The presence of multiple introduced organisms can facilitate invasion and break down of mutualisms among native species. Dispersal by animal vectors of seeds of invasive plants is one such mutualism with the potential to modify the dynamics of plant invasions. In tropical systems, knowledge of dispersal networks is often limited and thus the consequences for plant invasions and native plant regeneration remain poorly understood. In the tropical high volcanic islands of the Society archipelago (French Polynesia), exotic and native birds consume the fruits of introduced and native plants. I am investigating their feeding habits and generating dispersal webs on two islands that are highly invaded by exotic plants and animals. Diets for the introduced Red-vented Bulbul (Pycnonotus cafer), the introduced Silvereye (Zosterops lateralis), and the native Gray-green Fruit Dove (Ptilinopus purpuratus) were determined through analysis of fecal samples and direct field observations. Seed viability was assessed using germination tests with seeds extracted from intact fruits and fecal samples.
Results/Conclusions
Birds consumed the fruits of 21 plant species, 13 of which are naturalized alien plants. The invasive tree Miconia (Miconia calvescens) was the most commonly consumed fruit for all three species. Nearly half of the total diet of the bulbul and the fruit dove and a quarter of the diet of the Silvereye was comprised of Miconia fruit. The native fruit dove consumed the fruit of 9 species of plant, 4 of which are introduced and naturalized. Germination trials showed that gut passage did not change Miconia germination rates for any of the three birds. In contrast, the seeds of two species of native plants showed significantly higher germination after passage by the native fruit dove. I suggest that both native and introduced birds in French Polynesia affect invasions by dispersing high numbers of viable seeds of many different alien plants over long distances. Additionally, the indirect impacts of changes in the diet of the native fruit dove in the presence of high concentrations of invasive fruit-bearing trees may have important consequences for the regeneration of native plant communities.
