ALLEE EFFECTS LIMIT POPULATION VIABILITY OF AN ANNUAL PLANT

Allee effects may be experienced by plants when populations are too sm all or isolated to receive sufficient pollinator services to replace t hemselves. This article reports experimental data from an annual herb, Clarkia concinna, documenting that small patches suffered reproductiv e failure due to lack of effective pollination when critical threshold s of isolation were exceeded. In contrast, sufficiently large patches attracted pollinators regardless of their degree of isolation. These d ata accord with data on patch extinctions showing that small and isola ted patches have a higher extinction rate than do large patches and wi th observations showing chronically low reproductive success in such p atches prior to extinction. While not conclusively demonstrating that Allee effects cause extinction in small and isolated patches, the data are suggestive. Although threshold effects have been postulated in se veral mathematical models of population viability, this is the first r eport of data from natural populations that display the occurrence of such thresholds. These results have implications for the management of endangered plants, which often are restricted to isolated, small popu lations, as well as suggesting a potential limit to spatial spread in plant populations dependent on animal vectors for reproduction.