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

These notes condense IB Diploma Programme Biology D4.2 Stability and change (D4.2.1 to D4.2.15), from the IB Biology guide, first assessment 2025. Statements D4.2.12 to D4.2.15 are HL only; the rest are SL and HL. For full explanations read the study guide, then test yourself with the practice set. Also useful: the IB DP Biology hub, the printable checklist, the free diagnostics and energy flow in ecosystems.

Key definitions

Term	Meaning
Ecosystem stability	An ecosystem keeping its character and processes over long periods despite fluctuations
Tipping point	A threshold beyond which change becomes self-reinforcing and hard to reverse
Mesocosm	A small enclosed model ecosystem, ideally a sealed glass vessel
Keystone species	A species with a disproportionate effect on community structure compared with its abundance
Sustainable harvest	Harvest rate lower than the rate of replacement
Eutrophication	Nutrient enrichment of water, leading to algal blooms and oxygen loss
BOD	Biochemical oxygen demand: oxygen used by microbes decomposing organic matter
Biomagnification	Rising concentration of a persistent toxin at each higher trophic level
Rewilding	Restoring natural processes with minimal human control
Succession (HL only)	Change in community composition over time

D4.2.1 to D4.2.2: stability

• Evidence: forests, deserts and other ecosystems that have continued for very long periods, some for millions of years.
• Four requirements: energy supply; nutrient recycling; genetic diversity; climatic variables within tolerance levels.
• Memory hook: energy flows through, matter cycles round, genes give options, climate sets the limits.

D4.2.3: Amazon tipping point

Chain of events:

1. Forest cleared, so less transpiration.
2. Less water vapour, so less cooling, weaker air flows and less rainfall.
3. Drier conditions, so more tree death and further loss.

• Uncertainty remains about the minimum forest area that keeps these processes going.
• Percentage change = (new − original) ÷ original × 100.
• Reminder: forest area falls from 250 to 200 million hectares, so (200 − 250) ÷ 250 × 100 = −20%.

D4.2.4: mesocosms

Feature	Why
Sealed glass	Stops matter entering or leaving; light still passes
Aquatic or microbial	More likely to stay viable than terrestrial
Replicates	Each vessel develops differently
One variable changed	Fair test
IB guidelines	Care of living organisms

Limitation: far simpler than nature.

D4.2.5: keystone species

• Small population, large effect.
• Removal can cause collapse: a predator vanishes, a herbivore booms, producers crash.
• Add to answers: name the species, say what it controls, describe the knock-on changes.
• Distinction: keystone is about impact; dominant is about numbers.

D4.2.6: assessing sustainability

Method in steps:

1. Estimate the replacement rate (births, growth, recruitment).
2. Measure the harvest rate.
3. Compare. Harvest below replacement is sustainable.
4. Check indicators over time: catch per unit effort, fish size and age, seedling counts, age structure.

Reminder: replacement 3000 t, catch 1500 t, so the catch is 1500 ÷ 3000 × 100 = 50% of replacement, which is sustainable if the estimate is sound.

Examples to know: one terrestrial plant species and one marine fish species.

D4.2.7: agricultural sustainability

Factor	Problem
Soil erosion	Loss of topsoil
Leaching	Nutrients lost, water polluted
Fertilizer and input supply	Finite or energy-hungry
Agrochemical pollution	Harm to non-target species
Carbon footprint	Emissions from fuel, manufacture, transport

D4.2.8: eutrophication

Cause and effect chain: leaching of nitrogen and phosphate, so algal bloom, so light blocked, so plants die, so decomposers multiply and respire, so BOD rises, so oxygen falls, so fish die.

• Decomposers use the oxygen, not the algae directly.
• Reminder: dissolved oxygen 8.0 falls to 3.5 mg dm⁻³ in a sealed bottle. BOD = 8.0 − 3.5 = 4.5 mg dm⁻³.

D4.2.9: biomagnification

Needed properties: persistent, not excreted, and stored in tissues (DDT is fat-soluble; mercury compounds bind to proteins).

• DDT: insecticide, eggshell thinning in some predatory birds.
• Mercury: accumulates in large predatory fish.
• Calculate the factor as concentration at higher level ÷ concentration at lower level. Reminder: 12 ÷ 0.3 = 40 times.

Distinction: bioaccumulation is build-up in one organism; biomagnification is the increase along the food chain.

D4.2.10: plastics

• Persistent because they are not biodegradable.
• Microplastics are small fragments, macroplastics are large items.
• Effects: entanglement, blocked guts, false feeling of fullness, transfer to consumers.
• Nature of science: clear research findings plus media coverage changed public attitudes and prompted action.

D4.2.11: rewilding

Method	Effect
Reintroduce apex predators and keystone species	Restores control of herbivores
Connect habitats over large areas	Allows movement and gene flow
Reduce human impact and manage ecologically	Lets natural processes resume

Example: Hinewai Reserve, Banks Peninsula, New Zealand, where degraded gorse-covered farmland is regenerating into native forest.

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

• Triggers: abiotic change (glacier retreat, volcano, fire, climate) and biotic change (new arrivals, competition, grazing).
• Primary succession (no soil at the start) shows increases in plant size, primary production, species diversity, food web complexity and nutrient cycling.
• Cyclical succession: a repeating cycle of communities, such as the phases of heather moorland.
• Climax community: the characteristic end community for the conditions.
• Arrested succession: human activity holds the community in an earlier stage, for example livestock grazing or wetland drainage.
• Primary succession sequence for a bare rock surface: lichens and mosses, then soil begins to form, then grasses and herbs, then shrubs, then trees. Each stage changes the environment (more soil, more shade, more nutrients) so that the next stage can establish.
• Cyclical succession is not a failure to reach a climax. The cycle itself is the stable pattern, so cite a named example and the phases in order.

Answering data and explain questions

Method in steps for a pollution or sustainability question:

1. Read the axis labels and units, and state the trend with figures from the data.
2. Name the process (leaching, eutrophication, biomagnification, overharvesting).
3. Give the chain of cause and effect in order, one link per sentence.
4. Finish with the consequence for the community or the stock.

Command terms in this unit usually ask you to do the following.

• Explain: give reasons, with the mechanism.
• Outline: give the main points briefly.
• Evaluate: weigh strengths and limitations, then reach a judgement.
• Suggest: apply knowledge to an unfamiliar situation; there may be several acceptable answers.

Worked reminders:

• A kelp-forest herbivore rises from 20 to 50 per quadrat after its predator is removed: (50 − 20) ÷ 20 × 100 = +150%.
• Fishing effort doubles while the catch stays the same, so catch per unit effort halves. This points to a declining stock.
• A farm lets 30 kg of nitrogen per hectare leach and the neighbouring pond blooms. The link is leaching, then eutrophication, then high BOD.

Quick self-test

1. Name the four requirements for stability.
2. A forest shrinks from 800 to 640 thousand km². Calculate the percentage change.
3. Why are sealed glass mesocosms preferred?
4. Define keystone species in one sentence.
5. A fish stock is replaced at 900 t per year and 1100 t is caught. Is this sustainable?
6. Which organisms use up oxygen after an algal bloom dies?
7. A toxin is 0.2 mg kg⁻¹ in small fish and 5 mg kg⁻¹ in a seabird. By what factor does it increase?
8. Why do plastics persist in the ocean?
9. State two rewilding methods.
10. (HL only) Give two changes seen during primary succession.

Answers

1. Energy supply; nutrient recycling; genetic diversity; climatic variables within tolerance levels.
2. (640 − 800) ÷ 800 × 100 = −20%.
3. Matter cannot enter or leave, but light energy can pass through.
4. A species with a disproportionately large effect on community structure relative to its abundance.
5. No. Harvest exceeds replacement by 200 t per year, so the stock will decline.
6. Aerobic decomposers (bacteria and fungi), which respire the dead material.
7. 5 ÷ 0.2 = 25 times.
8. They are not biodegradable, so decomposers cannot break them down.
9. Any two: reintroduce apex predators or keystone species; reconnect habitats; reduce human impact.
10. Any two: bigger plants; more primary production; more species; more complex food webs; more nutrient cycling.

Where marks are usually lost

• Forgetting to say “within tolerance levels” for climatic variables.
• Dividing by the new value in percentage change, or leaving out the sign.
• Describing a keystone species as simply the most numerous one.
• Saying algae use up the oxygen in eutrophication instead of decomposers.
• Leaving out leaching as the link between fertilizer use and the water pollution.
• Writing “bioaccumulation” when the question asks about increases along a food chain.
• Naming a plastic problem without a specific effect on a marine organism.
• Describing rewilding without a method or the example asked for.
• Treating arrested succession as a natural pause. It needs a human influence, such as grazing or drainage.
• Using a mesocosm result as proof about a whole natural ecosystem without noting the limits.

Official syllabus

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