International A Level Biology / Ecology

Succession: from colonisation to climax

Edexcel International A Level Biology (YBI11), chapter 15: Ecology, specification point 5.15. Free lesson.

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Succession is the gradual change in the species making up a community, as one community replaces another until a stable climax forms. Understanding how conditions change at each stage helps you explain how succession happens and distinguish its different types.

What succession means

Succession is a change in the species that make up a community over time. One community replaces another, stage by stage, until a stable climax community forms. This takes a long time. As succession continues, soil usually gets deeper, while biodiversity (the variety of species) and biomass (the mass of living material) increase.

How succession develops on bare rock

Primary succession begins where there is no soil, such as bare rock left by a glacier. Lichens are often the first colonisers. As the ground changes, other species can survive there, until a stable community forms. A lowland British climax community is usually woodland, often including oak.

Why each community is replaced

Each community changes the abiotic conditions, meaning the non-living conditions in the environment. This allows new species to survive. They grow and compete with earlier species, so the earlier ones may die out. The new community changes conditions again, and the process repeats towards the climax. Soil holds water and mineral ions, which bare rock cannot do. Humus holds water and releases mineral ions, such as nitrate, as it decomposes. It does not provide plants with carbon for photosynthesis. Plants take in carbon dioxide from the air through stomata, pores in their leaves.

How pioneer species differ from later species

Pioneer species are the first organisms to colonise an area. They are suited to harsh conditions, but are often replaced as conditions change. Pioneers tolerate drought, extreme temperatures and having no soil. Later species need soil, water and some shelter. Pioneers grow and reproduce quickly. Later species often grow more slowly, and many live for a long time. Pioneers produce many tiny spores or seeds that spread by wind. Later species often produce fewer, larger seeds. Pioneers are poor competitors. Later species compete better, for example by growing tall and shading others.

How to tell the three types of succession apart

The three types differ in where succession starts and whether it reaches the natural climax. Secondary succession is faster than primary succession because soil, and often seeds and roots, remain after the disturbance. Primary succession starts where there is no soil, such as bare rock after a glacier or new lava. Secondary succession starts after a community is removed but soil remains, such as after a forest fire or on abandoned farmland. Deflected succession is held before the natural climax, usually by people grazing, mowing or burning. The community maintained this way is…

More lessons in Ecology

• Population, community, habitat and ecosystem
• Abiotic factors and population size
• Biotic factors: predation, competition and disease
• How the niche explains distribution and abundance
• Core practical 11: random sampling with quadrats
• Core practical 11: transects and abiotic measurements
• Null hypotheses and choosing a statistical test
• Spearman's rank correlation coefficient
• Student's t-test
• The chi-squared test