D4.2 Stability and changeIB Biology SL: Revision notes

Stability is a property of natural ecosystems: many have shown continuity over very long periods — some tropical rainforests and deserts have persisted for millions of years.

Requirements for stability:

• a continuous supply of energy (usually sunlight);
• recycling of nutrients by decomposers;
• genetic diversity, so populations can adapt;
• climatic variables staying within the tolerance of the organisms.

Rainfall across the Amazon depends on a large area of forest: transpiration returns water vapour to the air, which causes cooling, drives air flows and falls again as rain further inland. If too much forest is removed, the remaining forest may dry out and switch to savanna — a tipping point. There is uncertainty over the minimum area of forest needed to keep these processes going.

Percentage change = (final − original) ÷ original × 100. Example: 4.10 → 3.28 million km² gives −20%.

A mesocosm is a small experimental ecosystem used to test how variables affect stability. Open tanks can be used, but sealed glass vessels are preferable because matter cannot enter or leave while light energy can still pass in. Aquatic or microbial mesocosms work better than terrestrial ones. Animals must be cared for according to IB experimental guidelines.

A keystone species has a disproportionate impact on community structure relative to its abundance; removing it risks ecosystem collapse (e.g. sea otters controlling sea urchins that graze kelp).

Harvesting is sustainable if the rate of harvesting is lower than the rate of replacement.

• Terrestrial plant: wild Brazil nuts are gathered from forest trees; sustainability is assessed by surveying tree age structure to check enough seedlings establish to replace old trees.
• Marine fish: Atlantic cod — sustainability is assessed by estimating stock size (surveys, catch data, age structure) and setting quotas. Northern cod off Newfoundland collapsed by 1992 through overfishing.

Factors affecting sustainability of agriculture: soil erosion, leaching of nutrients, the supply of fertilisers and other inputs, pollution by agrochemicals and the carbon footprint of farming.

Eutrophication: nitrate and phosphate fertilisers leach into water → algal bloom → algae die → aerobic bacteria decompose them → high biochemical oxygen demand (BOD) → dissolved oxygen falls → fish and invertebrates die.

Biomagnification: persistent toxins that are stored and not excreted become more concentrated at each higher trophic level, because consumers eat large amounts of food from the level below. Examples: DDT (thinned eggshells of birds of prey) and mercury (high in tuna and other top predators).

Plastics are non-biodegradable and persist. Macroplastics entangle animals (turtles, seals) and are swallowed, blocking guts (albatross chicks); microplastics are eaten by filter feeders and plankton and can carry toxins. Media coverage of plastic harm changed public perception worldwide and drove action such as bag charges — scientists influence citizens by communicating findings clearly.

Rewilding restores natural processes. Methods: reintroduction of apex predators and other keystone species (e.g. wolves in Yellowstone), re-establishing connectivity between habitats over large areas, and minimising human impact, including by ecological management. Example: Hinewai Reserve, New Zealand — former sheep farmland on Banks Peninsula left to regenerate with minimal intervention, where native forest has returned.