What Is Succession?

Succession is the gradual process of change in the structure and composition of a community in an ecosystem over time. It involves the replacement of one group of species by another, leading toward a stable climax community.

Primary Succession

Occurs on newly formed or exposed land where no previous community existed. There is no soil at the start.

Examples:

• Bare rock after volcanic eruption

• Sand dunes

• Retreating glacier ground

Stages of Primary Succession

1. Colonisation by pioneer species (e.g., lichens) that can survive harsh, abiotic conditions.

2. The pioneer species change the abiotic environment. They die and are decomposed by micro-organisms. This forms a basic soil.

3. Conditions are less hostile, there is some soil to retain water and some minerals such as nitrates.

4. Other species may now start to grow, such as moss.

5. Larger plants which need more water and minerals will begin to grow once the soil deepens- these are grasses and flowering plants. When these plants die and decompose more minerals, such as nitrates will enter the soil. 

6. Shrubs, ferns and small trees begin to grow as the soil holds more water and contains more minerals. They outcompete the grasses and smaller plants to become the dominant species. Diversity increases.

7. Finally, the soil is rich and deep enough that there are enough minerals and water to support large trees. These outcompete other species to become the dominant species. This is now called the ‘climax community’.

Secondary Succession

Occurs in areas where an ecosystem previously existed but has been disturbed or destroyed, and soil is already present.

Examples:

• After a forest fire, flood, or human activity (e.g., abandoned farmland)

Difference from Primary Succession:

• Faster than primary succession (soil and seed bank are present)

• Pioneer species are different (e.g., fast-growing grasses or weeds)

Occurs when human activity prevents succession from reaching its natural climax community. This results in an artificial climax community.

Examples:

• Grazing by sheep maintains grassland instead of woodland.

• Mowing prevents shrubs and trees from establishing.

• Controlled burning in heathlands prevents climax forest.

Human interference maintains an earlier stage of succession.

• To begin with the environmental conditions are harsh/ hostile.

• Only the pioneer species can grow in the area- they have adaptations to enable them to survive.

• As succession occurs the environmental conditions become less harsh/ hostile.

• The composition of soil changes to contain more water/ nitrates.

• Species move in and outcompete previous species.

• The abiotic and biotic factors gradually change over time.

• Eventually a climax community is reached- these are stable communities.

• The climax community reached depends on the particular climate.

• As succession continues, the ecosystem becomes more complex and species diversity increases- plants create more habitats for animals, the abiotic conditions become less hostile and the amount of biomass increases.