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Mammal numbers high in logged tropical forests, study finds

  • The study quantified mammal numbers in forests and landscapes with varying degrees of human impact in Malaysian Borneo.
  • Across 57 mammal species recorded with live and camera traps, the average number of all animals combined was 28 percent higher in logged forests — where hunting wasn’t an issue — compared to old-growth forests.
  • The findings demonstrate the importance of conserving degraded forests along with more pristine areas.

Logged tropical forests have a bit of a bad reputation in the conservation community. They’re often seen as sub-standard habitats for animals, especially when compared with the robust and varied ecosystem in an old-growth forest.

But a new study published this month in the journal Biological Conservation adds to the evidence demonstrating that these “degraded” forests are worth protecting from further damage.

As ecologists, we’ve known for a long time that these areas, despite being ugly to look at, actually have the potential to sustain large populations of lots of different mammal species, as long as hunting levels are low,” said Oliver Wearn of the Zoological Society of London in an email. “Pigs and deer find more food, small mammals might find more insects and fruit, and the predators of all of these species find a moving buffet.”

A long-tailed macaque (Macaca fascicularis) in Malaysian Borneo. Photo by John C. Cannon / Mongabay

Some of Wearn’s earlier work had shown that the diversity of mammal species in logged forests was similar to what’s present in old-growth forests. He and his colleagues then wondered how changes by humans to those forests would affect the numbers of each of those species.

To answer that question, they set up camera and live traps in a variety of landscapes in Malaysian Borneo, including pristine old-growth forests, reserves that have been logged multiple times, and oil palm plantations. They recorded 57 different mammal species in these areas, and based on the frequencies that each one appeared, they then estimated how abundant the animals were in each environment.

They found that average abundance across all of those species actually went up by 28 percent. Small mammal numbers were 169 percent higher in logged forests than in untouched spots (though Wearn did point out that a lot of the first colonizers were invasive species, like rats and feral dogs).

“What was more surprising was that this pattern was so widespread across the mammal species we looked at, including some of those that partly make their living in the treetops, like orangutans and clouded leopards,” Wearn said.

The clouded leopard (Neofelis nebulosa) is one of the threatened species that the study showed does reasonably well in logged forests without hunting pressure. Photo ©Charles Ryan / Sticky Rice Travel

Recent research on orangutans, also in Borneo, also found that orangutans do pretty well in disturbed environments.

Hunting isn’t much of a problem for the mammals who live in the study areas, which makes these forests unique. In other parts of the world, logging makes forests more accessible to hunters, which can decimate both the breadth of species and the sheer numbers that inhabit them.

But in forests where hunting isn’t a problem, the question also remains about how much of the forest we can cut down before mammal populations start to dwindle.

“We didn’t find evidence of a neat threshold beyond which the forest is useless,” Wearn said. “Basically, the lower the cutoff used to define forest, the more mammals that will be saved.”

In the absence of that type of metric, it’s difficult to determine whether companies that make zero-deforestation pledges are following through, he added.

“What does ‘zero deforestation’ actually mean for forests that have been logged?” Wearn said. “At some point, when so many trees have been removed from a hectare of forest, it effectively ceases to be a forest, and so clearing of this area doesn’t really constitute deforestation as we would normally imagine it.”

Bornean orangutans (Pongo pygmaeus) similarly seem to find homes in logged forests where they don’t have to contend with hunters, research has shown. Photo by John C. Cannon / Mongabay

One solution that’s gaining momentum is using the amount of carbon held in a forest as a tool for land use decisions. Under these “high carbon stock” guidelines, proponents argue that carbon-rich forests are good targets for protection. That’s because they not only keep substantial amounts of carbon from the atmosphere and thus from further warming the planet, but they also are more likely to protect a greater number of species.

“The currently-proposed threshold of 35-50 [metric tons] (39-55 tons) of carbon per hectare would yield really positive conservation outcomes,” Wearn said, “as long as the areas are as large and as connected as possible, and protected from hunting.”

Strategies to protect biodiversity based on conserving high carbon stocks aren’t universally accepted, however. Some scientists argue that high levels of carbon and biodiversity don’t always line up.

Still, Wearn’s research highlights the preference of having forests that have been heavily thinned over another type of use that requires outright clearing, such as timber plantations and oil palm plantations. In fact, the researchers found a 47 percent drop in the abundance of mammals in oil palm compared to old-growth forests.

In logged forests, even threatened mammal numbers remained high, which was something Wearn and his colleagues didn’t anticipate. But their abundance on oil palm plantations was considerably lower.

A red leaf monkey (Presbytis rubicunda) in Malaysian Borneo. Photo by John C. Cannon / Mongabay

“We still need far more research to be done in African and South American oil palm plantations, but it’s likely to be a very poor habitat for most mammals, wherever it’s put,” Wearn said.

He also cautioned that, while this specific research demonstrates that mammals can cope with some degree of relatively recent deforestation, we also need to keep an eye on how they do over the long term.

“We know that there are often time-delays in ecological responses,” Wearn said. “It could be the case that some species are living on ‘borrowed time’ in this habitat.”

In his view, that makes protecting old-growth forests that much more important.

“This increasingly rare habitat is the absolute priority for conservation in Southeast Asia — it is the only home on Earth for species which are old-growth specialists, and is the evolutionary home for species in the region,” he said. “Cut this down and there is no going back.”

CITATION

Wearn, O. R., Rowcliffe, J. M., Carbone, C., Pfeifer, M., Bernard, H., & Ewers, R. M. (2017). Mammalian species abundance across a gradient of tropical land-use intensity: A hierarchical multi-species modelling approach. Biological Conservation212, 162-171.

Banner image of a clouded leopard by Rhett A. Butler / Mongabay.

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