D4.2 Stability and changeIB Biology HL: Revision notes

Many natural ecosystems show stability over very long periods, some for millions of years. Stability needs an energy supply, recycling of nutrients, genetic diversity and climate within tolerance limits.

The Amazon's rainfall depends on transpiration from a large forest area (water vapour, cooling, air flows, rain inland). Deforestation could pass a tipping point where forest turns to savanna; the minimum area needed is uncertain. Measure deforestation as percentage change = (final − original) ÷ original × 100.

Mesocosms model ecosystems: sealed glass vessels stop matter entering or leaving but allow light energy in; aquatic or microbial systems work best; animal care follows IB guidelines.

Keystone species have a disproportionate impact on community structure; removal risks collapse. Harvesting is sustainable if its rate is below the rate of replacement — e.g. wild Brazil nuts (terrestrial plant) and Atlantic cod (marine fish, collapsed off Newfoundland in 1992).

Sustainable agriculture must consider soil erosion, leaching, fertiliser supply, agrochemical pollution and carbon footprint. Leached nitrate and phosphate cause eutrophication: algal bloom → decomposition → high BOD → low oxygen.

Biomagnification concentrates persistent toxins such as DDT and mercury at higher trophic levels. Plastics are non-biodegradable: macroplastics entangle or block guts; microplastics are eaten by filter feeders. Media coverage changed public perception and drove action.

Rewilding reintroduces apex predators and keystone species, reconnects habitats and minimises human impact — e.g. Hinewai Reserve, New Zealand.

Ecological succession is the sequence of changes in a community over time. It can be triggered by changes in the abiotic environment (e.g. a new volcanic island, glacial retreat, a fire) or in biotic factors (e.g. arrival of a species that alters the soil). Organisms change their environment, allowing other species to replace them.

Primary succession starts on ground with no soil, e.g. lava on Surtsey (Iceland) or rock exposed by a retreating glacier. Pioneer species (lichens, mosses) begin soil formation. As succession proceeds there are increases in:

• size of plants;
• primary production;
• species diversity;
• complexity of food webs;
• amount of nutrient cycling.

In some ecosystems there is a cycle of communities rather than one unchanging climax — cyclical succession. Example: heather (Calluna vulgaris) moorland passes through pioneer, building, mature and degenerate phases; the degenerate phase opens bare ground and the cycle restarts.

Under given conditions, succession tends towards a particular climax community, but human influences can cause arrested succession: grazing by farm livestock (sheep eat tree seedlings, holding grassland) and drainage of wetlands (preventing wet woodland).