Summary

We distinguish between dispersal and migration, and within dispersal between emigration, transfer and immigration.

Various categories of active and passive dispersal are described, including especially passive dispersal in the seed rain and the guerrilla and phalanx strategies of clonal dispersers.

Random, regular and aggregated distributions are explained, and the importance of scale and patchiness in the perception of such distributions is emphasized, especially in the context of environmental 'grain'. Forces favoring and diluting aggregations are elaborated, including the theory of the selfish herd and density-dependent dispersal.

We describe some of the main patterns of migration at a range of scales - tidal, diurnal, seasonal and intercontinental - including those that recur repeatedly and those that occur just once.

We examine dormancy as migration in time in both animals (especially diapause) and plants. The importance of photoperiod in the timing of dormancy is emphasized.

The relationship between dispersal and density is examined in detail. The roles of in- and outbreeding in driving density dependences are explained, including especially the importance of avoiding kin competition on the one hand and the attractions of philopatry on the other.

We describe a variety of types of variation in dispersal within populations: polymorphisms and sex- and age-related differences.

We turn to the demographic significance of dispersal and introduce the concept of the metapopulation composed of a number of subpopulations. Dispersal can be incorporated into the dynamics of populations, and modeled, in three different ways: (i) an 'island' or 'spatially implicit' approach; (ii) a spatially explicit approach that acknowledges that the distances between patches vary; and (iii) an approach treating space as continuous and homogeneous.

Probably the most fundamental consequence of dispersal for the dynamics of single populations is the regulatory effect of density-dependent emigration. It is important also, though, to recognize the importance of rare long-distance dispersers in invasion dynamics.

Metapopulation theory developed from the earlier concept of the uninhabited habitable patch. Its origin as a concept in its own right was the Levins' model, which established the most fundamental message: that a metapopulation can persist, stably, as a result of the balance between random extinctions and recolon-izations, even though no subpopulations are stable in their own right.

Not all patchily distributed populations are metapopulations, so we address the question 'When is a population a metapopulation?', which may be particularly problematic with plant populations.

Finally, we explore the dynamics of metapopulations, emphasizing especially the likely importance of alternative stable equilibria.

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