Animal extinction upsets forest balance: UC study


Project Summary

By Marcus Butler

The extinction of some native seed-spreading animals has had a serious impact on Guam’s tree population, according to a University of Canberra-led study published today.

The introduction of the Brown Treesnake about 60 years ago almost entirely wiped out its native birds, unbalancing the Pacific island’s ecosystem.

Scientists with the University’s Institute for Applied Ecology (IAE) have examined the cascading effects that the devastation of the island’s bird life has had on its forests, particularly the spread of new trees.

Their results have been published by the highly-esteemed Proceedings of the National Academy of Sciences, the official journal of the United States National Academy of Sciences.

According to lead author and IAE’s postdoctoral fellow Dr Elizabeth Wandrag many tropical plants use fruit-eating animals, especially birds, to spread their seeds across large areas of land.

“The situation on Guam is unique; most of the island’s fruit-eating bird species have been wiped out since the introduction of the Brown Treesnake,” Dr Wandrag said. “But the nearby islands of Rota and Saipan are snake-free and have very similar trees, birds and bats to those that existed on Guam.

“This has given us the perfect comparative opportunity to see how seed dispersal by animals affects the plant diversity of the islands’ forests,” she said.

“We focused on tree seedlings growing under gaps in the forest canopy, because these are hotspots for tree regeneration, and found that the diversity of seedlings within these gaps was double when animals were involved in spreading the seeds.”

Studies of tree seed-dispersal have been notoriously difficult because in most places it is impossible to completely remove animal seed dispersers and see how forests change without them. However, the loss of birds in Guam has allowed important insights into the ecological role of dispersal.

Along with new perspectives on the importance of animals to tropical forests, Dr Wandrag said the study should raise the alarm for places like Guam where their native wildlife is under threat.

“Losing native seed dispersers from the landscape could irrevocably change the way these forests look. Introduced species such as the Brown Treesnake could end up threatening not just local animals, but ultimately entire ecosystems.”

“The gaps in tropical forests are nurseries for the future, where seedlings have a chance to grow,” Dr Wandrag said. “We found that without animals spreading seeds to these gaps there wasn’t just a decrease in diversity, but the seeds of some species that can only survive in high light areas like forest gaps simply weren’t getting to them.”

Collaborators in this research include IAE Centenary Professor Richard Duncan, Amy Dunham, Assistant Professor in BioSciences with the Department of Biosciences at Rice University in Houston, Texas and Dr Haldre Rogers, Assistant Professor with the Department of Ecology, Evolution, and Organismal Biology at Iowa State University.

Image: Christa Shen taking a hemispherical photograph of a forest canopy (Christa Shen collection)