Dominancecontrolled communities

In patch dynamics models where some species are competitively superior to others, the effect of the disturbance is to knock the community back to an earlier stage of succession (Figure 16.16). The open space is colonized by one or more of a group of opportunistic, early successional species (p p2, etc., in Figure 16.16). As time passes, more species invade, often those with poorer powers of dispersal. These eventually reach maturity, dominating mid-succession (m1, m2, etc.) and many or all of the pioneer species are driven to extinction. Later still, the community regains the climax stage when the most efficient competitors (cl, c2, etc.) oust their neighbors. In this sequence, diversity starts at a low level, increases at the mid-successional stage and usually declines again at the climax. The gap essentially undergoes a minisuccession.

Some disturbances are synchronized, or phased, over extensive areas. A forest fire may destroy a large tract of a climax community. The whole area then proceeds through a more or less synchronous succession, with diversity increasing through the early colonization phase and falling again through competitive exclusion as the climax is approached. Other disturbances are much smaller and produce a patchwork of habitats. If these disturbances are unphased, the resulting community comprises a mosaic of patches at different stages of succession. A climax mosaic, produced by unphased disturbances, is much richer in species than an extensive area undisturbed for a very long period and occupied by just one or a few dominant climax species. Towne (2000) monitored the plant species that established in prairie grassland where large ungulates had died (mainly bison, Bos bison). Scavengers remove most of the body tissue but copious amounts of body fluids and decomposition products seep into the soil. The flush of nutrients combined with death of the previous vegetation produces a competitor-free, disturbed area where resources are unusually abundant. The patches are also exceptional because the soil has not been disturbed (as it would be after a ploughed field is abandoned or a badger makes a burrow); thus, the idea of a successional mosaic disturbance ... gaps . . . dispersal . . . recruitment dominance control and succession disturbance scale and phasing

Figure 16.16 Hypothetical minisuccession in a gap. The occupancy of gaps is reasonably predictable. Diversity begins at a low level as a few pioneer (p^ species arrive; reaches a maximum in mid-succession when a mixture of pioneer, mid-successional (mf) and climax (q) species occur together; and drops again as competitive exclusion by the climax species takes place.

Figure 16.16 Hypothetical minisuccession in a gap. The occupancy of gaps is reasonably predictable. Diversity begins at a low level as a few pioneer (p^ species arrive; reaches a maximum in mid-succession when a mixture of pioneer, mid-successional (mf) and climax (q) species occur together; and drops again as competitive exclusion by the climax species takes place.

the colonizing plants do not derive from the local seed bank. The unusual nature of the disturbed patches means that many of the pioneer species are rare in the prairie as a whole, and carcass sites contribute to species diversity and community heterogeneity for many years.

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