IB Biology HL D4.2 Stability and change Notes

Ecosystem stability means a community can remain recognizable over long timescales even while local populations fluctuate. Ancient forests, deserts, and long-lived communities show that stability is not the absence of change; it is persistence of structure and function despite normal variation.

Stable ecosystems need several supports working together. Energy input powers food webs, nutrient cycling keeps materials available, biodiversity and genetic diversity spread risk, and abiotic conditions must stay within tolerance ranges for key species. If one support fails, stability can weaken.

Amazon forest stability depends partly on the forest making its own wet conditions. Deforestation reduces transpiration, rainfall recycling, and regional cooling. If enough forest is lost, reduced rainfall and higher temperature can push rainforest toward a savanna-like tipping point.

The tipping-point risk is about losing enough forest that rainfall recycling can no longer maintain rainforest conditions.

Mesocosms are small model ecosystems used to test stability under controlled conditions. A closed bottle restricts matter exchange, so nutrients and gases must cycle inside. Light can still enter as an energy input. Good mesocosm answers mention what is controlled, what is measured, and what limitation the model has.

A mesocosm is useful only if one variable changes and the control stays comparable.

A keystone species has a disproportionate effect on community structure. Removing it can change populations at several trophic levels, called a trophic cascade. The exam chain is: keystone species removed, one population changes, feeding pressure changes, and community structure becomes less stable.

Sustainable harvesting removes biomass at or below the replacement rate. If harvest exceeds recovery, the population or stock declines. Managed plant harvests and marine fish stocks need monitoring because the safe harvest rate depends on population recovery, reproduction, and changing conditions.

Agricultural sustainability depends on maintaining soil, nutrients, biodiversity, water, and climate impacts while producing food. Erosion and leaching remove soil and nutrients. Fertilizers, agrochemicals, irrigation, and carbon footprint can improve yield but also create environmental costs.

Eutrophication follows a predictable oxygen-loss chain. Nitrate and phosphate enrichment stimulates algal blooms. When algae die, decomposers respire while breaking them down, increasing biochemical oxygen demand. Dissolved oxygen falls, causing hypoxia that harms aquatic life.

Eutrophication is easiest to explain as an ordered chain from nutrient enrichment to hypoxia.

Biomagnification happens when persistent toxins become more concentrated at higher trophic levels. Organisms take in toxins faster than they excrete or break them down, and predators eat many contaminated prey. DDT and mercury are classic examples because top predators receive the highest doses.

Persistent toxins become most concentrated in top predators because biomass and stored pollutants are consumed repeatedly.

Macroplastics and microplastics both persist in marine ecosystems, but their effects often differ by size. Macroplastics can entangle animals or be ingested. Microplastics form by fragmentation, move through food webs, and can transport toxins. Both can affect feeding, survival, and ecosystem stability.

Rewilding aims to restore natural processes rather than manage every detail. It can restore trophic interactions, habitat connectivity, and ecosystem dynamics. Keystone species reintroductions may restart trophic cascades, but good rewilding decisions consider risks, habitat suitability, and monitoring.

Ecological succession is community change over time after new habitat forms or disturbance occurs. Pioneer species alter abiotic conditions, such as soil, shade, moisture, or nutrient availability. Those changes allow different species to replace earlier ones through abiotic modification and species interactions.

Primary succession begins on bare surfaces without soil, such as new rock. Pioneer species help create soil and trap nutrients. As soil depth increases, biomass, diversity, food-web complexity, and nutrient cycling usually increase, allowing larger plants and more complex communities.

Primary succession changes both the community and the abiotic habitat.

Cyclical succession is not a one-way march to a final community. It repeats predictable community changes in the same area because of recurring disturbance or species interactions. The same stages can recur rather than ending at one fixed endpoint.

A climax community is relatively stable and shaped by local conditions such as climate, soil, and species interactions. Succession can be arrested before climax when repeated grazing, burning, mowing, or drainage prevents later stages. Arrested succession is not the same as a natural stable climax.

Core D4.2 is secure when students can judge whether a system is being stabilized or pushed toward change. The route is: identify the stability support or disturbance, explain the mechanism, and state the ecosystem consequence using evidence.

HL D4.2 is the long-term change story. Succession begins after new habitat or disturbance, pioneer species modify conditions, primary succession builds soil and complexity, and some systems repeat cycles or are held before climax by disturbance.

Core transfer questions ask students to explain why an ecosystem remains stable or why a disturbance pushes it toward change. Strong answers do not list threats; they explain mechanisms such as lost rainfall recycling, trophic cascade, overharvest, nutrient enrichment, toxin biomagnification, plastic movement, or restoration through rewilding.

Explain how a named factor can maintain ecosystem stability or push an ecosystem toward change.

HL succession questions focus on community pathways over time. The answer starts from the initial condition, shows how species modify abiotic conditions and interactions, and identifies whether the pathway is primary, cyclical, climax, or arrested by repeated disturbance.

Explain how ecological succession can produce community change or relative stability over time.