When Invasions Collide: Minimal Indirect Effects of Invasive Snakes on Rodent-Mediated Seed Predation

While invasive species can facilitate the establishment of other invasive species and exacerbate negative impacts on native ecosystems, they may alternatively control other invasive species, producing a net benefit for native species. In Guåhan (also known as Guam), the invasive brown treesnake, Boiga irregularis, is most famous for its role in the decline of native birds. However, the snake also reduced numbers of nonnative seed predators such as rats (Rattus spp.) and mice (Mus musculus) compared to nearby islands that lack snakes. Here, we explore whether, in Guåhan, the snake has indirectly benefitted trees by reducing seed predation by nonnative rodents. We placed trays of seeds from six forest tree species at multiple forest sites in the islands of Guåhan, which has snakes, and Saipan, which has no established population of snakes and approximately five times as many rats. Surprisingly, no significant differences in seed removal were detected between islands, except for one nonnative species, papåya (Carica papaya), which was removed in higher proportions from trays in Saipan. Overall, seed removal was low—less than half of the seeds of all species on both islands were removed from our trays, indicating that these tree species may not suffer from high rodent predation rates. Our results did not show indirect benefits of snakes for native forest tree species via reduced seed predation, at least in the species we tested. Thus, we did not find evidence of beneficial cascading effects from one invasive species controlling another.