The demographic significance of dispersal

The ecological fact of life identified in Section 4.1 emphasized that dispersal can have a potentially profound effect on the dynamics of populations. In practice, however, many studies have paid little attention to dispersal. The reason often given is that emigration and immigration are approximately equal, and they therefore cancel one another out. One suspects, though, that the real reason is that dispersal is usually extremely difficult to quantify.

The nature of the role of dispersal in population dynamics depends on how we think of populations. The simplest view sees a population as a collection of individuals distributed more or less continuously over a stretch of more or less suitable habitat, such that the population is a single, undivided entity. Dispersal is then a process contributing to either the increase (immigration) or decrease (emigration) in the population. Many populations, however, are in fact metapopulations; that is, collections of subpopulations.

We noted in Section 6.3.1 the ubiquity of patchiness in ecology and the importance of dispersal in linking patches to one another. A subpopulation, then, occupies a habitable patch in the landscape, and it corresponds, in isolation, to the simple view of a population described above. But the dynamics of the metapopulation as a whole is determined in large part by the rate of extinction of individual subpopulations, and the rate of colonization - by dispersal - of habitable but uninhabited patches. Note, however, that just because a species occupies more than one habitable site, each of which supports a population, this does not mean that those populations comprise a metapopulation. As we shall discuss more fully below, 'classic' metapopulation status is conferred only when extinction and recolonization play a major role in the overall dynamics.

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