Context-dependent spatial sorting of dispersal-related traits in the invasive starlings (Sturnus vulgaris) of South Africa and Australia

Species undergoing range expansion frequently experience increased dispersal rates, especially among invasive alien species. Such increased dispersal rates have been attributed to ‘spatial sorting’, where traits enhancing dispersal assort towards the expanding range edge while traits enhacing competitiveness are favoured within the core range. To date no single study has compared patterns of spatial sorting across multiple continents for the same species. Here we compared patterns of spatial sorting in Sturnus vulgaris, the European starling (hereafter referred to as starlings), in its invasive ranges in South Africa and Australia. Starlings have experienced similar residence times in these two countries. Using multi-scale pattern analyses and generalized additive models, we determine whether dispersal and foraging traits (i.e. the morphological attributes of wings and bills) were sorted along the distance from introduction site. We found apparent patterns of spatial sorting in Australia, but not in South Africa. This difference may be attributed to differences in dispersal rates, clinal variation, environmental heterogeneity, and thus population demography on the two continents. Genetic data suggests that starlings in South Africa have experienced frequent long distance dispersal events, which could have diluted or overridden patterns of spatial sorting.