Overhunting in tropical forests may not put carbon storage capacity at risk

In tropical rain forests, as many as two-thirds of all tree species rely on native, fruit-eating mammals such as capuchin monkeys who are able to travel fairly large distances and carry any ingested seeds far from their parent trees. What happens to the forests when these seed dispersing animals are over hunted? Credit: Varun Swamy
27 February 2018

BONN, Germany (Landscape News) — Some scientists predict that due to the significant role animals play in seed dispersal, overhunting will cause irreparable harm to the capacity of tropical rainforests to store carbon, increasing the planet’s vulnerability to global warming.

Now, new research published in the Journal of Ecology indicates this might not be the case.

Although it cautions that overhunting is still very harmful, the study casts some doubt on previous theoretical models indicating that plant species that store significant amounts of carbon will die off at a faster rate.

The theory was based on the fact that without large birds and animals distributing them widely through droppings or their fur, seeds fall into a “kill zone” near the parent tree or plant where crowding leaves them vulnerable to natural predators. The tree species dispersed by wildlife store more carbon than the plants that replace them, potentially mitigating the impact of climate change.

Some two-thirds of tropical tree species rely on animal activities to spread seeds, the report states. Because they can travel great distances, larger birds and mammals do the best job, giving the seeds a much better chance at germinating and surviving. Those animals are often the prey of local hunters and used as bushmeat.

The researchers from the University of Connecticut (UConn) and the San Diego Zoo Institute for Conservation and Research, looked at what happened in six Amazonian rainforest tracts when large, fruit-eating animals and birds were no longer present.

The scientists used a recently developed statistical technique to collect extensive data on tree communities in Peru’s Madre de Dios river basin. While they found that saplings were forming denser clumps closer to other plants of the same species, “their density-dependent mortality did not increase as a result of them being more clustered,” said Robert Bagchi, assistant professor at UConn’s Department of Ecology and Evolutionary Biology.

“The lack of increased density-dependent thinning indicates that reduced dispersal did not increase mortality of large-vertebrate dispersed tree species that contribute disproportionately to forest biomass,” the paper states.

“There have been some suggestions based on modeling results that we are going to see these massive declines in carbon storage, and to some extent our paper is pushing back in the other direction – it is saying, yes, there are changes but they don’t seem to be quite as dramatic as those models assume or predict,” Bagchi said.

One theory that might explain the research finding is that, with their larger competitors out of the picture, smaller animals are increasing in number and picking up the slack. That would imply some dispersal even it is not as far from parent trees.

A similar study in Borneo led by Rhett Harrison, a tropical forest ecologist with the World Agroforestry Centre, found a serious decline in tree diversity in 2013, but there, even smaller animals, such as bats and flying squirrels, had disappeared as a result of overhunting.

The site in Borneo, said Bagchi, “is a properly defaunated forest, whereas I would say that the forests where we are working are altered. It is an altered fauna, as opposed to defaunation, per se.”

Hunting affects forests in quite subtle ways, he added, and further study is needed on the trees at earlier stages of growth.

“Maybe some dispersal is happening, but if you have these massive clusters of conspecific (single species) big trees then maybe dispersal is going to become less and less effective over time because you just can’t get away from parent trees of the same species, he said. “And if that is playing a role – which we think it is from other data – then that could become a more serious problem.”

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