Ecosystem stability
Stability = the ability of a natural ecosystem to persist over long periods while maintaining its overall structure and function.
Some ecosystems such as forests and deserts have shown continuity over very long timescales; some may persist for millions of years.
In exam answers, link stability to long-term persistence, not to “no change at all”. Stable ecosystems still show fluctuations, but remain functional overall.
Requirements for stability in ecosystems
A constant supply of energy is needed, usually ultimately from sunlight.
Nutrients must be recycled effectively so matter is reused within the ecosystem.
Genetic diversity helps populations adapt to change and resist collapse.
Climatic variables must remain within the tolerance limits of organisms in the ecosystem.
Stability depends on interactions between abiotic conditions and biotic communities.
Amazon rainforest tipping point
Deforestation of the Amazon rainforest is a key example of a possible tipping point in ecosystem stability.
A large area of rainforest is needed for transpiration to release water vapour into the atmosphere.
This atmospheric moisture contributes to cooling, air flows, and rainfall.
If too much forest is removed, these feedbacks may weaken, pushing the ecosystem toward major change.
The exact minimum area needed to maintain these processes is uncertain.
Exam link: reduced forest cover → less transpiration → less rainfall generation → greater risk of ecosystem shift.
Models and mesocosms
A model can be used to investigate how variables affect ecosystem stability.
Mesocosms are simplified ecosystem models; they may be aquatic, microbial, or less commonly terrestrial.
Sealed glass vessels are often preferred because matter cannot enter or leave, but energy can still be transferred.
This makes mesocosms useful for testing how changes in factors such as light, nutrient input, or species presence affect stability.
In practical work, follow IB experimental guidelines for care and maintenance.
Best exam point: models are simplified systems, so they are useful for testing variables but have limitations when representing full natural ecosystems.
Keystone species
A keystone species has a disproportionately large effect on community structure relative to its abundance.
Removal of a keystone species can trigger major changes in population sizes, food webs, and ecosystem stability.
Loss of a keystone species can increase the risk of ecosystem collapse.
Keystone species often regulate other populations and help maintain biodiversity.
Do not confuse keystone species with dominant species: a keystone species is important because of its effect, not necessarily because it is common.
This resource explains that a keystone species can define an entire ecosystem and that removal of one species can drastically alter biodiversity and food-web structure. It is ideal for linking keystone species to ecosystem stability and possible collapse. Source
Sustainable harvesting of natural resources
Sustainability means the rate of harvesting is lower than the rate of replacement.
Renewable resources can only be harvested sustainably if populations can recover fast enough.
Be ready to discuss one terrestrial plant species and one marine fish species as examples of renewable resources.
Key judgement in exams: compare rate of removal with rate of replacement/regeneration.
Unsustainable harvesting reduces population size and can destabilize ecosystems.
Sustainability of agriculture
Sustainable agriculture must consider soil erosion.
It must also consider leaching of nutrients from soil.
Dependence on fertilizers and other external inputs affects sustainability.
Pollution due to agrochemicals can damage ecosystems.
Carbon footprint is also an important measure of sustainability.
Strong exam answers link agriculture to both food production and environmental costs.
Eutrophication
Eutrophication is caused by leaching of nitrogen and phosphate fertilizers into aquatic or marine ecosystems.
Extra nutrients cause rapid growth of algae and other producers.
When this biomass dies, decomposers increase respiration, raising biochemical oxygen demand (BOD).
Increased BOD reduces dissolved oxygen, which can kill aquatic organisms.
Typical sequence: nutrient enrichment → algal bloom → decomposition → high BOD → oxygen depletion.
In data questions, look for evidence such as high nitrate/phosphate, algal bloom, and low dissolved oxygen.
Biomagnification and pollution
Biomagnification = increasing concentration of a toxin in organisms at higher trophic levels.
Toxins accumulate in tissues over time and become more concentrated as predators consume many contaminated prey.
IB examples: DDT and mercury.
Top consumers are often most affected because they occupy the highest trophic levels.
Distinguish bioaccumulation (build-up within one organism over time) from biomagnification (increase between trophic levels).
The illustration shows methylmercury moving from plankton to small fish, then to larger predatory fish and finally to humans/animals, clearly demonstrating biomagnification. It is a strong visual for explaining why pollutant concentration increases at higher trophic levels. Source
Plastic pollution in oceans
Microplastics and macroplastics affect marine ecosystems.
Plastics are persistent because they are non-biodegradable.
Macroplastics can cause entanglement, blockage, or physical injury.
Microplastics can be ingested, entering food chains and harming marine organisms.
Plastic pollution is a human-caused threat to ecosystem stability.
A strong exam point is that persistence in the environment means effects can be long-lasting.
Rewilding and ecosystem restoration
Rewilding aims to restore natural processes in ecosystems.
Methods include reintroduction of apex predators and other keystone species.
It also includes restoring connectivity of habitats over large areas.
Human impact should be minimized, including through ecological management.
Named example required by syllabus: Hinewai Reserve, New Zealand.
In exams, link rewilding to restoring self-sustaining ecosystem processes, not just adding more organisms.
HL only: Ecological succession
Ecological succession = gradual change in community structure over time.
Succession can be triggered by changes in abiotic factors or biotic factors.
During primary succession, general trends include increases in plant size, primary production, species diversity, food-web complexity, and nutrient cycling.
Pioneer species colonize first, helping create conditions for later species.
In some ecosystems, succession is cyclical rather than moving toward one permanent endpoint.
A climax community is the relatively stable community expected under given environmental conditions.
Arrested succession occurs when human activity prevents development of the expected climax community.
Syllabus examples of arrested succession: grazing by farm livestock and drainage of wetlands.
This page summarizes how communities change over time, from pioneer species to more complex communities, and explains climax communities. It is useful for HL content on primary succession, cyclical succession, and arrested succession. Source
Checklist: can you do this?
Define stability, tipping point, keystone species, eutrophication, BOD, and biomagnification accurately.
Explain how Amazon deforestation can reduce transpiration, alter rainfall, and threaten ecosystem stability.
Calculate percentage change from data on deforestation or ecosystem area.
Interpret why increasing BOD after fertilizer runoff leads to low oxygen and death of aquatic organisms.
Apply the idea of sustainable harvesting by comparing rate of removal with rate of replacement.
Exam traps and quick links
Stability does not mean no change; it means long-term persistence of function and structure.
Biomagnification is about increasing toxin concentration across trophic levels; bioaccumulation is within one organism.
Eutrophication is driven by nutrient leaching, not just “pollution” in general.
Keystone species are not necessarily the most abundant species.
Sustainable harvesting depends on replacement exceeding harvest, not just on a species being renewable in principle.
Percentage change = ((new - original) / original) × 100.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.