The impact of bird loss on seed dispersal in the forests of Guam

Background/Question/Methods

Birds make up a small percentage of the biomass of forest ecosystems, yet are thought to provide essential services including pollination, seed dispersal and control of insect herbivores. However, few studies have measured the importance of birds on a community-wide basis. The Mariana Islands of Guam, Saipan, Tinian and Rota offer a unique opportunity to study the role of birds in tropical forests. Virtually all forest birds were extirpated from Guam between 1945 and 1990 by the introduction of the Brown Treesnake (Boiga irregularis) whereas the nearby islands of Saipan, Tinian and Rota support relatively healthy bird populations. We are investigating how the loss of frugivorous birds has affected plant recruitment by comparing seed rain and seedling recruitment patterns on Guam to those on Saipan, Tinian and Rota. We measured the seed shadows for Premna obtusifolia, a bird-dispersed tree species, on all islands using seed rain traps placed under the canopy and at 1, 5, 10 and 20 meters from the canopy. To determine if differences in dispersal distance could be observed at the seedling stage, we randomly selected points in native forest on all islands, found the first seedling of each species within 10 meters from that point, and measured the distance between the seedling and its nearest conspecific adult (i.e. most likely parent). We present results only for species with sufficient sample sizes across all islands.
Results/Conclusions

Results from the seed rain traps indicate a reduction in long-distance dispersal on Guam compared to nearby islands with birds. The effect of disperser loss is also apparent at the seedling stage. Seedlings of the native bird-dispersed tree species (Aglaia mariannensis) were found primarily within 2 meters and exclusively within 5 meters of conspecific adult trees on Guam, whereas seedlings were two to three times further from the nearest conspecific tree on islands with birds (t-test: p<0.001). These species-specific changes in spatial pattern due to the loss of birds are likely to have large effects on the forest community, as roughly 70% of native forest trees have bird-dispersed seeds. Our results suggest that the full effects of the Brown Treesnake on the forests of Guam’s may far exceed their direct impact on the bird fauna; the indirect impacts caused by the loss of ecosystem services performed by frugivorous birds are more subtle, but have potential to be equally devastating to the forest community.