Rewilding to restore ecological function is gaining attention as a conservation tool. Guam is a prime candidate for rewilding given the invasion of Brown Treesnakes (BTS) and subsequent extirpation of almost all native avian species has severely altered ecosystem structure and function. The loss of avian frugivores resulted in major disruptions to the seed dispersal network, leading to altered forest dynamics as the majority of tree species rely on birds for seed dispersal. Prior to BTS introduction there were four main frugivorous birds native to Guam: Bridled White-eye, Micronesian Starling, Mariana Fruit-dove, and White-throated Ground-dove. We sought to evaluate the effectiveness of each frugivore in order to prioritize restoration efforts.
BTS have yet to reach the nearby island of Saipan, leaving the avian community relatively intact. In addition to the four frugivore species found on Guam, an additional important frugivorous species (Golden White-eye) also occurs on Saipan. To assess the diet of all five frugivores, we collected fecal samples from wild-caught birds and fed seeds to captive birds. We paired movement patterns from radio telemetry with gut passage times (GPT) to build species-specific dispersal kernels and compare potential seed dispersal distances among species.
Results/Conclusions
Fecal samples revealed that Mariana Fruit-doves, Micronesian Starlings and Golden White-eyes account for the majority of avian interactions with plant species (32 of 37 interactions). Fecal samples along with captive feeding trials showed White-throated Ground-doves mainly act as seed predators and Bridled White-eyes only ingest seeds of a limited subset of tree species. For the two remaining frugivores historically on Guam, GPT ranged from 2-141 min, with a mean GPT across tree species of 36 ± 20.5 min for Micronesian Starlings and 53 ± 35.3 min for Mariana Fruit-doves. Even though GPT was shorter in starlings, they moved farther than fruit-doves over any time interval resulting in higher mean seed displacement for starlings (163.4± 148.7 m) relative to fruit-doves (95.0 ± 104.7 m).
In summary, two species formally found on Guam are effective seed dispersers on Saipan, with starlings dispersing seeds over 1.5 times further away from the source tree than fruit-doves. Two additional frugivores likely disperse few seeds because they act as seed predators or have morphological restrictions to seed ingestion. Finally, Golden White-eyes were not historically found on Guam but represent a major seed disperser on Saipan. Our study illustrates the importance of studying candidate species before instigating rewilding to restore ecological functions.