IB Biology · Theme D: Continuity and change · SL and HL
D4.2 Stability and change
A one-page summary of D4.2 Stability and change, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What features of ecosystems allow stability over unlimited time periods?
What changes caused by humans threaten the stability of ecosystems?
What D4.2 covers
What keeps an ecosystem stable
- D4.2.1Stable, but never still
- D4.2.2Energy flows through, nutrients go round
- D4.2.2Diversity and a tolerable climate
- D4.2.3The forest that makes its own rain
- D4.2.3Past the tipping point
- D4.2.3Measuring loss: percentage change
- D4.2.4A model ecosystem in a jar
Keystone species and sustainable use
- D4.2.5Small numbers, huge impact
- D4.2.6Harvest rate versus replacement rate
- D4.2.6Brazil nuts: harvest without felling the tree
- D4.2.6Atlantic cod: when catch outruns replacement
- D4.2.7Can farming go on forever?
Pollution and restoration
- D4.2.8From fertilizer to fish kill
- D4.2.9Toxins that climb the food chain
- D4.2.9DDT and mercury
- D4.2.10Plastic that never goes away
- D4.2.10Harm to marine life, and to public opinion
- D4.2.11Rewilding: restore, then step back
- D4.2.11Hinewai: let the gorse do the work
HL: Ecological succession
- D4.2.12 · HLCommunities that change over time
- D4.2.13 · HLPrimary succession at Glacier Bay
- D4.2.13 · HLFive things that increase
- D4.2.14 · HLCycles instead of endpoints
- D4.2.15 · HLClimax communities, and how humans stop them
- D4.2.15 · HLDraining a wetland ends its succession
D4.2 Stability and change: summary
Stability & its requirements
- Stability is long-term persistence of structure and function despite change; some ecosystems have lasted millions of years.
- Four requirements: energy supply, nutrient recycling, genetic diversity, climate within tolerance levels.
Tipping points & models
- Amazon: forest recycles its own rain by transpiration; deforestation feeds a positive feedback loop toward a tipping point. % change = (new − original) ÷ original × 100.
- A sealed-glass mesocosm lets energy through but not matter.
Keystone species & sustainable use
- Keystone species (sea otter) have effects far larger than their abundance.
- Harvest must stay below replacement (Brazil nut, Atlantic cod); farming: erosion, leaching, inputs, agrochemicals, carbon footprint.
Pollution
- Eutrophication: leached nitrate and phosphate, algal bloom, high BOD, low oxygen.
- Biomagnification of persistent toxins (DDT, mercury); non-biodegradable plastic fragments into microplastics.
Restoration
- Rewilding: restore natural processes by reintroducing apex predators and keystone species and reconnecting habitats over large areas, then minimize human impact, keeping ecological management to a minimum.
- Hinewai Reserve (New Zealand): gorse acted as a nurse canopy for native forest with minimal management.
HL · Succession
- Succession is triggered by abiotic or biotic change; primary succession raises plant size, production, diversity, food-web complexity and nutrient cycling (Glacier Bay).
- Cyclical (heather) and arrested (grazing, wetland drainage) succession show a climax is not guaranteed.
Key terms
- Tipping point
- A threshold beyond which a self-reinforcing change pushes an ecosystem into a different state.
- Mesocosm
- A small enclosed model ecosystem used to test how one variable affects stability.
- Keystone species
- A species whose effect on community structure is far larger than its abundance suggests.
- Sustainable harvesting
- Harvesting at a rate lower than the rate at which the resource is replaced.
- Eutrophication
- Nutrient enrichment of water, leading to algal blooms, a high BOD and low oxygen.
- BOD
- Biochemical oxygen demand: the oxygen used by microorganisms in a water sample.
- Biomagnification
- The rise in concentration of a persistent toxin at each higher trophic level.
- Rewilding
- Restoring an ecosystem's natural processes (bringing back apex predators and keystone species, reconnecting habitats over large areas) and then minimizing human impact, keeping ecological management to a minimum.
- Percentage change
- (new − original) ÷ original × 100, used to measure the extent of deforestation.
- Ecological succession HL
- Change in the species composition of a community over time.
- Primary succession HL
- Succession that begins on bare substrate with no soil.
- Climax community HL
- The relatively stable community that succession tends toward under given conditions.
- Cyclical succession HL
- A repeating sequence of communities instead of one unchanging climax.
- Arrested succession HL
- Succession held at an early stage by human influence, such as grazing or drainage.
Sample exam questions
Three of the 34 multiple-choice questions for D4.2. Try each one before opening the answer.
Question 1. Which statement best describes ecosystem stability as a natural property?
- Stable ecosystems never experience any disturbance
- Some natural ecosystems (e.g. certain forests and deserts) have persisted with broadly the same structure and species composition for very long periods, in some cases millions of years, despite ongoing disturbance
- Only ecosystems created or managed by humans can be stable
- Stability means an ecosystem's species composition is fixed and can never change at all
Show the answer
Answer: B. The guide asks students to illustrate stability with real evidence — e.g. old-growth forest or desert ecosystems that have persisted, in recognisable form, for very long timescales, sometimes millions of years, despite fires, storms, and other regular disturbances.
Question 2. Which is real evidence that has been used to demonstrate long-term ecosystem stability?
- A single year of rainfall data from one weather station
- The number of species listed on an endangered species register
- The market price of timber over the last decade
- Fossil pollen and sediment records showing broadly similar vegetation types persisting in an area over geological timescales
Show the answer
Answer: D. Palaeoecological evidence (pollen cores, sediment layers) allows scientists to reconstruct past vegetation and show that some ecosystem types have persisted for very long periods.
Question 3. The four requirements for ecosystem stability listed in the syllabus are:
- Supply of energy, recycling of nutrients, genetic diversity, and climatic variables remaining within tolerance levels
- High rainfall, high temperature, low altitude, and low latitude
- Absence of predators, absence of disease, absence of competition, and absence of humans
- Large population size only
Show the answer
Answer: A. D4.2.2 lists exactly these four requirements. Note that nutrients must be recycled (an ecosystem is not a source of new matter), while energy must be continuously supplied (energy flows through and is dissipated, it cannot be recycled).
Linking questions
Questions that connect D4.2 to other parts of the course, the kind that come up in Paper 2.
- Genetic diversity is listed as one requirement for ecosystem stability (D4.2.2), and evolution by natural selection depends on heritable variation within a gene pool (D4.1). Explain how a loss of genetic diversity within a population (e.g. through a population bottleneck) could reduce an ecosystem's overall stability. (see D4.1)
- Eutrophication (D4.2.8) disrupts the normal flow of energy and cycling of matter described in C4.2. Using the concept of biochemical oxygen demand, explain how excess nutrient input increases decomposer respiration and why this can collapse an aquatic food web. (see C4.2)
- Keystone species maintain community structure through top-down control (D4.2.5, C4.1.17). Using the sea otter/sea urchin/kelp example, explain how removing a keystone predator can shift a community from top-down to a different, degraded stable state. (see C4.1)
- Deforestation of the Amazon is presented as a possible ecological tipping point (D4.2.3). Climate change (D4.3) can also push ecosystems past tipping points via positive feedback loops. Compare the feedback mechanism in Amazon dieback with one climate-change-driven tipping point studied in D4.3. (see D4.3)
- A keystone species interacts with other species through its ecological niche (B4.2). Explain, using the concept of niche, why removing one keystone species can free up resources that trigger competitive release in several other species simultaneously. (see B4.2)
Practise D4.2
Study notes, every question and full markschemes for D4.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.