The role of bird-mediated dispersal in plant invasiveness

When species are introduced to new environments, their interactions with native species may determine their invasiveness. Bird-mediated dispersal facilitates plant establishment and spread by directing seed deposition to suitable habitats and creating long-distance dispersal events. The prevalence of existing mutualisms between birds and introduced plants, as well as implications for plant invasions, were assessed at three scales: national, regional, and by individual bird. A questionnaire distributed to ornithologists in four U. S. states yielded 1143 reports of unique interactions between particular bird and plant species. Of these, 539 involved potential seed dispersal. Reports enabled identification of information gaps where introduced species appear to be bird-dispersed but have not been studied for invasiveness. This analysis guided the selection of three non-native plants (Olea europaea, Ligustrum lucidum, and Triadica sebifera) for regional field observations of their interactions with resident birds in California's Sacramento Valley. All three species are invasive elsewhere and widely planted in California, but their invasiveness in the study region was unknown. Field observations included a native species, Heteromeles arbutifolia, for comparison. Most non-native fruit removal was performed by flocking species that travel long distances and by birds likely to serve as dispersers rather than seed predators. Although native fruits received the highest rate of bird visitation, birds consumed fruits of all species at levels sufficient to promote plant spread. To further probe bird feeding choices and their implications for plant invasiveness, an aviary study examined the individual preferences of birds representing three species. Native birds preferred native fruits independent of familiarity vwith the three target non-native fruits. This ruled out neophobia as the cause of the native fruit preference. This preference is thus likely to persevere as these non-native fruits become more common in the region. No such preference existed when native fruits were briefly compared with superficially similar non-natives, however, indicating that dispersal of such plants may exceed that of dissimilar non-natives. In combination, the three studies demonstrate widespread mutualisms between birds and introduced plants and suggest that increased use of native plants in landscaping and restoration might reduce the bird-mediated dispersal of some non-native plants.