Avian dispersal of exotic shrubs in an archipelago

Abstract

The distribution of exotic species bearing fruits may depend in part on their palatability to birds, because species attracting a more diverse array of dispersers should broadcast seed to a wider range of habitats, including those otherwise assumed to represent refuges from exotic species invasion, such as small islands. To test these ideas, we compared the palatability of 11 exotic shrub species to their occurrence on 91 islands of varying size and isolation in the Gulf Islands of British Columbia, Canada and the San Juan Islands of Washington, USA. The estimated palatability of fruits for 10 common frugivorous birds was positively related to the number of islands on which they occurred. Rubus armeniacus and R. laciniatus were the most widely consumed exotic species and the only such species prevalent on small islands, suggesting that they have benefited by attracting a wide array of frugivores that have, in turn, facilitated their rapid colonization of the region. Our results suggest that protecting small islands from human visitation will be insufficient to prevent exotic species establishment and the loss of native biodiversity.