Rodents gnawing on oil palm crops can be a costly business. One study estimated that they can reduce yields by as much as 10 per cent.
Among the means used to tackle rat infestations: deploying barn owls and domestic cats, or putting out bait laced with rat poison. A recent review, published in the journal Ecotoxicology, has assessed the latter method’s risk to the wildlife that are known to hunt on oil palm plantations in Southeast Asia.
“There are numerous reports of non-target environment poisoning from other regions of the world, namely Europe, USA, Australia and New Zealand, however there is a glaringly low number of such reports or research in Asian, especially Southeast Asian, countries,” study co-author Hasber Salim, of the Barn Owl and Rodent Research Group (BORG) at Universiti Sains Malaysia, wrote in an email. “As anticoagulant rodenticides are used by most plantations, there is a high exposure risk to non-target wildlife and [this is] why a baseline study is needed.”
Costly pest, risky response
Two main pathways of exposure exist. Wildlife may consume the bait laid out for rats or they may consume rats that have eaten this bait. Rats can accumulate the chemicals in their bodies for several days before it proves fatal.
Rafael Mateo, a wildlife toxicologist at the Institute for Game and Wildlife Research, Spain, told Mongabay that while rodenticides have proved effective at controlling infestations, the possibility of bioaccumulation in species that eat rodents is cause for concern.
“Anticoagulant rodenticides tend to bioaccumulate in the liver [of] wild predators of rodents,” said Mateo, who was not involved in the study. “When the liver concentration exceeds certain levels, these predators become poisoned and die as a result of inhibition of blood clotting.”
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The Southeast Asian barn owl is a good example of the impact of such chemicals. There is a paradox in the fact that the natural predators of rodents are being poisoned by the chemicals that humans use to control the rodents. We are killing our allies.
Rafael Mateo, wildlife toxicologist, Institute for Game and Wildlife Research
Different types of anticoagulant rodenticides, which induce massive internal bleeding, are commonly used. Known as first- and second-generation rodenticides, they contain a variety of different chemicals. In previous studies, Salim and his colleagues investigated the effects of rodenticides on barn owls (Tyto alba).
Rats account for up to 90 per cent of the owls’ diet on oil palm plantations. Exposure to some rodenticides can result in mortality and affect reproduction. Outside of this species, however, little is known about the possible impacts on other “non-target” wildlife on plantations.
“The Southeast Asian barn owl … is a good example of the impact of such chemicals,” Mateo said. “There is a paradox in the fact that the natural predators of rodents are being poisoned by the chemicals that humans use to control the rodents. We are killing our allies.”
Salim and his team carried out a risk assessment based on current knowledge of several other species’ use of oil palm plantations. A host of domestic animals, macaques, grain-eating birds, and small mammals were assessed as being at medium to high risk of primary exposure, through eating poisoned bait.
Leopard cats (Prionailurus bengalensis), palm civets (Paradoxurus hermaphroditus), Asian water monitors (Varanus salvator), and several rodent-eating raptor species were all assigned as being at high risk for secondary exposure, by eating contaminated rodents. Larger carnivores, such as the Sunda clouded leopard (Neofelis diardi) or Malayan tiger (Panthera tigris jacksoni), were considered at low to medium risk of tertiary exposure, as they may eat larger prey, such as wild pigs, which had consumed bait.
Nadine Ruppert, a primatologist at the School of Biological Sciences, Universiti Sains Malaysia, studies pig-tailed macaques (Macaca nemestrina) closely. These monkeys are listed as vulnerable on the IUCN Red List, and for her, their potential exposure to harmful chemicals is troubling.
“A group of pig-tailed macaques at our study site consumes around 3,000 plantation rats per year, making it an important part of their diet,” Ruppert, who was not involved in the study, told Mongabay. The macaques regularly visit plantations for a couple of hours each day, she added. “The plantations are an integral part of their core foraging area, and they actively hunt for rats in the plantation during the day.”
Ruppert pointed to research in Africa that has connected pesticide exposure to facial deformities, known as facial dysplasia, in chimpanzees and baboons, as well as other effects such as infertility and poor health. Similar studies in Malaysian wildlife are lacking, Ruppert said. “[B]ut we have observed similar symptoms in a study population of wild pig-tailed macaques and plan to examine potential risks of agricultural chemicals on the macaques,” she said.
Mourad Gabriel, a wildlife biologist with the US Department of Agriculture’s Forest Service, who was not involved in the study, said the possible mix of chemicals that wildlife are exposed to when different rodenticides are used is another open question.
“We’re having difficulty just trying to tease out a single compound’s impact on a prey species or non-target species through secondary [or] tertiary exposure,” he said in an interview.
Reducing rodent use
One of the first steps toward reducing the risk of exposure is for plantation managers and oil palm smallholders to understand the potential harm to non-target wildlife, Salim said. One alternative method is to only place bait when damage to fruit is above 5 per cent, as part of integrated pest management strategies, he said.
“This however requires managers [to] be comfortable with a certain level of damage as well as accepting the presence of a rat population,” he said. “[T]his change in mindset is not an easy thing to push for, as of course, plantations would like there to be zero loss of fruit.”
So-called biological rodent controls, such as barn owls, can also play an important role. A study published earlier this year found that a higher presence of barn owls on plantations allowed for rodenticide use to be cut by nearly half, for instance. Barn owls, as Salim underlined, need rats on plantations to survive.
Ruppert said macaques can also play a part in limiting rodent numbers. She said that on plantations near forests that house pig-tailed macaques, the species can be a complementary pest control as they hunt in different ecological niches.
Whereas owls hunt at night while rats are scurrying on the plantation floor, macaques search for them in and among the oil palm trees during the day while they are sleeping, she said. A 2019 study by Ruppert and her team found that areas with macaques suffer less yield loss of approximately $100 per hectare ($40 per acre) per year.
“Barn owls may be introduced to a plantation that has no connecting forest habitat by providing sufficient nest boxes, but macaques cannot survive in plantation areas as they depend on intact forest for shelter, a diverse diet, and to conduct their full range of social behaviours,” Ruppert said. “The solution is to keep forests intact, replant forest corridors to allow wildlife to enter plantations and retreat to the forest, and to practice environmentally friendly plantation management without harmful pesticides.”
Salim and his colleagues are planning to carry out a study to explore the effects of anticoagulant rodenticides on non-target wildlife more closely.
This story was published with permission from Mongabay.com.
