Case study: The impact of complete frugivore loss on the island of Guam
All native vertebrate frugivores (mostly birds) were functionally extirpated from the island of Guam by the invasive Brown Treesnake (Boiga irregularis). Our research takes advantage of this unique mass extirpation by comparing Guam to three nearby islands that have relatively intact frugivore communities. We investigate how the loss of vertebrate frugivores affects plant recruitment on a population-level, and then consider how these effects scale up to the forest community. In other systems, vertebrate frugivores have been shown to increase germination rates through handling, move seeds away from high mortality near the parent tree, move seeds to microsites suitable for germination, lay the template for the spatial pattern of trees, and assist in colonization of new areas. The loss of vertebrate frugivory thus is likely to have widespread effects on the populations of fleshy-fruited tree species. We used nursery experiments to determine the effect of handling on germination and seed and seedling additions to assess whether survival is increased when seeds are moved away from the parent tree. We also compared the abundance of pioneer species that necessitate high-light microsites, the spatial pattern of trees, and native seed rain into degraded areas between islands with and without vertebrate frugivores.
Vertebrate frugivore loss has led to widespread effects at the population level, and may be responsible for changes in the overall forest community. Ingestion by vertebrate frugivores doubled to quadrupled the chance of germination for both species we tested. In addition, seeds were dispersed significantly farther from the parent tree on islands with birds than on Guam, where seeds rarely landed beyond the canopy of the parent tree. Four out of five species of seedlings planted away from conspecifics experience higher survival, suggesting that disperser loss will have demographic effects. Pioneer species are significantly less abundant in the forests on Guam, although there are twice as many forest gaps present where pioneers could grow. There is some indication that spatial pattern of trees is starting to differ on Guam compared to Saipan, Tinian and Rota. Colonization of new habitat by native, fleshy-fruited tree species is unlikely to occur on Guam due to complete cessation of avian seed dispersal, although non-native pigs may be dispersing some species. Collectively, the impact of disperser loss on Guam is likely to produce major changes in the spatial pattern, abundance and diversity of trees within the forest, as well as changes in overall forest structure.