A forest, or a rangeland, that appears to have reached a stable community structure when studied on a scale of hectares, will always be a mosaic of miniature successions. Every time a tree falls or a grass tussock dies, an opening is created in which a new succession starts. One of the most seminal papers in the history of ecology was entitled 'Pattern and process in the plant community' (Watt, 1947). Part of the pattern of a community is caused by the dynamic processes of deaths, replacements and microsuccessions that the broad view may conceal. Thus, although we can point to patterns in community composition in space (see Section 16.3) and in time (see Section 16.4), it is often more meaningful to consider space and time together.
We have already seen that disturbances that open up gaps are common in all kinds of community. The formation of gaps is obviously of considerable significance to sessile or sedentary species that have a requirement for open space, but gaps have also proved to be important for mobile species such as invertebrates on the beds of streams (Matthaei & Townsend, 2000). The patch dynamics concept of communities views the habitat as patchy, with patches being disturbed and recolonized by individuals of various species. Implicit in the patch dynamics view is a critical role for disturbance as a reset mechanism (Pickett & White, 1985). A single patch without migration is, by definition, a closed system, and any extinction caused by disturbance would be final. However, extinction within a patch in an open system is not necessarily the end of the story because of the possibility of reinvasion from other patches.
Fundamental to the patch dynamics perspective is recognition of the importance of migration between habitat patches. This may involve adult individuals, but very often the process of most significance is the dispersal of immature propagules (seeds, spores, larvae) and their recruitment to populations within habitat patches. The order of arrival and relative recruitment levels of individual species may determine or modify the nature and outcome of population interactions in the community (Booth & Brosnan, 1995).
In Section 16.4.1 we identified two fundamentally different kinds of situations within communities: those in which some species are strongly competitively superior are dominance controlled (equivalent to succession) and those in which all species have similar competitive abilities are founder controlled. Within the patch dynamics framework, the dynamics of these two situations are different and we deal with them in turn.
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