Conservation news

Hunting stymies rainforest regrowth, says a new study

  • The study compared sites with different degrees of hunting pressure, ranging from a national park to timber concessions.
  • Areas with fewer large mammals tended to have more rodents, which in turn ate and destroyed more seeds of tree species valuable to the timber industry.
  • The findings suggest that regulating hunting on timber concessions may help to encourage the regeneration of economically valuable trees.

The global rise in bushmeat hunting isn’t great news for the animals that are the focus of the pursuit. But how hunting affects the broader ecosystem remains an open ecological question.

Now, a new study finds that the consequences could be enough to hurt timber interests, demonstrating that we’re only just beginning to understand the rippling effects of hunting pressure.

“When I started doing this research myself, I expected one or two little effects” from bushmeat hunting, said Cooper Rosin, a tropical ecologist at Duke University and lead author of the study.

For example, he said he expected that hunting bigger mammals would mean that seeds aren’t scattered quite as well throughout the forest, since animals such as duikers, primates and wild hogs are often important seed distributors.

Bushmeat hangs for sale at a roadside village stand, near Makokou, Gabon. Photo by Ruby Harrison

But instead, he and his colleague John Poulsen found that taking these large mammals out of the equation on timber concessions in Gabon led to a bump in the number of rodents, which then changed their behavior and stepped up their consumption of seeds.

Rodents are known as “seed predators.” Rather than targeting fruits and ingesting seeds whole as bigger mammals do, rodents go after the seeds themselves, destroying them and any possibility of that seed becoming a tree in the process.

Rosin and Poulsen found that that was enough to slow down the natural regeneration of several commercially important timber species, including okoume (Aucoumea klaineana), adjouaba (Dacryodes buettneri) and azobe (Lophira alata). They published their work online in October with the journal Forest Ecology and Management.

Rosin said that he, like other researchers doing similar work in other parts of the world, was surprised to see “that a drop in a bucket might cause a wave like that.”

“These ecological webs are very complex, and changes at any level are likely to perturb other levels.”

For this study, Rosin picked six sites in northeastern Gabon that represented a range of hunting pressures, from very little inside the boundaries of Ivindo National Park, to much higher within working timber concessions.

The study’s lead author, Cooper Rosin, and field assistant Jeannot Mbelassanga set up the experimental seed exclosures, Ivindo National Park, Gabon. Photo by Rachel Rosin

In order to tease apart what was happening to the seeds, they set up exclusion experiments at each of the six sites, allowing three levels of access: After scattering a handful of seeds around an area, they left the spot open, set up a fence around it that kept out all but the smallest mammals, or closed it off completely. They then recorded how many seeds the animals ate and which of the seeds sprouted for each of the treatments at the different sites.

They found that rodents got more of the seeds available to them in the more heavily hunted logging concessions. That discovery has important implications for timber companies, which often don’t regulate hunting where they operate. While it might seem a cheap way to keep their workforce fed, “hunting can be detrimental to the bottom line,” as Rosin points out.

“The outcome of that for a timber company would be a loss of future harvestable timber,” he said.

Seeds from the African oil bean tree (Pentaclethra macrophylla) was one of eight tree species used in the study. Photo by Cooper Rosin

The potential incentive to stop hunting on timber plantations also presents the companies with an opportunity to benefit conservation, Rosin said, adding that conservation can be more difficult with subsistence hunters.

“It’s hard to be really critical of hunting that supports someone’s family,” he added. “That’s a livelihood for them and actually a meal on their table.

“But hunting in those areas where that meat is not so critical is where we could spend a little more time thinking of appropriate conservation solutions,” Rosin said. Companies could truck in frozen meat, for example, or set up livestock programs for their employees.

A giant pouched rat (Cricetomys emini) is ‘caught’ on a camera trap removing a seed from the rodents-only partial seed exclosure. Photo by Cooper Rosin

But, Rosin said, it remains to be seen whether those issues resonate with concession managers.

“I’m hoping that [our conclusions] may lend an economic argument for timber companies to reduce hunting within concessions,” he said.

Citation:

Rosin, C., & Poulsen, J. R. (2016). Hunting-induced defaunation drives increased seed predation and decreased seedling establishment of commercially important tree species in an Afrotropical forest. Forest Ecology and Management.

Banner image: The forest of Gabon, as seen from the Ivindo River by Rachel Rosin