PATTERNS OF SUCCESS IN PASSERIFORM BIRD INTRODUCTIONS ON SAINT-HELENA

Ecologists have long attempted to predict the success of species that are introduced into foreign environments. Some have emphasized qualiti es intrinsic to the species themselves, whereas others have argued tha t extrinsic forces such as competition may be more important. We test some of the predictions made by both the extrinsic and intrinsic hypot heses using passeriform birds introduced onto the island of Saint Hele na. We found direct evidence that extrinsic forces are more important predictors of successful invasion. Species introduced when fewer other species were present were more likely to be successful. In a direct t est of the alternative hypothesis that intrinsic forces play a more pr ominent role in success or failure, we found a tendency for species wh ich successfully established on Saint Helena to be also successful whe n introduced elsewhere. However, the vast majority of species unsucces sful at establishing on Saint Helena had probabilities of success outs ide Saint Helena of 50% or greater, making this result somewhat equivo cal. Finally, we found no evidence to support the hypothesis that spec ies that are successful early are those that are intrinsically superio r invaders. These results are consistent with similar analyses of the introduced avian communities on Oahu, Tahiti, and Bermuda.