Explaining and predicting the range of introduced plants: Perspectives from South Africa

Once established in a new region, the fate of an introduced species is mediated by complex interactions between traits of the species, various environmental factors, and the ways that human activities affect dispersal and regeneration opportunities.   This paper reports on several recent and ongoing studies aimed at elucidating the roles of these factors using the natural experiment of plant introductions and invasions in South Africa.  Data from the South African Plant Invader Atlas (SAPIA) database and from other sources were used to explore fundamental determinants of invasive success expressed as the size of the adventive range.  We found that the range size of an invasive species depends on how much time it has had to spread (its residence time). Importantly however, range size and spread rates are also mediated by the total extent of suitable (i.e. potentially invasible) habitat and the frequency and intensity of introductions (propagule pressure), the position of founder populations in relation to the potential range, and the spatial distribution of the potential range.  Relationships between environmental factors and intrinsic and human-mediated attributes of invasive plant species were investigated using a novel multivariate approach that enabled us to define clusters of species with similar determinants of geographic range.  After accounting for environmental factors, the spatial pattern of invasive plants in South Africa can be explained by human uses, life forms, and reproductive traits.  Further insights are emerging from studies of phylogenetic patterns in invasions at different spatial scales and from phylogeographic studies of key taxa.