fail and savanna may replace forest.
The minimum forest area needed to keep this going is uncertain; estimates differ.
Percentage change = (new − original) ÷ original × 100; a loss gives a negative value, such as −17%.
Students often think Amazon rain comes only from the ocean. In fact much of it is water the forest itself transpired.
Students often divide by the new value, or report the area remaining. In fact divide the change by the original area.
D4.2.4 Mesocosms as models of stability
A mesocosm is a small enclosed ecosystem with producers, consumers, decomposers and abiotic parts.
A sealed glass vessel stops matter entering or leaving, but light enters and heat leaves.
Oxygen and carbon dioxide are recycled between photosynthesis and respiration inside.
Aquatic or microbial mesocosms work best; care must follow IB experimental guidelines.
Students often think a sealed vessel is closed to energy too. In fact light passes through glass; only matter is sealed in.
Students often think a mesocosm shows exactly what a real ecosystem will do. In fact it is a simplified model; apply its results with caution.
D4.2.5 Keystone species hold the community together
A keystone species has an effect on community structure far larger than its abundance suggests.
Examples: sea otters in kelp forests, the starfish Pisaster, wolves in Yellowstone, beavers.
Removing one can cascade through the food web and cause ecosystem collapse.
Students often think the keystone species is the most abundant. In fact many are rare or small in biomass.
Students often think removing a predator affects only its prey. In fact the changes cascade through the whole community.
D4.2.6 Is the harvest sustainable?
A renewable resource is replaced by growth and reproduction, yet can still be depleted.
Harvesting is sustainable only if the harvest rate is below the net replacement rate.
For Atlantic cod, rising effort for the same catch and younger fish signal over-harvesting.
For the Brazil nut tree, seedlings and young trees show enough seeds are left.
Replacement is net: growth and recruitment minus natural mortality, and it falls when breeding adults are removed.
Students often think renewable means it cannot run out. In fact harvest faster than replacement and the stock collapses.
Students often judge by catch alone. In fact a steady catch can hide a falling stock; check effort and age structure.
D4.2.7 What makes farming unsustainable
Soil erosion: soil forms so slowly that lost soil is not replaced.
Leaching washes nitrate and phosphate down through soil into water, depleting and polluting.
Fertilisers and other inputs must be supplied, and their manufacture carries a large carbon footprint.
Agrochemical pollution: pesticides harm non-target organisms and drift or leach off the farm.
The carbon footprint also includes machinery fuel, methane from livestock, nitrous oxide from soils and land clearance.
Students often think more fertiliser is always more sustainable. In fact excess leaches away, causing eutrophication, and costs carbon to make.
Students often think a pesticide stays on its field. In fact residues reach pollinators, amphibians and streams.
D4.2.8 Eutrophication and biochemical oxygen demand
Eutrophication is nutrient enrichment of water, mainly nitrate and phosphate leached from farmland.
An algal bloom grows, dies, and is decomposed by aerobic bacteria.
Their respiration removes dissolved oxygen, so fish and other aerobes die.
BOD is the oxygen microbes consume decomposing the organic matter; eutrophication raises it.
Students often think fertiliser poisons fish directly. In fact fish die of oxygen shortage after the bloom decomposes.
Students often think high BOD means oxygen-rich water. In fact it means oxygen is being removed fast.
D4.2.9 Biomagnification: DDT and mercury
Biomagnification is a persistent toxin's concentration rising at each trophic level.
A consumer keeps the toxin from every prey item in a much smaller body mass.
DDT, fat-soluble and persistent, thinned eggshells in birds of prey; many countries banned it.
Mercury becomes methylmercury in water and concentrates in tuna; at Minamata it damaged nervous systems.
Students often think top consumers just live longer. In fact accumulation in one individual is bioaccumulation; magnification is between levels.
Students often think predators have most toxin because they eat most. In fact it is concentration: many prey retained in one small body.
D4.2.10 Plastics persist, and public understanding drove action
Macroplastics such as bags and nets entangle animals and block guts.
Microplastics, under about 5 mm, are eaten by filter feeders and pass along food chains.
Plastics are non-biodegradable: decomposers lack enzymes for these polymers, so pieces shrink but persist.
Clear communication of research, through popular media, changed global perception and drove measures.
Students often think plastics biodegrade slowly. In fact they only fragment; the material stays.
Students often think plastic bans were emotion without science. In fact they answered clearly communicated research findings.
D4.2.11 Rewilding: let natural processes do the work
Rewilding restores natural processes so an ecosystem maintains itself.
Methods: reintroduce apex predators and other keystone species; re-establish habitat connectivity over large areas.
Minimise human impact, including ecological management such as removing livestock.
At Hinewai Reserve, New Zealand, farmland has reverted to native forest since 1987, barely touched.
Gorse was left as a nurse canopy: native seedlings grew through it and shaded it out.
Students often think rewilding is replanting and managing. In fact it is stepping back so regeneration happens naturally.
Students often think invasive gorse must be cleared first. In fact at Hinewai it sheltered the native seedlings that replaced it.
D4.2.12 Succession and what triggers it HL
Ecological succession is one community replacing another over time towards a climax.
Primary succession starts on bare rock or sand with no soil.
Secondary succession starts where a community was removed but soil remains, as after fire.
Students often think only abiotic events cause succession. In fact organisms altering soil, light and nutrients drive it too.
Students often think pioneers prepare the ground on purpose. In fact their side effects happen to favour species that then outcompete them.
D4.2.13 What changes as primary succession proceeds HL
The pioneer community of lichens, mosses and hardy plants begins to form soil.
Plant size increases, and so does primary production.
Species diversity rises and food webs become more complex.
Nutrient cycling increases as soil depth and organic content build.
Students often think the pioneer stage is the most diverse. In fact diversity is greatest in later stages.
Students often think production falls as slow trees take over. In fact leaf area and soil increase, so production rises.
D4.2.14 Some ecosystems cycle instead of settling HL
In cyclical succession the community repeats a cycle of phases, not a single climax.
Heather moorland: heather passes through pioneer, building, mature and degenerate phases.
Dying heather leaves gaps colonised by lichens and mosses, then heather seedlings return.
Students often think succession must end in one permanent climax. In fact heather moorland cycles through phases repeatedly.
Students often think a cycling community is unstable. In fact the cycle itself is the stable state.
D4.2.15 Climax communities and how humans arrest succession HL
A climax community is the stable end point for a given climate and soil.
Arrested succession is a human influence holding the community at an earlier stage.
Grazing by livestock removes tree seedlings and keeps grassland; drainage of wetlands does likewise.
Remove the influence and succession resumes towards the climax.
Students often think long-standing grazed grassland is the natural climax. In fact grazing holds it there; woodland would follow.
Students often think an arrested succession can never recover. In fact stop the grazing or re-wet the land and it resumes.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 What evidence indicates that an ecosystem has persisted, and so shown stability, for millions of years?
Answer and reasoning
Fossil pollen from dated layers of sediment shows the same type of plant community persisting through geological time. — Dated sediment cores containing fossil pollen, leaves and animals record the community present at each time. Where the same type of community appears throughout, the ecosystem has shown continuity, in some cases for millions of years.
The oldest individual trees now growing in the ecosystem have been dated at several thousand years old. — A student who takes the age of living individuals as the age of the ecosystem picks this. A few thousand years is far short of millions, and no individual lives that long; the evidence comes from fossils in dated sediments.
Census data show that the population size of every species has stayed exactly constant from year to year. — A student who thinks stability means no change picks this. Populations in a stable ecosystem fluctuate; it is the persistence of the same type of community that shows stability.
The ecosystem has a high rate of primary production and supports a very large number of species. — A student who equates stability with productivity and richness picks this. Stability is persistence over time, and sparsely populated deserts have persisted for as long as any rainforest.
2 Which set of conditions is required for an ecosystem to remain stable?
Answer and reasoning
Recycling of both energy and nutrients, genetic diversity and climatic variables staying within tolerance levels. — A student who thinks energy cycles like carbon or nitrogen picks this. Energy flows through an ecosystem and is lost as heat at each trophic level, so it must be supplied continuously and cannot be recycled.
A continuing supply of energy, recycling of nutrients, large population sizes and climatic variables staying within tolerance levels. — A student who thinks population size can stand in for genetic diversity picks this. A large but genetically uniform population can be destroyed by a single disease; it is diversity of alleles that gives resilience.
A continuing supply of energy, recycling of nutrients, genetic diversity and climatic variables staying within tolerance levels. — These are the four requirements the guide lists. Energy must be supplied continuously because it is lost as heat; nutrients must be recycled because their supply is finite; genetic diversity allows populations to survive change; and climate must stay within the tolerance of the species present.
A continuing supply of energy, recycling of nutrients, genetic diversity and climatic variables that are held perfectly constant. — A student who reads 'stable' as 'constant' picks this. Climatic variables fluctuate in every ecosystem; the requirement is that they remain within the tolerance levels of the organisms.
3 Why is a sealed glass vessel preferable to an open tank for a mesocosm used to investigate ecosystem stability?
Answer and reasoning
It forms a completely closed system, so neither matter nor energy can be exchanged with the surroundings. — A student who thinks sealing shuts out energy as well as matter picks this. Glass transmits light, so energy enters continuously; only matter is prevented from entering or leaving.
The lid can be removed briefly each day to replace the oxygen that the organisms inside have used up. — A student who thinks oxygen must be supplied from outside picks this. In a balanced mesocosm oxygen released by photosynthesis is recycled through respiration, and opening the vessel would defeat the purpose of sealing it.
It reproduces the natural ecosystem exactly, so any result obtained can be applied directly to the real ecosystem. — A student who thinks a model is an exact copy picks this. A mesocosm is a simplified model; sealing it does not make it identical to a natural ecosystem, and its results must be interpreted with caution.
Matter cannot enter or leave the vessel, but light can still enter and heat can still leave. — Sealing makes the mesocosm closed to matter, so nutrients must be recycled within it as in a stable ecosystem, while the glass allows energy transfer: light enters to drive photosynthesis and heat leaves.
4 Sea otters make up a tiny fraction of the biomass of a kelp forest. Where otters were hunted to extinction, sea urchins multiplied and grazed the kelp down to bare rock, and the fish, seals and seabirds that depended on the kelp disappeared. What does this show about sea otters?
Answer and reasoning
They cannot be a keystone species, because a keystone species is one contributing most of the community's biomass. — A student who thinks keystone means dominant or most abundant picks this. The defining feature is disproportionate impact, and sea otters are the classic example of a keystone species with small biomass.
They are a keystone species, because their effect on community structure is far greater than their abundance suggests. — A keystone species has a disproportionate impact on community structure relative to its abundance. Otters control urchin numbers; without them the urchins destroy the kelp and the whole community collapses.
Their removal should only have affected the urchins they ate, so the loss of fish and seabirds must have had a different cause. — A student who traces effects one step along a food chain picks this. The urchin increase destroyed the kelp, and the loss of kelp removed the habitat of fish, seals and seabirds: a cascade through the food web.
The kelp forest was not a stable ecosystem, since a stable ecosystem does not change when a species is removed. — A student who thinks stability means immunity to any change picks this. Stable ecosystems depend on their keystone species; the guide's point is that removing one risks ecosystem collapse.
5 Which practice increases the sustainability of agriculture on a farm?
Answer and reasoning
Applying extra inorganic fertilizer every year so that all the nutrients lost by leaching are replaced. — A student who thinks more fertilizer means more sustainability picks this. Fertilizer beyond what the crop takes up is itself leached, pollutes water and carries a large carbon footprint from its manufacture.
Growing cover crops between harvests so that the soil is protected from erosion and less nitrate is leached away. — Soil erosion and leaching of nutrients are two of the factors affecting the sustainability of agriculture. Plant cover holds the soil, takes up nitrate that would otherwise leach, and reduces the need for fertilizer inputs.
Ploughing sloping fields deeply each year, because soil is replaced by weathering as fast as it is lost. — A student who thinks soil regenerates quickly picks this. Soil forms over centuries, and bare ploughed slopes lose it rapidly to erosion, which permanently reduces the land's capacity to grow crops.
Replacing manure with manufactured nitrogen fertilizer, since the fertilizer adds nothing to the farm's carbon footprint. — A student who thinks carbon footprint means only fuel burned on the farm picks this. Manufacturing nitrogen fertilizer uses large amounts of energy from natural gas, so it adds substantially to the carbon footprint.
6 Nitrate and phosphate leached from farmland enter a lake. Which sequence of events leads to the death of fish in the lake?
Answer and reasoning
Fertilizer enters the lake → fish absorb nitrate across their gills → it poisons them → fish die — A student who thinks the fertilizer is directly toxic picks this. At the concentrations involved nitrate does not poison fish; the deaths are caused by lack of oxygen after the bloom decomposes.
Algal bloom → algae die → BOD falls as the algae decay → less oxygen dissolves in the water → fish suffocate — A student who confuses BOD with dissolved oxygen picks this. Decay of the algae raises BOD, because bacteria demand oxygen to decompose the organic matter; it is the dissolved oxygen concentration that falls.
Algal bloom → algae die → aerobic bacteria decompose them → dissolved oxygen falls → fish suffocate — Nutrients cause an algal bloom; when the algae die, saprotrophic bacteria decompose them using aerobic respiration, which raises biochemical oxygen demand and depletes dissolved oxygen, so fish and other aerobic animals die.
Algal bloom → light is blocked → submerged plants and algae die → fish have nothing to eat → fish starve — A student who reasons only through food chains picks this. Fish in a eutrophic lake die of oxygen depletion, usually suddenly and in large numbers, not of starvation.
7 Which statement about plastic pollution of the oceans is correct?
Answer and reasoning
Marine bacteria biodegrade plastics, but so slowly that the material persists for a few decades before disappearing. — A student who thinks plastics decompose slowly picks this. Fragmentation is a physical process; the polymer is not decomposed, which is why plastic persists for centuries and is found in the tissues of marine organisms.
Microplastics are too small to harm marine animals, so only macroplastics have effects on marine life. — A student who equates harm with entanglement or blockage picks this. Microplastics are ingested by filter feeders and zooplankton, damage the gut, carry adsorbed pollutants and pass along food chains.
Macroplastics fragment into microplastics but are not broken down by decomposers, so the material persists. — Plastics are non-biodegradable, because decomposers lack enzymes that can break down these synthetic polymers. Sunlight and waves fragment items into microplastics, but the plastic remains in the environment and enters food chains.
Microplastics enter the oceans only as manufactured particles such as microbeads, not from larger plastic items. — A student who associates microplastics only with microbeads picks this. Most microplastic comes from the fragmentation of macroplastics such as bags, bottles and nets.
8 Which statement describes the approach used to restore native forest at Hinewai Reserve in New Zealand?
Answer and reasoning
Grazing livestock were removed and introduced gorse was left as a nurse canopy, under which native trees regenerated with minimal interference. — Hinewai's approach is minimal interference: with livestock gone and introduced browsers controlled, native seedlings grew up through the shade-intolerant gorse, overtopped it and formed native forest without planting.
Native tree seedlings were raised in nurseries and planted across the whole reserve in place of the former pasture and scrub. — A student who thinks rewilding means planting picks this. At Hinewai the forest regenerated naturally from seed once the human impacts that suppressed it were removed.
The introduced gorse was cleared with herbicide and machinery before native forest was allowed to return to the area. — A student who thinks invasive plants must always be removed picks this. The gorse was deliberately left; it sheltered native seedlings and died out once they shaded it.
Native forest was allowed to recover in scattered small patches across the peninsula, since connecting habitats over one large area is not needed. — A student who thinks total area is all that matters picks this. Hinewai is managed as one continuous, expanding area of more than 1000 hectares; re-establishing connectivity over a large area is one of the methods of rewilding.
9 Surveys were made on land exposed by a retreating glacier 10, 50, 100 and 200 years earlier. Mean plant height was 0.1 m, 3 m, 15 m and 30 m; the number of plant species was 6, 18, 25 and 31; and net primary production (kg m⁻² yr⁻¹) was 0.05, 0.4, 0.9 and 1.2. Soil depth and organic content increased throughout. Which conclusion do the data support? HL
Answer and reasoning
The rise in species number is misleading, because the hardy pioneer community at 10 years was the most diverse stage. — A student who thinks pioneers are the most diverse picks this. The data show only 6 species at 10 years and 31 at 200 years; diversity increases as habitats and food webs become more complex.
Net primary production should have fallen after 100 years because mature trees grow slowly, so the 200-year value must be wrong. — A student who confuses the growth rate of individual trees with production per square metre picks this. A forest has far more photosynthetic tissue than a pioneer community, so its production is higher.
The site surveyed at 10 years must already have had a proper soil, since plants cannot colonise ground until soil is present. — A student who thinks soil must pre-exist picks this. The survey reports that soil depth and organic content increased through the sequence, showing that soil formed during succession, beginning with pioneers on bare ground.
Plant size, species diversity and primary production all increased as the primary succession proceeded. — All three measures rise steadily across the sequence, as the guide describes for primary succession: larger plants, more species and greater primary production as soil builds up and nutrient cycling increases.
10 On a heather moor, heather plants pass through pioneer, building, mature and degenerate phases. In the degenerate phase the central branches die, opening a gap that is colonised by lichens and mosses before heather seedlings re-establish there. What does this show? HL
Answer and reasoning
The community follows a cycle of phases driven by the heather's own ageing, rather than settling into one unchanging climax. — This is cyclical succession. The ecosystem persists, but its communities cycle: ageing heather opens gaps, other species occupy them, and heather re-establishes, so there is no single fixed climax community.
The moor has not yet reached its climax, which will be a permanent heather community with no gaps in it. — A student who expects every succession to end in a single permanent climax picks this. The cycle repeats indefinitely; the moor's stable state is the cycle itself, not a gap-free end point.
The repeated dying back of the heather shows that the ecosystem is unstable and is degrading towards collapse. — A student who reads change as instability picks this. Heather moorland has persisted through many such cycles; the degenerate phase is a normal part of the pattern, not a sign of collapse.
Each change of phase must be triggered by an external abiotic disturbance such as fire, frost or a drought. — A student who thinks only abiotic events cause succession picks this. The cycle is driven by a biotic factor, the ageing and die-back of the heather plants themselves.
Read the ones marked not yet in Learn, then Verify.
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18 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 Sediment cores from a desert basin contain pollen in every layer. The layers have been dated to span the last 30 million years, and the deepest layers contain pollen of the same drought-adapted plant families as the surface layers. What is the best conclusion from this evidence?
Answer and reasoning
The region cannot have supported a stable ecosystem for so long, because a desert is too harsh to persist for millions of years. — A student who thinks only productive ecosystems can be stable picks this. The evidence shows precisely that the desert did persist; harshness of conditions does not prevent continuity.
A desert ecosystem has persisted in this region for tens of millions of years, so it has shown long-term stability. — The same type of community, identified from its pollen, is present throughout 30 million years of dated sediment. That is direct evidence of an ecosystem persisting over a very long period, which is what stability means.
The plants whose pollen is in the deepest layers must still be alive today for the ecosystem to count as stable. — A student who thinks persistence must be shown by living individuals picks this. No plant lives for 30 million years; continuity of the community type, not survival of individuals, is the evidence of stability.
The ecosystem was not stable, because the numbers of each species must have varied over 30 million years. — A student who equates stability with constant populations picks this. Population sizes fluctuate in every ecosystem; the persistence of the same kind of community is what shows stability.
2 A plantation forest consists of a single tree species, all grown from cuttings of one parent tree, so the trees are genetically almost identical. A fungal disease that kills this species arrives. Which statement about the stability of this forest is correct?
Answer and reasoning
It is as stable as a natural forest, because stability depends on the number of trees, not on their genetic variation. — A student who confuses population size with genetic diversity picks this. However many trees there are, if all are susceptible the disease can destroy them all.
It will remain stable, because the energy in the trees killed by the fungus is recycled to support new tree growth. — A student who thinks energy is recycled picks this. Energy in dead trees is released as heat by decomposers; only nutrients are recycled, and recycling nutrients cannot replace a population with no resistance.
It is stable because a forest is a highly productive ecosystem with a large rate of photosynthesis per hectare. — A student who equates stability with productivity picks this. High production does not protect a genetically uniform population from a disease to which none of its members is resistant.
It is vulnerable to collapse because few or no trees carry alleles that give resistance to the disease. — Genetic diversity is a requirement for stability. In a genetically uniform population, no individuals are likely to carry resistance alleles, so the disease can kill almost every tree and the ecosystem that depends on them collapses.
3 In 1970 the area of a rainforest was 4 000 000 km². By 2020 the area was 3 320 000 km². What is the percentage change in the area of forest?
Answer and reasoning
−17.0% — Percentage change = (new − original) ÷ original × 100 = (3 320 000 − 4 000 000) ÷ 4 000 000 × 100 = −680 000 ÷ 4 000 000 × 100 = −17.0%. The negative sign shows a decrease from the original area of forest.
−20.5% — A student who divides the change by the final area picks this: 680 000 ÷ 3 320 000 × 100 = 20.5%. Percentage change is always expressed relative to the original value, 4 000 000 km².
−83.0% — A student who calculates the percentage of the original forest that remains, 3 320 000 ÷ 4 000 000 × 100 = 83%, and reports it as the size of the decrease picks this. The question asks for the change, which is 83% − 100% = −17%.
−1.70% — A student who misplaces the decimal point when converting the fraction −0.17 to a percentage picks this. Multiplying by 100 gives −17%, not −1.7%.
4 Which statement correctly explains the link between the area of the Amazon rainforest and the rainfall it receives?
Answer and reasoning
Rain over the forest comes from water evaporated from the Atlantic Ocean, so the area of forest has little effect on the rainfall. — A student who pictures the water cycle as sea-to-land only picks this. Ocean evaporation supplies some rain, but transpiration from the forest itself is a major source of the water vapour that falls as rain over the Amazon.
Transpiration from a large area of trees adds water vapour to the air, which cools, drives air flows and returns as rain over the forest. — The forest generates a large proportion of its own rainfall. Water vapour from transpiration produces cooling, air flows and rainfall, so a large area of forest is needed to keep these processes going.
Every hectare of forest contributes a fixed share of the rainfall, so rainfall falls in exact proportion to the area of forest that is cleared. — A student who expects a linear response picks this. Moisture recycling may continue until a threshold area is lost and then fail rapidly; this possible tipping point is why the relationship is not simply proportional.
Scientists have established that rainfall stays the same until exactly 20% of the forest has been cleared, and then declines sharply. — A student who treats an estimate as an exact known value picks this. Some researchers have suggested a threshold in the region of 20–25% deforestation, but the minimum area of forest needed is uncertain.
5 Different research groups have estimated that the Amazon rainforest could reach a tipping point when 20%, 25% or 40% of its original area has been deforested. About 17% has been cleared so far. What does this range of estimates indicate?
Answer and reasoning
Only one of the estimates can be correct, so nothing can be concluded until the other two have been shown to be wrong. — A student who thinks a threshold is a precisely known value picks this. A range of estimates is informative in itself: it shows that the threshold is uncertain and that the lowest estimate is uncomfortably close to current deforestation.
Rainfall must already have fallen by 17%, in proportion to the area cleared, so there is no threshold left to estimate. — A student who assumes a linear response picks this. The concern about a tipping point is precisely that rainfall may be largely maintained until a threshold is crossed and then fall abruptly.
The minimum area of forest needed to maintain moisture recycling is uncertain, so the tipping point could be close. — The estimates differ because models and assumptions differ, which is exactly the uncertainty the guide describes. Since the lowest estimate is only a few percentage points above the area already cleared, the tipping point may be near, which argues for caution.
Rainfall over the forest comes from the ocean, so deforestation cannot affect it and no tipping point can exist. — A student who thinks the forest does not generate its own rain picks this. Transpiration from the forest supplies much of the water vapour that returns as rain, so loss of forest can reduce rainfall.
6 A class plans to use mesocosms to investigate the effect of light intensity on ecosystem stability. Which design is most likely to succeed and to follow IB experimental guidelines?
Answer and reasoning
Sealed glass jars containing pond water, algae, small aquatic invertebrates and sediment, placed at different distances from a lamp for several weeks. — Aquatic mesocosms are more likely to be successful than terrestrial ones, sealing prevents exchange of matter while light still enters, and small invertebrates in pond water can be kept without causing suffering, in line with IB guidelines.
Open terraria containing soil, plants and a small lizard, placed at different distances from a lamp, since a terrestrial model is more realistic. — A student who equates realism with size and animals picks this. Terrestrial mesocosms are less likely to succeed, and exposing a vertebrate to treatments that may harm it does not follow IB experimental guidelines.
Open tanks of pond water and algae with an air pump running in each, so that the organisms do not run out of oxygen during the investigation. — A student who thinks oxygen must be supplied from outside picks this. Photosynthesis by the algae supplies the oxygen; an open, aerated tank allows matter to enter and leave, so it no longer tests whether the system can sustain itself.
A single large sealed aquarium containing fish, because a bigger and more realistic model gives results that apply directly to real lakes. — A student who thinks a model reproduces reality exactly picks this. One vessel cannot test the effect of a variable, and no mesocosm, however large, gives results that transfer directly to a natural lake.
7 A stock of Atlantic cod has a biomass of 200 000 tonnes. Each year, growth of the fish and recruitment of young fish add 15% to the biomass, while natural mortality removes 5% of it. What is the annual rate of replacement of the stock, below which the catch must stay for harvesting to be sustainable? The rate of replacement is the net change in biomass from these processes.
Answer and reasoning
10 000 tonnes — A student who thinks the harvest can take the fish that would have died naturally picks the 5% mortality figure. Natural deaths are a loss from the stock, not a surplus available for catching.
30 000 tonnes — A student who counts only the additions picks 15% of 200 000 tonnes. Natural mortality removes 5% of the biomass each year and must be subtracted to give the net rate of replacement.
20 000 tonnes — Net replacement = additions − natural losses = 15% − 5% = 10% of 200 000 tonnes = 20 000 tonnes per year. A catch below this leaves the biomass unchanged or rising, so harvesting is sustainable.
40 000 tonnes — A student who adds natural mortality to the growth, as though fish that would die anyway could be harvested as well, picks 20% of 200 000 tonnes. Natural mortality reduces the stock; it does not add to what can be taken.
8 Records for a cod fishery show that the annual catch stayed close to 250 000 tonnes for 20 years. Over the same period the number of fishing days needed to land that catch doubled, and the mean age of the cod caught fell from 8 years to 4 years. What do these data indicate?
Answer and reasoning
Harvesting is sustainable, because the catch has remained constant for two decades and so the stock must be replacing itself. — A student who judges sustainability from the catch alone picks this. The catch has been maintained only by doubling effort and by taking younger fish, which are signs of a declining stock.
Harvesting is sustainable, because cod are a renewable resource that reproduce each year and so cannot be depleted by fishing. — A student who thinks renewable means inexhaustible picks this. A renewable resource is depleted whenever the rate of harvesting exceeds the rate of replacement, and the effort and age data show this is happening.
The fall in the mean age of the catch cannot affect replacement, because the reproductive rate of any species is a fixed value. — A student who thinks replacement rate is fixed picks this. Replacement depends on the number of mature fish left to breed; removing fish at younger ages reduces the breeding stock and so reduces replacement.
The stock is being harvested unsustainably: more effort is needed for the same catch and few fish now survive to old age. — A constant catch obtained with doubled effort means catch per unit effort has halved, so the stock has declined. A falling mean age shows that fish are being removed before they grow old, reducing the breeding stock. Both indicate harvesting above the replacement rate.
9 Brazil nuts are collected from wild trees in the Amazon rainforest. Surveys found that in areas where nuts have been harvested intensively for decades there are many old trees but almost no young trees or seedlings, whereas in lightly harvested areas trees of all ages are present. What does this indicate about the intensive harvesting?
Answer and reasoning
It is unsustainable, because nuts are being removed faster than new trees are being recruited to replace the old ones as they die. — Sustainability depends on the rate of harvesting being lower than the rate of replacement. Removing most of the nuts leaves too few seeds to produce seedlings, so when the old trees die there will be no replacements and the harvest will collapse.
It is sustainable, because the large number of mature trees means that the yield of nuts can be maintained for many years. — A student who judges sustainability from the current yield picks this. The yield will continue only until the old trees die; the absence of young trees shows that replacement has already failed.
It is sustainable, because the trees are a renewable resource that will continue to produce a crop of nuts every year. — A student who thinks a renewable resource cannot be depleted picks this. The trees renew the population only through seedlings, and intensive harvesting has prevented that renewal.
It has no effect on the population, because a tree species produces a fixed number of seedlings however many nuts are taken. — A student who thinks replacement rate is fixed picks this. Seedlings come from nuts that are left uncollected; removing nearly all the nuts removes nearly all the potential seedlings.
10 Water was sampled at three points on a river: A, upstream of a farm; B, 2 km downstream of the farm's drainage outlet; and C, 15 km downstream. Nitrate concentration (mg dm⁻³) was 1 at A, 12 at B and 3 at C. BOD (mg dm⁻³) was 2 at A, 14 at B and 4 at C. The number of fish species found was 8 at A, 1 at B and 6 at C. Which explanation fits these data?
Answer and reasoning
Algae stimulated by the nitrate at B released extra oxygen by photosynthesis, so the fish must have been lost for another reason. — A student who thinks an algal bloom enriches the water with oxygen picks this. Whatever the algae produced while alive, their decomposition consumes far more oxygen, as the high BOD at B shows.
The BOD of 14 at B shows that the water there holds the most oxygen, so lack of oxygen cannot explain the loss of fish there. — A student who reads BOD as the amount of oxygen in the water picks this. BOD is the oxygen that decomposers will consume; a high value means oxygen is being removed rapidly, so B is the site most likely to be short of oxygen.
The nitrate at 12 mg dm⁻³ is directly toxic to fish, which by itself explains why only one fish species was found at B. — A student who thinks leached fertilizer poisons fish picks this. Nitrate at this level is not directly lethal to fish; it drives eutrophication, and the fish are lost because decomposition of the bloom removes oxygen.
Nitrate at B caused an algal bloom whose decomposition raised BOD and removed oxygen, so fish were lost. — Leached nitrate is highest just below the outlet, and the high BOD there shows a large amount of decomposing organic matter, the dead algae. Oxygen depletion explains the loss of fish species at B, and the partial recovery at C as nutrients are diluted and used up.
11 In a study of an estuary, DDT concentrations were measured in parts per million (ppm): water 0.00005, plankton 0.04, small fish 0.5, larger predatory fish 2, fish-eating birds 25. Which statement explains this pattern?
Answer and reasoning
DDT is fat-soluble and not excreted, so each consumer retains the DDT from all its prey and the concentration rises at each trophic level. — This is biomagnification. The toxin taken in with every prey item is retained in fatty tissue, so a consumer's tissues hold a higher concentration than its food; over four trophic levels the concentration rose about half a million times above that in the water.
Birds live longer than fish, so the pattern is due only to each bird having had more years in which to accumulate DDT. — A student who confuses biomagnification with accumulation over a lifetime picks this. Longevity contributes, but the step-up occurs at every trophic level, including from plankton to short-lived small fish.
Each organism absorbed DDT directly from the water, so the birds must have been exposed to more contaminated water. — A student who thinks pollutants are taken up only from the surroundings picks this. All these organisms live in or beside the same water; the difference comes from DDT concentrated in food.
Birds at the top of the food chain eat the greatest mass of food, so they take in the largest total amount of DDT. — A student who confuses total intake with concentration picks this. Eating a large mass of prey is not itself the explanation; the concentration is high because the DDT from all the prey eaten is retained in a much smaller mass of bird tissue, and this step-up occurs at every trophic level.
12 Why does mercury released from industry reach high concentrations in large predatory fish such as tuna?
Answer and reasoning
Large fish swallow the greatest volume of water while feeding, so they take in more mercury than smaller fish do. — A student who thinks the top consumer simply takes in more picks this. The concentration in tuna comes from mercury already concentrated in the prey fish it eats, not from the water it swallows.
Mercury taken in with prey is retained in tissues rather than excreted, so its concentration rises at each trophic level up to the tuna. — This is biomagnification. Mercury in water and sediments is converted by bacteria to methylmercury, which is taken up by small organisms, retained rather than excreted and passed on at increasing concentration along the food chain, so long-lived top predators such as tuna carry the highest concentrations.
Tuna swim through polluted water for years and absorb mercury through their gills rather than from their food. — A student who thinks uptake is direct from the environment picks this. Uptake across the gills is minor; the high concentration comes from eating prey that already contain methylmercury.
Tuna live for many years, so they accumulate mercury over time; smaller fish would reach the same level if they lived as long. — A student who equates biomagnification with lifetime accumulation picks this. Age contributes, but a tuna's food is itself already more concentrated than the water, and each trophic level adds a further step.
13 A television series showed footage of albatross chicks that had died with their stomachs full of plastic fragments. Within two years, many countries had banned microbeads and single-use plastic bags. Which statement best describes the role of scientists in this outcome?
Answer and reasoning
Publication of the findings in scientific journals caused the policy change, and the television coverage added nothing to it. — A student who thinks published facts act by themselves picks this. Journal papers reach few citizens; it was the clear communication of the findings to a global audience that changed perception and behaviour.
The bans were an emotional response to distressing images and so cannot be counted as an outcome of scientific research. — A student who sets science against the media picks this. The images illustrated documented research on ingestion of plastic by seabirds; communicating that research clearly is a legitimate scientific influence on citizens.
The bans on microbeads were unjustified, because particles that small cannot cause harm to marine animals of any size. — A student who thinks only large items harm marine life picks this. Microplastics are ingested by animals from zooplankton to fish, reducing feeding and growth and carrying pollutants into food chains.
Their findings, communicated clearly to the public through the media, changed perceptions and drove policy action. — Scientists can influence the actions of citizens if they provide clear information about their research findings. Popular media coverage of the effects of plastic on marine life changed public perception globally and led to measures against plastic pollution.
14 Which of these is a method used in rewilding to restore natural processes in an ecosystem?
Answer and reasoning
Removing apex predators from the area so that the populations of herbivores and other prey species can increase in size. — A student who thinks predators reduce biodiversity picks this. Rewilding reintroduces apex predators, because as keystone species they restore the natural control of herbivores and allow vegetation and dependent species to recover.
Re-establishing connectivity between areas of habitat over a large region so that animals can move between them. — Re-establishment of connectivity of habitats over large areas is one of the methods the guide lists, alongside reintroduction of apex predators and other keystone species and minimization of human impact.
Replanting the whole area with nursery-grown trees and maintaining it by continuous intensive management. — A student who equates restoration with planting picks this. Rewilding minimizes human impact so that natural regeneration and natural processes maintain the ecosystem.
Protecting several small separate reserves instead of one large area, since total area matters more than connection. — A student who thinks isolated fragments are as good as a connected area picks this. Rewilding re-establishes connectivity over large areas so that populations can move, interbreed and support wide-ranging species such as apex predators.
15 Alder trees colonising ground exposed by a retreating glacier have nitrogen-fixing bacteria in their root nodules. Within decades the soil nitrogen content rises and spruce trees, which cannot grow on the bare ground, replace the alders. Which statement about the cause of this change is correct? HL
Answer and reasoning
It is an abiotic cause, because only changes in the physical environment such as glacial retreat can drive succession. — A student who thinks succession has only abiotic causes picks this. The glacial retreat exposed the ground, but the replacement of alder by spruce was driven by the alders' effect on the soil, a biotic factor.
The alders enriched the soil in order to prepare the site for the spruce trees that follow them in the succession. — A student who thinks succession is purposeful picks this. Nitrogen fixation benefits the alders themselves; soil enrichment is a side effect that happens to favour their competitors.
It is a biotic cause: the alders changed the soil so that other species could establish and then outcompete them. — Succession can be triggered by biotic factors as well as abiotic ones. The alders enrich the soil with nitrogen as a consequence of their symbiosis; the enriched soil allows spruce to establish, and spruce then shade out the alders.
This is secondary succession, because soil had already formed on the site before the spruce trees arrived. — A student who applies the soil test to each stage picks this. The sequence began on bare ground with no soil, so the whole sequence, including the spruce stage, is primary succession.
16 Sheep have grazed a hillside for centuries and it is covered by grassland. When grazing was stopped on a fenced plot, shrubs appeared within ten years and young trees within twenty. What does this show? HL
Answer and reasoning
Grassland is the natural climax community here, and fencing disturbed it and began a new succession. — A student who equates a long-standing community with a climax picks this. The grassland persisted only because grazing removed woody seedlings; the appearance of shrubs and trees once grazing stopped shows the climax lies beyond grassland.
Grazing had arrested succession, holding the hillside at a grassland stage below its climax community. — Under the environmental conditions of the hillside, succession tends towards woodland. Grazing livestock remove tree and shrub seedlings, which is a human influence that prevents the climax from developing; when it stops, succession resumes.
The shrubs and trees must have been planted, because succession cannot resume once it has been arrested by human activity. — A student who thinks arrested succession is permanent picks this. Arrested succession is maintained only while the influence continues; removing the sheep allowed seedlings to survive and succession to resume naturally.
The fence altered the plot's climate, and only such an abiotic change can make a community change. — A student who thinks succession has only abiotic causes picks this. The fence changed nothing about the climate; it removed a biotic influence, grazing, which had been preventing woody plants from establishing.
17 How does drainage of a wetland for agriculture affect ecological succession there? HL
Answer and reasoning
The drained farmland is now the climax community for the new, drier conditions, so no further succession is possible there. — A student who thinks any persistent community is a climax picks this. Farmland is maintained by continued drainage and cultivation, not by the environment alone, so it is an arrested stage, not a climax.
The wetland community cannot re-establish afterwards, because succession stops permanently once the wetland has been drained. — A student who thinks arrested succession is irreversible picks this. When drainage is stopped and the land re-wets, wetland communities return, as wetland restoration projects have shown.
It prevents the community that the wet conditions would naturally lead to from developing, so the land stays as farmland. — Drainage of wetlands is the guide's second example of a human influence that prevents a climax community from developing. The community that the waterlogged conditions would have produced is replaced by farmland that persists only while drainage and cultivation continue.
It starts a new succession that will lead back to the same wetland climax once the crops are harvested, since the climax for an area is fixed. — A student who thinks every area has one fixed climax it must return to picks this. The climax depends on the environmental conditions; while the land is kept drained the waterlogged conditions that produced the wetland community no longer exist, so succession is held at farmland and cannot lead to the wetland climax.
18 A farm sprays insecticide and herbicide several times each season. Downstream, insect and amphibian numbers have fallen and residues of the sprays are found in well water. Which factor affecting the sustainability of agriculture do these observations illustrate?
Answer and reasoning
Supply of inputs, since the sprays are inputs that protect the crop yield and so make the farm more sustainable. — A student who sees inputs only as benefits picks this. Supply of inputs is a sustainability factor because producing and applying them has costs; here the observations show damage caused by the inputs, which is pollution due to agrochemicals.
Pollution due to agrochemicals, since the sprays have harmed non-target organisms and contaminated water beyond the farm. — Insecticides and herbicides are agrochemicals. The fall in downstream insects and amphibians and the residues in well water show that the sprays have harmed non-target organisms and polluted water off the farm, which is the pollution factor the guide lists.
Leaching of nutrients, since sprays act only on pests within the field, so the downstream losses must be due to fertilizer. — A student who thinks pesticides act only on the target pests within the field picks this. No fertilizer is mentioned; the residues in the well water are from the sprays, and insecticides and herbicides kill many non-target species when they drift, run off or leach from the field.
Carbon footprint, since the only effect of agrochemicals beyond the field is from the fuel burned by the machinery that sprays them. — A student who thinks the impact of agrochemicals is only the fuel used to apply them picks this. Carbon footprint concerns greenhouse gas emissions; the observations describe toxic effects on organisms and contamination of water, which is pollution due to agrochemicals.
That was your twenty minutes. Real practice on D4.2 is past-paper questions marked against the mark scheme.
What the exam asks of D4.2
Paper 1A asks you to pick the requirement, pollutant or process a scenario describes, and to calculate a percentage change. Paper 1B gives fishery effort and catch data, BOD readings or succession graphs and asks you to interpret them. Paper 2 uses *outline* and *explain*: name the process, give the mechanism step by step, then the consequence for the ecosystem. HL questions use *describe* for succession trends and *discuss* for climax versus arrested communities; use a named example each time.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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