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Since this paper deals with dispersals of large mammals, it is useful to reflect on what causes a dispersal and how. Many use the word migration for what is called here dispersal. However, it has been argued to restrict the term migration to the cyclic, generally seasonal, geographic movements of individuals or populations (Tchernov, 1992a). Also the words expansion and reduction are used to describe changes in the geographic distribution of a taxon. Here these words are taken as including any change in the distribution, even if gradual and minor, while dispersal is reserved for large scale expansions within a short time. When a species expands or disperses, it occupies a new area. In continental mammals, this generally does not imply that any individual travels a long distance, though there are cases when this may have occurred, e.g. when an individual or group of individuals accidentally crossed an extensive area with a habitat that was not favourable (Simpson, 1940, Tchernov, 1992a). These types of dispersal are also called diffusion and jump dispersal (Brown and Lomolino, 1998). Dispersal across barriers can be through corridors (geographically restricted areas), filters (areas that can be crossed by only part of the species under consideration) and sweepstake (accidental dispersal, which only occasionally is successful) (Simpson, 1940, Brown and Lomolino, 1998), but alternatively a suitable habitat may expand through environmental change and no barrier needs to be crossed.
A healthy population of a species that is well adapted to its environment produces more offspring than what is needed to maintain population size. The individuals that are best fitted to this environment have the best chances to become adult, to obtain good territories and good resources and are likely to procreate, while the less fit individuals have least chances to obtain these. A species is likely to “fill” an area with a favourable environment, and in a stable situation the excess of births over deaths is balanced by individuals that will not arrive to procreate. This excess will allow a healthy population to recover from a disaster like an epidemic or a drought, but also will allow expansion when new areas or niches become available. The expansion into a new area is derived from the birth excess in the border area of the population. The width of this border area is determined by the distance which individuals may travel from their birthplace to find a new territory to occupy. The foregoing implies that diffusion into new areas is driven by the availability of new habitats and not by “demographic pressure” somewhere in the distant hinterland.