Effective dispersal is not straightforwardly density dependent at least in part because there are also selective forces in favor of not dispersing, but instead showing so-called philopatry or 'home-loving' behavior (Lambin et al., 2001). This can come about because there are advantages of inhabiting a familiar environment; or individuals may cooperate with (or at least be prepared to tolerate) related individuals in the natal habitat that share a high proportion of their genes; or individuals that do disperse may be confronted with a 'social fence' of aggression or intolerance from groups of unrelated individuals (Hestbeck, 1982). These forces, too, may become more intense as the environment becomes more saturated. Thus, for example, Lambin and Krebs (1993) found in Townsend's voles, Microtus townsendii, in Canada, that the nests or centers of activities of females that were first degree relatives (mother-daughters, littermate sisters) were closer than those that were second degree relatives (nonlittermate sisters, aunt-nieces), which were closer than those that were more distantly related, which in turn were closer than those not related at all.
And in a study of Belding's ground squirrels, Spermophilus beldingi, even when females dispersed, they tended to settle near their sisters (Nunes et al., 1997). Moreover, there are examples of fitness being higher when close kin are nearby. For instance, Lambin and Yoccoz (1998) manipulated the relatedness of groups of breeding females of Townsend's vole, mimicking either a situation where the population had experienced philopatric recruitment followed by high survival ('high kinship'), or where the population had experienced either low philopatric recruitment or high mortality of recruits ('low kinship'). Survival of pups, especially early in their life, was significantly higher in the high kinship than in the low kinship treatment.
Overall, then, the relationship between dispersal and density will depend, just like all other adaptations, on evolved compromises to conflicting forces, and also on which aspect of dispersal (emigration, effective dispersal, etc.) is the focus of attention. It is no surprise either that, as we shall see below, the balance of advantage works out differently for different groups: males and females, old and young, and so on. Such variation also argues against broad generalizations suggesting that dispersal is 'typically' at presaturation densities (i.e. before resource limitation is intense) or for that matter at saturation densities (Lidicker, 1975).
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