As elephants go, so go the trees

A first-of-its-kind study led by researchers at the University of Florida shows that the dramatic loss of elephants, which disperse seeds after eating vegetation, is leading to the local extinction of a dominant tree species, with likely cascading effects for other forest life.

Their work shows that loss of animal seed dispersers increases the probability of tree extinction by more than tenfold over a 100-year period.

"The entire ecosystem is at risk," said Trevor Caughlin, a UF postdoctoral student and National Science Foundation fellow. "My hope for this study is that it will provide a boost for those trying to curb overhunting and provide incentives to stop the wildlife trade."

Caughlin, joined by colleagues from UF, the Conservation Ecology Program in Thailand, Wageningen University in the Netherlands and the Royal Thai Forest Department, published their study in the Proceedings of the Royal Society B showing how vital these animals are to maintaining the biodiversity of tropical forests in Thailand. The team looked specifically at seed dispersal and how elephants contribute to moving the seeds around the forest.

The elephant has long been an important spiritual, cultural and national symbol in Thailand. At the beginning of the 20th century, their numbers exceeded 100,000. Today, those numbers have plunged to 2,000. Elephants, as well as other animals such as tigers, monkeys and civet cats, are under attack from hunters and poachers, mostly for fabled properties of their organs, teeth and tusks.

Caughlin spent three years gathering tree data in Thailand. He looked at the growth and survival of trees that sprouted from the parent tree and grew up in crowded environs, compared with seeds that were transported and broadcasted widely across the forest by animals. The data were supplemented with a dataset from the Thai Royal Forest Department that contained more than 15 years of data on trees to create a long-term simulation run on UF's supercomputer, HiPerGator.

The team discovered that trees that grow from seeds transported by those animals being overhunted are hardier and healthier.

"Previously, it's been unclear what role seed dispersal plays in tree population dynamics," Caughlin said. "A tree makes millions of seeds during its lifetime, and only one of those seeds needs to survive to replace the parent tree. On the surface, it doesn't seem like seed dispersal would be that important for tree population. What we found with this study is that seed dispersal has an impact over the whole life of a tree."

Jeremy Lichstein, an assistant professor of biology at UF and one of the paper's authors, said, "Our study is the first to quantify the decades-long effects of animal seed dispersal across the entire tree life cycle, from seeds to seedlings to adult trees."

Richard Corlett, director of the Center for Integrative Conservation at the Xishuangbanna Tropical Botanical Gardens in Yunnan, China, underscored the study's importance.

"This study fills a major gap in our understanding of how overhunting affects forest trees, particularly in tropical forests," he said. "We knew hunting was bad, but we were not sure why it was bad, and therefore could not predict the long-term impacts. Now we know it is really, really bad and will get worse. The message that 'guns kill trees too' should help put overhunting at the top of the conservation agenda, where it deserves to be."