| The economic and environmental damage wrought by plant invaders fosters much interest in identifying the rare combinations of attributes, environmental factors and circumstances that transforms an immigrant population into an invasion. Environmental stochasticity in the recipient range is a major component in this transformation because immigrant plant populations are likely vulnerable to environmental stochasticity. These populations are usually small and cannot withstand recurring plant losses from random environmental events. I hypothesized that founder population characteristics, cultivation, and disturbance can mitigate hazardous abiotic and biotic factors; providing a prolonged opportunity for population growth, dispersal, and eventual persistence in a new range.; Using a series of three-year factorial experiments, I investigated the effects of different cultivation regimens on the persistence of founder populations of four alien species (Fagopyrum esculentum, Trifolium incarnatum, Helianthus annuus, and Echinochloa crus-galli). To evaluate the role of disturbance, populations of Hordeum vulgare were introduced during autumn and spring into four habitats with different regimes of disturbance intensity and frequency (undisturbed native communities, vacant lots, roadsides, plowed fields). To evaluate the effects of different microenvironments on establishment, populations of F. esculentum, Pisum sativum, and H. annuus were buried in soil, sown on the soil surface or deposited in cow feces, in both native steppe and forest communities.; Larger founder population size resulted in a greater likelihood of establishment; however, a minimum viable population size was not demonstrated. Cultivation (i.e. protection from vertebrates, irrigation) increased the likelihood of establishment as gauged by the net reproductive rate, R0. Disturbance increased propagule survival and R0. Microsites differences (i.e. seed burial or deposition in cow feces) altered survival, but did not result in significant increases in R0. In each experiment, the level of cultivation or disturbance required for R0 > 1.0 differed among years and species. These results underscore the importance of environmental stochasticity in determining the fate of founder populations and the potential of cultivation, disturbance, and large population size to counter the long odds against naturalization. An experimental framework for prediction will need to pay ample attention to the extent and character of cultivation and disturbance that the immigrants might receive. |
