Is bird loss a big deal?
How bird loss reduces forest diversity
Bird loss on the island of Guam has reduced forest diversity and changed its structure according to research undertaken by Dr Lizzie Wandrag and Professor Richard Duncan of the IAE, in collaboration with Dr Amy Dunham (Rice University) and Dr Haldre Rogers (Iowa State University). The team investigated the importance of vertebrate seed dispersers on tropical forest diversity across the Mariana Islands in the North Pacific Ocean.
The Brown Tree Snake (Boiga irregularis) was introduced to the island of Guam in the 1940s and has since decimated local bird populations, driving all of the native species extinct. As a result Guam is unique in having lost all of its forest birds. These birds and other vertebrates would have previously played a key role in dispersing the seeds of more than 85% of Guam’s forest trees. The absence of birds on Guam provides a remarkable opportunity to understand how forests are affected by the loss of vertebrate seed dispersers by comparing forests on Guam with those of its neighbouring islands, Saipan and Rota, where birds are still present.
Dr Lizzie Wandrang and her colleagues found that canopy gaps on Saipan and Rota had a much greater diversity and more even distribution of seedlings than canopy gaps on Guam. In fact, the presence of seed-dispersing birds doubled local seedling species richness within gaps, and halved species turnover among gaps. On Guam, seedlings were much more patchily distributed and gaps had lower seedling diversity because seeds were not being moved around by birds.
“This study demonstrates the critical role of seed dispersal by birds in structuring tropical forests,” says Dr Lizze Wandrag. “Vertebrate species are declining in most tropical systems so this provides an example of what could happen in other places, such as Australian rainforests, if they lose their vertebrate seed dispersers.”
So yes, bird loss is a big deal. This study not only shows us what can happen when we lose critical ecosystem components, such as seed dispersers, but stresses the importance of keeping invasive predators out of local environments.
Image: Christa Shen taking a hemispherical photograph of a forest canopy (Christa Shen collection)