Pest control with monkeys and lizards? Indeed, on some oil palm plantations

Across vast stretches on the island of Borneo, which is shared by Malaysia and Indonesia, lush rainforests have been felled for timber and agricultural land. In their place have sprouted endless stretches of oil palm plantations.

In the Malaysian state of Sabah, a mere fifth of the land is now covered in forests. The area’s storied biodiversity has suffered greatly as a result. Numerous endemic species like orangutans and hornbills are being pushed ever closer to the edge of extinction.

Yet even as some species have suffered, others have thrived. Rodents like rats are proliferating at palm oil plantations and logged forests where they pose grave threats to small mammals with which they end up sharing the same underbrush habitats.

New predators have arrived at those plantations, however. Namely: macaques.

The monkeys are known for raiding crops, but according to researchers it isn’t just the fruits of oil palm trees they are after. They are also after rats.

Macaques can reduce the number of rats by more than 75% at some plantations. That’s bad news for the rats but good news for farmers as the monkey can effectively control populations of the rodents without the use of pesticides.

“By uncovering cavities in oil palm trunks where rats seek shelter during the day, one group of pig-tailed macaques can catch more than 3,000 rats per year,” explains Anna Holzner, a scientist at the University of Leipzig and the Max Planck Institute for Evolutionary Anthropology who was an author of a new study.

According to the researchers, a group of pig-tailed macaques that live in nearby forests consume more than 12 tons of oil palm fruits a year at some oil palm plantations in an area. Yet that seemingly large sum amounts to a mere 0.56% of overall production.

And the monkeys make up for what they pilfer by eating rats and plenty of them. That’s helping cultivators since rats cause far more losses than monkeys. The rodents consume or otherwise damage 10% of oil palm fruits.

“I was stunned when I first observed that macaques feed on rats in plantations,” says Nadine Ruppert, of Universiti Sains Malaysia whose team has been studying the ecology and behavior of southern pig-tailed macaques since 2013.

“I did not expect them to hunt these relatively large rodents or that they would even eat so much meat,” Ruppert adds. “They are widely known to be frugivorous primates who only occasionally feast on small birds or lizards.”

And it isn’t just macaques that are taking advantage of rats infesting oil palm plantations. Malayan water monitor lizards (Varanus salvator) have been doing that too. They are highly adaptive creatures and ubiquitous from mangrove forests to city parks across Malaysia.

The lizards are opportunistic scavengers that feed on everything from dead fish to live birds and from frogs to street cats. They aren’t picky and will eat almost anything. This is “what enables water monitors to survive in the wasteland of oil palm [plantations],” explains Joshua P. Twining, a researcher specializing in altered ecosystems at Queen’s University in Belfast. Twining has been investigating how scavengers are adapting to the ecological wastelands of oil palm plantations in a part of Borneo.

“In the natural forests that surround the plantations, they face competition from mammal scavengers and predators such as sun bears, otters, civets and mongooses,” he adds. “There, water monitors are found only in relatively low numbers and at significantly smaller sizes.”

Not so on the plantations, however. In the relatively inhospitable monoculture of oil palm plantations the giant lizards have come to dominate. That does not mean, however, that all is well in such degraded environments.

“The increased parasite load that come from living at high densities reduces overall fitness of individuals, and in addition to energetic costs to adults, increased densities of a species that cannibalises juveniles of the same species, greatly deduces the chance of young making it to adulthood,” the researcher notes.

“For now, these giant lizards appear healthy,” he elucidates. “Of Borneo’s large native scavengers, they are the only species that has successfully adapted to the plantations. But, in the long run, due to the culmination of adverse effects, water monitors may be doomed to the same fate as the bears and other mammals that once inhabited the space now dominated by oil palm.”

Yet oil palm producers can help ensure that monkeys and lizards can thrive at their plantations if only out of self-interest.

“We expect that our results will encourage both private and public plantation owners to consider the protection of these primates and their natural forest habitat in and around existing and newly established oil palm plantations,” says Anja Widdig, a scientist who was involved in the study on predatory macaques.

“In collaboration with local palm oil companies and NGOs, we will work towards the realization of a plantation design that maintains viable macaque populations and higher levels of biodiversity via wildlife corridors while increasing the plantations’ productivity and sustainability by effective and environmentally friendly pest control,” Widdig adds.

“This ultimately can lead to a win-win situation for both biodiversity and the oil palm industry.”