Changes in ecosystemsEdexcel A-Level Biology B: Revision notes

An ecosystem is a community of organisms together with the abiotic environment in which they live. Ecosystems change over time.

Colonisation is the arrival and establishment of species in a new or newly cleared area. Succession is the process by which the community of an ecosystem changes over time, with one set of species being replaced by another.

• Primary succession begins on bare surfaces with no soil, for example new volcanic rock or sand.
• Secondary succession begins where soil is already present, for example after a fire or abandoned farmland, so it is faster.

The first species to colonise are pioneer species, such as lichens or marram grass. They tolerate harsh abiotic conditions. Each stage in the sequence is a seral stage.

Succession occurs because the organisms change the abiotic environment.

1. Pioneers tolerate extreme conditions. Their death and decomposition adds organic matter (humus), and they weather rock, so soil forms.
2. Soil retains more water and nutrients, and nitrogen fixers raise nitrate levels, so the environment becomes less hostile.
3. New species colonise and outcompete the earlier ones, for example for light, so the pioneers are replaced.
4. Species diversity and biomass increase, and taller plants create shade, shelter and microhabitats.
5. Change slows as the community approaches a stable end point.

The abiotic factors, such as soil depth, water retention and light at ground level, change in step with the biotic community.

The final, stable community of a succession is a climax community. Its species composition remains fairly constant and it is in balance with the abiotic conditions.

• A climatic climax is the community that develops naturally when succession is determined by the regional climate, for example deciduous woodland in much of the UK.
• A plagioclimax is a community held at an earlier stage because of human activity, for example grazing, burning or mowing. Examples are chalk grassland and heathland. If the human activity stops, succession resumes towards the climatic climax.

Conservation managers sometimes maintain a plagioclimax deliberately, because it can have higher species diversity and rare species than the climatic climax.

Abiotic factors are non-living, such as temperature, light, pH and water. Biotic factors are living, such as competition, predation and disease.

• Density-independent factors affect a population regardless of its size, for example a severe frost, drought or fire.
• Density-dependent factors have a greater effect as population size rises, for example competition, predation, parasitism and disease.

Competition may be intraspecific (same species) or interspecific (different species). Competition for limited resources sets the carrying capacity, the maximum population size an environment can support over time.

Predator-prey populations show cycles, with the predator peak lagging behind the prey peak. More prey means more food for predators, which then eat more prey, so prey numbers fall. This is negative feedback.

Aim: investigate the effect of one abiotic factor, such as light intensity, soil pH or moisture, on the distribution or morphology of one species.

• Use a transect with quadrats at regular intervals to show distribution across a gradient, or compare two sites using randomly placed quadrats.
• Random sampling (random coordinates) avoids bias.
• Measure the abiotic factor at each point, for example with a light meter, soil pH probe or moisture meter, at the same time of day.
• For morphology, measure a feature such as leaf area on a large sample (at least 30) and compare means with a t-test, or use Spearman's rank to test correlation along a transect.
• Control other variables, such as leaf age.

Ethics: take as few samples as possible, avoid damaging organisms or the habitat, return organisms where appropriate, and follow site rules.