The effect of complete bird loss on herbivory and plant recruitment

Birds make up a small percentage of the biomass of forest ecosystems, yet are thought to provide essential ecosystem services such as seed dispersal and control of herbivores. While some studies have used disrupted seed disperser mutualisms or bird exclosures to investigate the role of birds, few have been able to do this on a community-wide basis. The Mariana Islands of Guam, Rota and Saipan offer a unique opportunity to study the role of birds in tropical forest since virtually all forest birds were extirpated from Guam within the last 30 years by the introduction of the Brown Treesnake while Saipan and Rota have maintained relatively healthy bird populations. We surveyed 25x25 meter plots in native limestone forest on these three islands for adult trees (>40mm dbh) and nine 3x3 meter subplots for juvenile trees (<40mm dbh) within each 25m² plot. We measured herbivory on three species common to all islands using branch sampling. Seventy-five percent (3/4) of the plots on Saipan and Rota contained at least one species as a juvenile but not as an adult, while 0% (0/2) of the plots on Guam contained juveniles not represented as adults, indicating that long-distance dispersal events may be rarer on Guam than on the other islands, particularly in bird-dispersed species. We also found species-specific differences in recruitment rates and herbivory between Guam and the northern islands. This comparative study suggests strong differences between Guam and nearby islands, possibly due in part to the loss of birds. Future experimental studies will address the mechanism responsible for these differences in recruitment and herbivory.