The fate of a silent forest: The effects of complete bird loss on the forest of Guam.

摘要

The abundance and diversity of birds are declining around the world, with unknown consequences for the communities they inhabit. Frugivorous birds provide an ecosystem service by consuming fruit and dispersing seeds, and insectivorous birds are top predators of arthropods, potentially with effects cascading down the food web. However, the impact of the decline of birds on a community-wide basis is unknown. Here, we take advantage of an unplanned experiment, the functional extirpation of birds by the invasive Brown Tree Snake (Boiga irregularis) from the Pacific Island of Guam, compared with three nearby islands with intact bird communities (Saipan, Tinian and Rota), to examine the ecological importance of frugivorous and insectivorous forest birds. First, we deploy seed traps to show that seeds on Guam are not dispersed away from the parent tree and that bird ingestion of seeds increases germination rates (Ch.1). Next, we use a manipulative field experiment to estimate seedling mortality near and far from a conspecific tree, suggesting that the loss seed dispersers will impact seedling recruitment by increasing losses to density-dependent mortality close to parent trees (Ch.3). Finally, we show that the loss of insectivorous birds is correlated with an increase in spider abundance on the island of Guam when compared to Saipan, Tinian and Rota (Ch.2). Collectively, this work demonstrates that the loss of birds from the forests of Guam has triggered a suite of indirect impacts which may dramatically change the spatial distribution and species composition of Guam's forests in the years to come.