Naturalization of plant populations: the role of cultivation and population size and density.

Field experimentation is required to assess the effects of environmental stochasticity on small immigrant plant populations-a widely understood but largely unexplored aspect of predicting any species' likelihood of naturalization and potential invasion. Cultivation can mitigate this stochasticity, although the outcome for a population under cultivation nevertheless varies enormously from extinction to persistence. Using factorial experiments, we investigated the effects of population size, density, and cultivation (irrigation) on the fate of founder populations for four alien species with different life history characteristics (Echinochloa frumentacea, Fagopyrum esculentum, Helianthus annuus, and Trifolium incarnatum) in eastern Washington, USA. The fate of founder populations was highly variable within and among the 3 years of experimentation and illustrates the often precarious environment encountered by plant immigrants. Larger founder populations produced more seeds (P < 0.001); the role of founder population size, however, differed among years. Irrigation resulted in higher percent survival (P < 0.001) and correspondingly larger net reproductive rate (R(0); P < 0.001). But the minimum level of irrigation for establishment, R(0) > 1, differed among years and species. Sowing density did not affect the likelihood of establishment for any species. Our results underscore the importance of environmental stochasticity in determining the fate of founder populations and the potential of cultivation and large population size in countering the long odds against naturalization. Any implementation of often proposed post-immigration field trials to assess the risk of an alien species becoming naturalized, a requisite step toward invasion, will need to assess different sizes of founder populations and the extent and character of cultivation (intentional or unintentional) that the immigrants might receive.