A biogeographical approach to plant invasions : ecology and evolution of an invasive ruderal in native and introduced ranges

Abstract

Most theory and empirical research on exotic invasions is based on the assumption that problematic exotics are much more abundant in regions where they invade than in regions where they are native. The overwhelming majority of studies on exotics, however, have been conducted solely within the introduced range. I argue and demonstrate that our understanding of invasions is greatly enhanced by comparative studies of exotics in both introduced and native ranges. The role of disturbance in plant invasions illustrates particularly well the need for studying invasions in a biogeographical context. In field experiments conducted in the native range of Centaurea solstitialis and in two regions of its non-native range, I show that disturbance increased C. solstitialis abundance and performance far more in the non-native ranges than in the native range. Stronger positive effects of disturbance on C. solstitialis abroad than at home indicate that disturbance alone cannot explain the remarkable success of this species in disturbed sites in its non-native regions. The powerful effects of disturbance must act in concert with other factors, allowing C. solstitialis to attain community dominance only where it occurs as exotic. A second study further reveals the importance of studying exotics in their native and introduced range. In common garden experiments, I demonstrate a genetically-based shift in the germination strategy of invasive populations of C. solstitialis. When introduced to a region with a Mediterranean climate, resembling that in the native range, C. solstitialis exhibits similar germination rates as native populations; in contrast, populations introduced to a region with spring and summer rainfall have higher levels of seed dormancy than native populations. Genetic differentiation among these populations, in combination with high genetic variation in non-native populations, a history of multiple introductions from largely overlapping sources, and the outcrossing mating system of C. solstitialis , strongly suggest that increased seed dormancy in non-Mediterranean populations is the result of rapid evolution in response to novel selection pressures. I believe this is just the tip of the iceberg; major developments in invasion biology, ecology, and evolution are likely to come from the study of organisms in native and non-native ranges.