Invasion dynamics

In almost every aspect of life, there is a danger in imagining that what is usual and 'normal' is in fact universal, and that what is unusual or eccentric can safely be dismissed or ignored. Every statistical distribution has a tail, however, and those that occupy the tail are as real as the conformists that outnumber them. So it is with dispersal. For many purposes, it is reasonable to characterize dispersal rates and distances in terms of what is typical. But especially when the focus is on the spread of a species into a habitat that it has previously not been occupied, those propagules dispersing furthest may be of the greatest importance. Neubert and Caswell (2000), for example, analyzed the rate of spread of two species of plants, Calathea ovandensis and Dipsacus sylvestris. In both cases they found that the rate of spread was strongly dependent on the maximum dispersal distance, whereas variations in the pattern of dispersal at lesser distances had little effect.

This dependence of invasion on rare long-distance dispersers means, in turn, that the probability of a species invading a new habitat may have far more to do with the proximity of a source population (and hence the opportunity to invade) than it does on the performance of the species once an initial bridgehead has been established. For instance, the invasion of 116 patches of lowland heath vegetation in southern England by scrub and tree species was studied for the period from 1978 to 1987 (Figure 6.15) and also from 1987 to 1996 (Nolan et al., 1998; Bullock et al., 2002). There were four types of heath - dry, humid, wet and mire - and with the importance of eccentric dispersers

Figure 6.14 Diagrammatic representation of variations in mortality and seed production of Cakile edentula in three areas along an environmental gradient from open sand beach (seaward) to densely vegetated dunes (landward). In contrast to other areas, seed production was prolific at the seaward site. Births, however, declined with plant density, and where births and deaths were equal, an equilibrium population density can be envisaged, N*. In the middle and landward sites, deaths always exceeded births resulting from local seeds, but populations persisted there because of the landward drift of the majority of seed produced by plants on the beach (seaward site). Thus, the sum of local births plus immigrating seeds can balance mortality in the middle and landward sites, resulting in equilibria at appropriate densities. (After Keddy, 1982; Watkinson, 1984.)

Figure 6.14 Diagrammatic representation of variations in mortality and seed production of Cakile edentula in three areas along an environmental gradient from open sand beach (seaward) to densely vegetated dunes (landward). In contrast to other areas, seed production was prolific at the seaward site. Births, however, declined with plant density, and where births and deaths were equal, an equilibrium population density can be envisaged, N*. In the middle and landward sites, deaths always exceeded births resulting from local seeds, but populations persisted there because of the landward drift of the majority of seed produced by plants on the beach (seaward site). Thus, the sum of local births plus immigrating seeds can balance mortality in the middle and landward sites, resulting in equilibria at appropriate densities. (After Keddy, 1982; Watkinson, 1984.)

Figure 6.15 The invasion (i.e. increase in abundance) of most of the 116 patches of lowland heath in Dorset, UK, by scrub and tree species between 1978 and 1987. Coastland is to the south and the county boundary to the east. (After Bullock et al., 2002.)

Figure 6.15 The invasion (i.e. increase in abundance) of most of the 116 patches of lowland heath in Dorset, UK, by scrub and tree species between 1978 and 1987. Coastland is to the south and the county boundary to the east. (After Bullock et al., 2002.)

two periods, eight data sets on which an analysis could be carried out. For six of these, a significant proportion of the variation in the loss of heath to invading species could be accounted for. The most important explanatory variables were those describing the abundance of scrub and tree species in the vegetation bordering the heath patches. Invasions, and thus the subsequent dynamics of patches, were being driven by initiating acts of dispersal.

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