Trees need birds: forest fruitfulness depends on seedy living
A tree snake’s devastation of Guam’s birds provides a rare opportunity to study the effect of animal seed dispersal on forest diversity, writes Liz Drummond.
An unwinding ecological disaster on the Pacific island of Guam has provided a team of Australian and American scientists with a rare opportunity to quantify the importance of forest fruit lovers to maintaining the diversity and health of arboreal habitats.
The introduction of the brown tree snake (Boiga irregularis) to Guam in the 1950s led to the extinction of all the island’s native vertebrate seed dispersers apart from one small population of Micronesian starlings (Aplonis opaca) and a few bats. This species loss has provided a chance to gauge how Guam’s forests have been affected by the almost complete absence of animal seed dispersers, comparing to the forests the neighbouring Micronesian islands of Saipan and Rota, where animal seed dispersers remain relatively abundant.
About 70% of tree species across the three islands have fleshy fruits adapted for dispersal by birds. The research teams led by Elizabeth Wandrag of the University of Canberra created “canopy gaps” on each island, counting tree seedlings that grew in each gap after one year. Seedlings were classified as either having come from seedfall from nearby trees or from further afield via active dispersal.
Seedling diversity was consistently higher in the gaps on Rota and Saipan compared to Guam, according to the research published in the Proceedings of the National Academy of Science. The loss of vertebrate seed dispersers on Guam, the paper estimates, resulted in about half the number of tree species taking root.
On Guam the gaps were also generally populated by one species or another, while on Saipan and Rota species were more evenly distributed.
“Gaps in tropical forests are nurseries for the future,” Wandrag says. “These seedling species simply aren’t getting to the gaps and will therefore struggle to survive.”
“Losing native seed dispersers from the landscape could irrevocably change the way these forests look. Introduced species such as the brown tree snake could end up threatening not just local animals but ultimately entire ecosystems.”