Succession

Communities of plants and animals change with time. Within the same habitat, the species composition will change, as will the number of individuals within each species. This process of change is known as 'succession'. Two types of succession are recognized.

• Primary succession is seen in a situation of uncolonized rock or exposed subsoil. Sand dunes, disused quarries and landslide locations are examples. Primary succession runs in parallel with the development of soils (see p300) or peat (see p328). It can be seen that plant and animal species from outside the new habitat will be the ones involved in colonization.

The term 'sere' is often used instead of 'succession' when referring to a particular habitat. Lithosere refers to a succession beginning with uncolonized rock, psammosere to one beginning with sand (often in the form of sand dunes).

• Secondary succession is seen where a bare habitat is formed after vegetation has been burnt, or chopped down, or covered over with flood silt deposit. In this situation, there will often be plant seeds and animals which survive under the barren surface, which are able to begin colonization again by bringing topsoil, or at least some of its associated beneficial bacteria and other micro-organisms, to the surface. This kind of succession is the more common type in the British Isles. A hydrosere refers to succession occurring in a fresh water lake.

Influences on succession can come in two ways. 'Allogenic succession' occurs when the stimulus for species change is an external one. For example, a habitat may have occasional flooding (or visits from grazing animals) which influence species change. In contrast, 'autogenic succession' occurs when the stimulus for change is an internal one. For example, a gradual change in pH (or increased levels of organic matter) may lead to the species change.

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