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

This study guide covers IB Diploma Programme Biology, theme D (Continuity and change), subtopic D4.2 Stability and change, statements D4.2.1 to D4.2.15. It follows the IB Biology guide, first assessment 2025. D4.2.1 to D4.2.11 are for SL and HL. D4.2.12 to D4.2.15 (succession) are additional higher level, so they are HL only. The page applies to examinations from the first assessment in 2025.

Use it with the revision notes and the practice set. Course links: IB DP Biology hub, printable checklist and free diagnostics. Background on energy and trophic levels is in energy flow in ecosystems, and conservation approaches are in evolution, speciation and conservation of biodiversity.

What this unit covers

Statement	You must be able to	Level
D4.2.1	Illustrate stability with ecosystems that have persisted for long periods	SL and HL
D4.2.2	State what stability requires	SL and HL
D4.2.3	Explain the Amazon tipping point; calculate percentage change	SL and HL
D4.2.4	Use mesocosms as models	SL and HL
D4.2.5	Explain the role of keystone species	SL and HL
D4.2.6	Assess sustainability of harvesting	SL and HL
D4.2.7	Evaluate factors affecting agricultural sustainability	SL and HL
D4.2.8	Explain eutrophication and BOD	SL and HL
D4.2.9	Explain biomagnification (DDT, mercury)	SL and HL
D4.2.10	Explain effects of plastic pollution	SL and HL
D4.2.11	Describe rewilding (Hinewai Reserve)	SL and HL
D4.2.12	Succession and its causes	HL only
D4.2.13	Changes during primary succession	HL only
D4.2.14	Cyclical succession	HL only
D4.2.15	Climax communities, and succession arrested by people	HL only

Stability and what it needs (D4.2.1, D4.2.2)

A stable ecosystem keeps its overall character, species and processes over a long time, even though individuals are born and die and conditions fluctuate. Evidence comes from forests and deserts whose type has continued over very long periods, and the guide notes that some ecosystems have persisted for millions of years. Stability is not stasis: populations rise and fall, but around a steady pattern.

Four requirements must be met:

• A supply of energy, normally light for photosynthesis, because energy is lost as heat at each trophic level and cannot be recycled.
• Recycling of nutrients, because matter is finite. Decomposers return elements to the soil or water for producers.
• Genetic diversity, so that populations contain variation and can adapt when conditions change.
• Climatic variables within tolerance levels for the species present, such as temperature and rainfall.

Remove any one and the ecosystem can shift to a different state.

Tipping points: the Amazon (D4.2.3)

A tipping point is a threshold beyond which a change becomes self-reinforcing and hard to reverse. In the Amazon, trees release water vapour by transpiration. Over a large area this vapour cools the air, helps drive air flows and returns as rainfall. Clearing forest reduces transpiration, so rainfall falls, so more trees die, so transpiration falls again. This is positive feedback.

Scientists are uncertain about the minimum area of forest needed to keep these processes running. Say so in an answer: the threshold is possible but its position is not known precisely.

Worked example (percentage change). A region originally held 120 000 km² of rainforest. Satellite data now show 93 000 km².

percentage change = (new − original) ÷ original × 100 = (93 000 − 120 000) ÷ 120 000 × 100 = −27 000 ÷ 120 000 × 100 = −22.5%

The forest has declined by 22.5% from its original area. Always measure against the original area, not the current one. Keep the sign or the word “decrease”.

Models of stability: mesocosms (D4.2.4)

A mesocosm is a small, enclosed model ecosystem. Sealed glass vessels are preferred to open tanks because matter cannot enter or leave, while light energy still passes through the glass. Aquatic or microbial systems are more likely to work than terrestrial ones, for example pond water with algae, small invertebrates and decomposing microbes.

To investigate a variable, set up several identical vessels and change one factor, such as light intensity, keeping the rest constant. Measure outcomes such as dissolved oxygen, population counts or turbidity at regular intervals. Replicates are needed because each mesocosm develops differently. Follow IB experimental guidelines for the care and maintenance of the organisms.

Worked example. Two sealed jars contain the same pond community. Jar B is kept in darkness. Within days producers die in B. The energy supply has been removed, so consumers and decomposers lose their food source and the community collapses, while jar A with light stays stable. This links a variable to a stability requirement.

Limitation: a mesocosm is much simpler than a natural ecosystem, so results cannot be applied directly to it.

Keystone species (D4.2.5)

A keystone species has a disproportionate impact on community structure relative to its abundance. If it is removed, the community can change greatly and may collapse.

Worked example. A predatory fish is removed from a pond. Its prey, a grazing snail, increases from 40 to 62 per m², a 55% rise. The snails graze algae and plants, and plant species fall from 14 to 6, a change of (6 − 14) ÷ 14 × 100 = −57.1%. The predator was controlling the herbivore, so its removal let one species dominate.

Sea otters controlling urchins in kelp forests and apex predators controlling herbivores on land are further examples. Do not confuse a keystone species with the most abundant species.

Sustainable harvesting (D4.2.6)

Harvesting is sustainable when the rate of harvesting is lower than the rate of replacement. The guide asks for one terrestrial plant and one marine fish as examples of renewable resources.

Marine fish. Assess sustainability with stock estimates and catch records. Rising fishing effort for the same catch means the stock is shrinking. Catch per unit effort (CPUE) is useful.

Worked example. A fishery landed 450 t with 900 boat-days of effort, then later 380 t with 1100 boat-days.

• Earlier CPUE = 450 ÷ 900 = 0.50 t per boat-day.
• Later CPUE = 380 ÷ 1100 = 0.345 t per boat-day.
• Change = (0.345 − 0.50) ÷ 0.50 × 100 = −30.9%.

Each boat-day catches almost a third less, which suggests the stock is falling. Other evidence: fish getting smaller and younger, and catches exceeding the estimated annual replacement.

Terrestrial plant. For a wild-harvested species such as a bark or nut tree, compare harvest rate with recruitment of new mature trees, and survey seedlings and age structure. If 4 trees per hectare per year are harvested and 2.5 mature in their place, the population is falling by 1.5 per hectare per year, so the harvest is unsustainable.

Sustainability of agriculture (D4.2.7)

Consider each of these:

• Soil erosion: bare soil is lost to wind and water, reducing fertility and depth.
• Leaching of nutrients: water carries soluble ions below the root zone or into waterways.
• Supply of fertilizers and other inputs: some are finite or energy-intensive to make.
• Pollution from agrochemicals: pesticides and herbicides can harm non-target organisms.
• Carbon footprint: fuel, fertilizer production, machinery and transport release carbon dioxide.

Evaluate by weighing yield now against long-term damage. Practices such as cover crops, crop rotation and reduced tillage lower erosion and inputs.

Eutrophication (D4.2.8)

Eutrophication is nutrient enrichment of water. Nitrogen and phosphate fertilizers leach from fields into rivers, lakes and coastal seas. The sequence:

1. Nutrients increase, so algae and plants grow rapidly (an algal bloom).
2. The bloom blocks light, so submerged plants die.
3. Dead material feeds bacteria and other decomposers, which respire aerobically and use oxygen. Biochemical oxygen demand (BOD) rises.
4. Dissolved oxygen falls, so fish and aerobic invertebrates die. Biodiversity drops.

BOD is the oxygen used by microorganisms breaking down organic matter in a water sample, usually found by the fall in dissolved oxygen over a set time at constant temperature.

Worked example. A sample from a stream beside farmland has dissolved oxygen 9.0 mg dm⁻³ at the start and 2.4 mg dm⁻³ after five days sealed in the dark at 20 °C. BOD = 9.0 − 2.4 = 6.6 mg dm⁻³. A higher value than a clean-water sample indicates more organic matter.

Biomagnification (D4.2.9)

Some toxins are not broken down or excreted, and are either fat-soluble or bound to proteins, so they accumulate in tissues. At each trophic level each consumer takes in many prey from the trophic level beneath, so concentration increases along the food chain. Top consumers receive the highest dose.

Examples: DDT, an insecticide, which thinned the eggshells of some birds of prey; and mercury, which builds up in large predatory fish.

Worked example. Pollutant concentrations in tissue: plankton 0.04 mg kg⁻¹, small fish 0.5, large fish 2.0, fish-eating bird 25. From plankton to bird the concentration rises by 25 ÷ 0.04 = 625 times. From large fish to bird it rises 25 ÷ 2.0 = 12.5 times.

Plastic pollution (D4.2.10)

Plastics are persistent because they are not biodegradable: decomposers cannot break the polymers down. Macroplastics are large items; microplastics are tiny fragments formed as larger pieces break up. Effects on marine life include entanglement, ingestion that fills the gut and reduces feeding, and the transfer of plastic particles up food chains. If 18 of 60 turtles examined contain plastic, that is 18 ÷ 60 × 100 = 30%.

Nature of science: clear information from scientists, amplified by media coverage, changed public perception worldwide and drove action.

Rewilding (D4.2.11)

Rewilding restores natural processes. Methods:

• Reintroducing apex predators and other keystone species.
• Re-establishing connectivity of habitats over large areas, for example with corridors.
• Minimizing human impact, including through ecological management.

Hinewai Reserve, on Banks Peninsula in New Zealand, was degraded, gorse-infested farmland that is regenerating into native forest with minimal intervention. Gorse is treated as a shelter under which native seedlings establish.

Succession (HL only: D4.2.12 to D4.2.15)

Ecological succession is the change in a community over time. It is triggered by changes in the abiotic environment (a retreating glacier, a volcanic eruption, climate) and by biotic factors (new species arriving, competition, grazing).

Primary succession starts on bare ground with no soil. Lichens and mosses colonize rock and form soil, then grasses, shrubs and trees follow. Across a sequence, expect increases in plant size, primary production, species diversity, food web complexity and nutrient cycling.

Worked example (glacier foreland). Plant species: 4 at 10 years after ice retreat and 17 at 100 years, an increase of (17 − 4) ÷ 4 × 100 = 325%. Soil organic matter rises from 0.5% to 14%, which is 28 times higher: accumulating humus supports bigger plants.

Cyclical succession: some ecosystems show a repeating cycle of communities instead of one climax. In a heather moorland, young, building, mature and degenerate heather phases follow each other, and gaps then restart the cycle.

Climax community and arrested succession. Given particular conditions, succession tends toward a characteristic climax community. Human influences can prevent it. Grazing by farm livestock holds grassland and stops tree seedlings growing. Draining a wetland stops wetland succession and the area is held in another state.

Common errors

• Calling any abundant species keystone.
• Calculating percentage change from the new value.
• Saying eutrophication directly uses oxygen: it is decomposer respiration that does so.
• Confusing biomagnification with fertilizer enrichment.
• Writing about primary succession in an SL answer: it is HL only.

Official syllabus

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