When range expansion rate is faster in marginal habitats

Knowledge about the potential spatial dynamics of species is important. Already in 1922, the American ornithologist Joseph Grinnell (Grinnell 1922) noted that in order to understand the population consequences of dispersal, its role in the stochastic nature of rare longdistance movements and the interplay between innate and environmental factors in triggering dispersal, have to be known. Furthermore, Grinnell claimed, we need knowledge of the role of dispersal in the colonisation (or re-colonisation) of unoccupied areas of suitable habitats. Although many animals are regarded as relatively sedentary and specialised in marginal parts of their geographical distribution (Thomas et al. 2001), several species (Spivak et al. 1991, Hill et al. 1999) develop more dispersive forms at range fronts, which increases rates of range expansion. Such physiologically plastic responses to new environments are unlikely to occur in larger species (Weisser 2001). Here we report that a medium-sized cervid / the roe deer (Capreolus capreolus ) increases its expansion rate 3to 20-fold in marginal habitats by performing a conditional dispersal strategy in accordance with the balanced-exchange dispersal model (Doncaster et al. 1997). A bi-sexual dispersal behaviour (Wahlström and Liberg 1995a, Linnell et al. 1998) and an inverse relationship between local patch quality and local dispersal probability are shown to have profound implications for expansion rates, by decreasing the rate of local extinction (i.e. the rescue effect, sensu Brown and Kodric-Brown 1977) and increasing the rate of colonisation (i.e. the establishment effect, sensu Lande et al. 1998).