Photo: Nadine Ruppert
A recent scientific study is showing that keeping the habitat of forests near palm oil plantations intact is important if macaques were to naturally help to reduce the population of rats, a plantation pest that is usually dealt with expensive and often toxic chemical poisons.
The study, conducted under the Macaca Nemestrina Project by the University of Sains Malaysia, showed that pig-tailed macaques (Macaca nemestrina) included rats from oil palm plantations in their omnivorous diet.
“Our study showed that one group of macaques in our study site consumed circa 3,000 rats per year,” said Nadine Ruppert, a senior lecturer at the USM School of Biological Sciences who took part in the study at the Segari Melintang Forest Reserve and adjacent oil palm plantations in Perak, Malaysia. A tribe of macaques on average consists of about 40 individuals, big and small.
If used for rodent control in place of the conventional method of poison, macaques could provide an important ecosystem service and enhance palm oil sustainability.
Ruppert told The Palm Scribe in an email, that this species, which was widespread in Peninsular Malaysia, Borneo and Sumatra, is often found in oil palm plantations near forests within their distribution range. She said that groups of macaques observed in the study held from January 2016 to September 2018, had a fixed home range and roam inside plantations and forests of around 90 hectares.
In a correspondence authored by Ruppert and published by the Current Biology Magazine this month, she said that the pig-tailed macaque was not a crop pest in oil palm plantation as their impact on yield was minor.
“More importantly, our data suggest that wild macaques have the potential to act as biological pest control by feeding on plantation rats, the major pest for oil palm crops,” said the correspondence. “If used for rodent control in place of the conventional method of poison, macaques could provide an important ecosystem service and enhance palm oil sustainability.”
Indonesia is the world’s top producer, exporter, and consumer of palm oil. Official statistics put the country’s palm oil plantation at more than 14 million hectares. The expansion of oil palm plantations has had negative ecological impacts, as forests became isolated fragments, leading to reduced species and genetic diversity, impaired climate regulation, and lower resilience.
In fragmented forests macaques increasingly divert their foraging activities into oil palm plantations, where they are widely regarded as crop pests.
The annual rat consumption by a tribe of macaques could mitigate annual losses of $112 per hectare. A report by Chung G.F. on vertebrate pests of oil palm published in 2000 showed that rats can cause losses of up to 10 percent of the yield. The use of rodenticides in pest control is also not only expensive but has proven to be harmful to non-target wildlife and the environment.
Ruppert, however, said that to keep these macaques to continue to help reduce the rat population in a plantation effectively, efforts were needed to make sure that their habitat remained undisturbed and that they have access to the plantation from their habitat.
“These macaques need the forest habitat to survive and they only visit plantations nearby the forest for a few hours a day. Thus, the most important way to make them come into plantations is to keep the forest habitat largely intact and to establish forest corridors or buffers along and inside plantations,” Ruppert told the Palm Scribe.
She said that unlike owls that are kept in plantation to catch rats, macaque should be allowed to roam free between their natural habitat and the plantation and that forest corridors were good solutions to connect the two if they did not share a common border.
Ruppert said that by allowing macaques to keep the rat population in plantation down, resulting in not only saving in the cost of often toxic rodenticides but also in crop damage reduction, palm oil cultivation would be more sustainable.
During observation time, the macaque primary diet consists of fruits, seeds, leaves, and mushrooms but they also devour arthropods and vertebrates such as rats.
“So they fed 60 percent of the observation time on fruits and 5 percent on meat. But, since they are cheek pouch feeders, they quickly snatch a rat and then keep it in the cheek pouch to be consumed somewhere else later. Thus, the actual time feeding on rats is much higher than the recorded 5 percent,” Ruppert said.
Since macaques were omnivores, it went without saying that they ate meat, including rats. “But I was ’surprised’ to see how much meat they actually consume,” she said.
“Now, 5 percent does not sound a lot but I was not expecting them to actively and frequently hunt these relatively large rodents and to forage for them in a very targeted, specific manner,” she added.
Her team was still trying to find out whether consuming rats was prompted by an abundance of rodents or the scarcity of their other normal diet elements. A long-term forest study was ongoing to assess seasonal fruit abundance, and its correlations with feeding time in the plantations, Ruppert added.
“With this study, we want to provide a strong argument for the conservation of rain forest habitat so that the primate and other forest species can be protected and to allow the macaques to access plantations and in turn benefit growers through savings from rodenticides purchases,” Ruppert said.
A recognition of the role of the pig-tailed macaque and the benefit this arrangement may bring, may prompt farmers and palm oil companies to protect primates in their natural habitat via wildlife corridors between forest patches and viable interfaces between forests and plantations. This, the correspondence in Current Biology said, could maintain functional connectivity and gene flow between macaque populations while increasing environmental sustainability and productivity of existing oil palm plantations, promoting win-win solutions for palm oil producers and biodiversity.
“This can make the management of oil palm plantation greener and more sustainable, and also perhaps cheaper,” Ruppert concluded.