SuccessionAQA A-Level Biology: Revision notes

Ecosystems are dynamic: communities change over time. Succession is the gradual change in the species of a community over time as the abiotic and biotic environment changes.

Primary succession begins on a newly formed or exposed area with no soil and no previous community, such as bare rock from a volcano, a retreating glacier or newly formed sand dunes. It takes place over many decades or centuries.

The sequence is a series of stages. Each stage is called a seral stage, and changes in the species present are driven by the organisms themselves.

The first species to colonise bare ground are pioneer species, such as lichens, algae and bacteria on rock, or marram grass on sand. The conditions are hostile: no soil, little water and few nutrients, and extreme temperatures or wind.

Pioneers are adapted to these conditions. They may be able to photosynthesise with little water, tolerate drought, or fix nitrogen. They are small, and they spread by wind-blown spores or seeds.

Pioneers change the environment. They weather the rock, trap particles, and when they die they decompose to form a thin soil rich in organic matter (humus). This is the start of a less hostile environment.

At each stage the organisms present alter the abiotic environment, making it more suitable for other species with different adaptations.

• Soil gets deeper, with more organic matter, which increases water retention and mineral ion content.
• Plant cover reduces temperature extremes and wind speed at ground level.
• Nitrogen-fixing species increase the supply of nitrate.

This allows mosses, then grasses, shrubs and finally trees to colonise. Each new group of species is better suited to the new conditions.

As conditions become less hostile, biodiversity usually increases because there are more habitats and niches and more species can survive.

The changes made by new species can make the environment less suitable for the species that came before.

• Taller plants shade the ground, so light-demanding species receive too little light and are outcompeted.
• Litter from some trees acidifies the soil.
• Deeper, richer soil favours species that outcompete the pioneers.

Earlier species decline and are replaced. This is why the community changes continuously until a stable stage is reached. Because the dominant trees can reduce the light available to the ground flora, species richness may be highest in the middle of the succession and lower at the climax.

Eventually a climax community forms. It is a stable community in which the species present stay in balance with each other and with the abiotic environment, and in which there is no further change in the dominant species. In Britain this is usually woodland.

The climax community depends on the climate and soil of the area. It is stable but not static: populations still fluctuate around their carrying capacities, and individual trees die and are replaced.

A worked sand dune example: marram grass on the shifting dunes, then other grasses and lichens on fixed dunes, then shrubs such as gorse and finally woodland. Moving inland, the soil is deeper, has more humus and holds more water, and there is less salt spray.

A good answer follows a sequence:

1. Pioneer species colonise, adapted to hostile conditions.
2. They change the environment, for example by forming soil.
3. The environment is now more suitable for new species, which colonise.
4. These may outcompete or shade earlier species, which decline.
5. Biodiversity usually increases at first.
6. A stable climax community forms.

When you are given data from areas of different ages, remember that the areas are assumed to show one sequence over time, but they may differ in other ways.