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It is not surprising that dispersal of individuals or propagules at some stage of the life cycle is a characteristic of most biological species. An organism requires some finite space in the environment from which resources neces sary for its survival are acquired. The size of this space may fluctuate with changes in resource availability and population density, but there exists some critical minimum space that must be maintained if the organism is to persist. Although resources within this space may be diverted into repro duction and the rearing of young, the continued occupancy of the parent's resource space by the offspring, or vice versa, is a threat to survival. Yet the movement of either parent or offspring away from the natal site is likely to place the disperser at risk. The dispersal of individuals can have profound effects on the genetic structure and demographic processes of a population. The movement of individuals between populations facilitates gene flow and the maintenance of genetic variability (56, 76). Since dispersal promotes the utilization of new or depopulated areas, it can facilitate the persistence of local popula tions and therefore the stability of the species over large geographic areas (136, 139). In spite of this, ecologists until recently have largely neglected dispersal as a demographic parameter that affects population density. This may be because we cannot distinguish between natality and immigration or
Gaines et al. (Sat,) studied this question.