Our focus is on successional patterns that occur on newly exposed landforms. If the exposed landform has not previously been influenced by a community, the sequence of species is referred to as a primary succession. Lava flows and pumice plains caused by volcanic eruptions (see Section 16.4.3), craters caused by the impact of meteors (Cockell & Lee, 2002), substrate exposed by the retreat of a glacier (Crocker & Major, 1955) and freshly formed sand dunes (see Section 16.4.4) are examples. In cases where the vegetation of an area has been partially or completely removed, but where well-developed soil and seeds and spores remain, the subsequent sequence of species is termed a secondary succession. The loss of trees locally as a result of disease, high winds, fire or felling may lead to secondary successions, as can cultivation followed by the abandonment of farmland (so-called old field successions - see Section 16.4.5).
Successions on newly exposed land-forms typically take several hundreds of years to run their course. However, a precisely analagous process occurs amongst the animals and algae on recently denuded rock walls in the marine subtidal zone, and this succession takes only a decade or so (Hill et al., 2002). The research life of an ecologist is sufficient to encompass a subtidal succession but not that following glacial retreat. Fortunately, however, information can sometimes be gained over the longer timescale. Often, successional stages in time are represented by community gradients in space. The use of historic maps, carbon dating or other techniques may enable the age of a community since exposure of the landform to be estimated. A series of founder control: many species are equivalent in their ability to colonize dominance control: some potential colonizers are competitively dominant primary succession: an exposed landform uninfluenced by a previous community secondary succession: vestiges of a previous community are still present communities currently in existence, but corresponding to different lengths of time since the onset of succession, can be inferred to reflect succession. However, whether or not different communities that are spread out in space really do represent various stages of succession must be judged with caution. We must remember, for example, that in northern temperate areas the vegetation we see may still be undergoing recolonization and responding to climatic change following the last ice age (see Chapter 1).
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