Deep in a rainforest on the Malaysian mainland, there was a longstanding mystery to solve. Since the late 1980s, scientists working in the Pasoh Research Forest, a 1,500-acre chunk of virgin forest connected to a vast protected reserve, have noticed that the understory was disappearing. Over time, the researchers found they could walk with increasing ease through the jungle, without having to bushwhack through a tangle of seedlings and saplings. The trend was worrisome, since those young trees represent the future forest canopy.
There was an obvious culprit: wild boars. They snap off saplings to use for nests, they trample seedlings, and they churn up the soil. But why was the forest teeming with pigs? Could a decline in predators like tigers be responsible?
Matthew Luskin was skeptical. He had spent months traipsing through the forests of nearby Sumatra to study tigers for his PhD. He knew that if a lack of predators was the problem, there should also be an excess of other prey species, like deer and tapir. There wasn’t.
In a study published this week in Nature Communication, Luskin and his colleagues suggest another explanation: palm oil.
A ubiquitous ingredient in all sorts of supermarket products, from cookies to cosmetics, palm oil is a booming business—and an environmental disaster. Land clearing to make way for palm plantations has been the direct cause of a massive loss of forest across Indonesia and Malaysia.
What Luskin’s team has found is that those plantations can also damage even the seemingly healthy forest around them. The reason out-of-control boars are destroying the understory in the Pasoh Forest, the researchers say, isn’t a dearth of tigers. It’s a plethora of nearby palms.
A Natural Experiment
Normally in southeast Asia’s forests, trees produce fruit only every few years, and animal populations rise and fall with that food supply. In most years, there’s very little fruit to eat, which keeps animal densities low.
The Pasoh Forest, however, is surrounded on three sides by oil palm plantations. Oil palm trees are the most productive fruit trees in the world—that’s why they’re so commercially important—and they fruit continuously for roughly 25 years. Luskin suspected that the wild pigs of Pasoh were commuting to the plantations to consume fallen fruit, then returning to the forest to wreak ecological havoc.
The crop cycle for oil palm—combined with copious data on tree growth, boar nests, and oil palm production collected by scientists working at Pasoh—provided a perfect natural experiment to test the hypothesis. After 25 years, oil palm trees begin to decline, so growers must rip up their plantations and start again. In the early 2000s, the growers around Pasoh cleared all their trees and replaced them.
All of a sudden there was no oil palm fruit—and though conditions hadn’t changed in Pasoh itself, the boar population crashed. In a 125-acre area of the forest, the number of boar nests plummeted, from more than 300 before the palms were cleared to just one nest a few years later. When the new oil palms started fruiting, the boars came back with the same speed: within a few years there were hundreds of nests again.
Wild boar are highly destructive in large numbers in part because they rip out thousands of trees and churn up the soil, but also because they eat anything on the forest floor—a seed, an egg, a lizard. They can also reproduce faster than any large animal in the world, with females giving birth to two litters of 9 to 12 piglets each per year. Previous research showed that during peak population years, boars can damage more than half of all the saplings in an area.
But Luskin, a research fellow at Nanyang Technological University in Singapore and the Smithsonian Tropical Research Institute (and also a National Geographic Society grantee), believes macaques may ultimately prove just as problematic. The monkeys’ numbers are also skyrocketing near oil palm plantations, and like the pigs they will eat anything available, from fruit to chicks to frogs. “No one has studied those effects yet,” Luskin says.
Bigger Patches Are Better
Ecologists have a name for this kind of phenomenon, he and his colleagues write: When animals that are benefitting from agriculture “extend the ecological impacts of cultivation into food webs far away, in seemingly unaltered areas,” it’s called a “subsidy cascade.”
What surprised the scientists, though, was how far this particular subsidy cascaded. All the study sites at Pasoh were deep inside the forest, at least .8 mile from the forest edge and the oil palms.
Oil palm plantations sometimes set aside patches of forest in order to be certified as “sustainable” by the Roundtable on Sustainable Palm Oil. These areas of “high-conservation value” forest are often small patches in a sea of palm. Luskin, who visited dozens of such forest patches in Sumatra, refers to them as “pig and macaque zoos.” He thinks they may be inadequate.
“A solution has been to keep these patches of forest as oases for nature,” he says. “But that strategy might not work over the long term because of these invisible processes that are happening and are slowly eroding the forest.”
To keep forest ecosystems from being unraveled by plagues of palm-fed pigs and monkeys, he goes on, we may need to set aside swaths that are “much larger than we thought before. This paper indicates that we really need to up the size of our forest reserves if we want to have them long-term.”
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