Rapid changes in dispersal on a small spatial scale at the range edge of an expanding population

In expanding populations, the allocation of resources to life-history traits is expected to change rapidly after the colonization of a new area. Understanding these changes is of crucial importance to predict the future changes in distribution ranges, and the possible impacts of expanding species on the colonized environments. Both theoretical and empirical studies have provided some evidence for an increased dispersal at the range edge of expanding populations. Because this phenomenon has been documented on large spatial scales, it seems difficult to determine what proportion of the phenotypic variation is due to the expansion process (spatial sorting) versus spatial variation in the environment (local adaptation). The aim of our study is to determine whether, in a small and recent expanding population, the dispersal capacity is increased at the range edge compared to the range core despite a minimal role of local adaptation. We studied the modification of dispersal capacity at the range edge of an expanding population of the African clawed frog, Xenopus laevis recently settled in Western France. To do so, we used experiments to compare movements of individuals depending on their location in the population, and capture–mark–recapture field surveys to estimate variation in traits related to dispersal across the invasive range. Both methods consistently showed significantly higher dispersal rates and distances, as well as in the relative length of the hind limb, at the range edge, for both sexes. This result is the first report suggesting that changes in allocation to dispersal, and not only traits associated with dispersal, can occur at a small geographical and temporal scale, where the role of local adaptation is likely minimal. This study fills a gap in an issue where it is currently crucial, for conservation purposes, to understand the mechanisms involved in range expansions.