Reconstructing 50 years of Opuntia stricta invasion in the Kruger National Park, South Africa:: environmental determinants and propagule pressure

Many factors influence the spread dynamics and distribution of invasive alien organisms. Despite progress in unravelling the determinants of invasiveness and invasibility, robust, spatially-explicit predictive models for explaining real-world invasion dynamics remain illusive. Reconstructing invasion episodes is a useful way of determining the roles of different factors in mediating spread and proliferation. In many cases, however, human-aided dispersal and other anthropogenic factors blur the roles of natural controlling factors. We describe the reconstruction of an isolated invasion event from a known source: the 50-year invasion history of Opuntia stricta in the Kruger National Park. Our aim was to explore the relative roles of environment and propagule supply in shaping the invasion pattern. Environmental variables (landscape heterogeneity and distance from water sources) were moderately useful for explaining the presence/absence of O. stricta in 1-ha cells across the 660 km(2) (53% of cells correctly classified). Adding fire frequency increased the accuracy of the model (68%). However, when we considered the role of propagule pressure (measured as the distance of sites from the known primary invasion focus and putative secondary invasion foci), model accuracy was greatly improved (77%). No environmental variables or propagule pressure correctly explained spatial variation in abundance (expressed as cladode density in 1-ha cells). We discuss implications of the importance of propagule supply for modelling and managing invasions.