The effect of complete bird loss on the forests of Guam

Background/Question/Methods

Birds are thought to provide essential ecosystem services including seed dispersal and control of insect herbivores. However, few studies have measured the importance of birds on a community-wide basis, in part because birds are difficult to manipulate experimentally at a scale relevant to their impact. We take advantage of a unique ecological catastrophe to study the role of birds in tropical forests.  Virtually all forest birds were extirpated from the island of Guam by the introduction of the Brown Treesnake, whereas three nearby islands support relatively healthy bird populations and thus, serve as suitable controls.  Our project investigates how the loss of frugivorous and insectivorous birds has affected Guam’s forests. 

Results/Conclusions

On Guam, about 70% of tree species have fruit adapted for bird-dispersal. Bird loss has consistent negative impacts on seed dispersal and seedling survival. First, in the two species we studied, germination of fruit unhandled by birds was significantly lower than germination of handled seeds. Second, seed dispersal distances are shorter on Guam compared to nearby islands with birds; this is likely to have a demographic impact due to higher seedling survival far from conspecific trees than near conspecifics. Third, gap dynamics may be altered due to the loss of dispersal by birds. Finally, using seed traps, we found complete cessation of seed rain from native forest into adjacent degraded forest; regeneration of degraded forest is expected to proceed slowly without native seed rain. 

The impact of bird loss on top down control of insect herbivores is less clear, possibly due to the complex response of the arthropod community. The strongest signal we have measured is in web-building spiders, which are more abundant on Guam than on islands with birds. However, bird loss does not consistently cascade to impact plants, as would be expected. In a bird exclosure study conducted across all four islands, we found reduced seedling survival when birds were excluded (on islands with birds) or missing (on Guam) in only one species, and no difference or increased survival in four other species. Additional research is needed to determine whether birds play a role in reducing insect outbreaks or preventing new invasive insects from establishing.

Collectively, our results show that impacts of bird loss reverberate through the forests of Guam, and that the effect of frugivorous bird loss has been more obvious and detrimental than that of insectivorous bird loss.