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In a sea of oil palms, even monitor lizards need islands of natural forest

An Asian water monitor lizard (Varanus salvator) walking along the banks of the Kinabatangan river, Sabah, Malaysian Borneo. Image ©Alexandre Schaal

  • Forest patches in and around oil palm plantations are crucial for the survival of Asian water monitor lizards, a generalist species that usually thrives in human-impacted landscapes.
  • A new study focused on the Kinabatangan floodplain in Malaysian Borneo found significantly more lizards in natural forest near oil palm plantations than in the plantations themselves.
  • The researchers suggest lizards are particularly dependent on forest patches for breeding sites and shelter.
  • The study adds to a growing body of evidence that demonstrates the vital role of natural forest in and around oil palm plantations and reaffirms the importance of buffer zones and habitat corridors that enable animals to negotiate oil palm-dominated landscapes.

The Kinabatangan River rises in the beating heart of Sabah, Malaysian Borneo, where ancient primary forest cloaks the slopes of steep-sided mountains. As it meanders east toward the Sulu Sea, the river traverses floodplains overrun by vast monocultures. Regimented rows of oil palms claw at the thin weft of natural forest that lines the Kinabatangan’s course. This fragile lifeline is home to some of the region’s most spectacular species, including Bornean elephants (Elephas maximus borneensis), orangutans (Pongo pygmaeus) and proboscis monkeys (Nasalis larvatus).

Now, scientists have found that the forested river corridor, designated in 2005 as the Lower Kinabatangan Wildlife Sanctuary, is also vital for the somewhat less glamorous Asian water monitor lizard (Varanus salvator).

As both predators and scavengers, Asian water monitor lizards are known to flourish in human-impacted landscapes, including oil palm plantations, where they thrive on an abundance of leftovers and rodent prey. Prior research has even shown that they fare better in plantations than natural habitats.

However, this doesn’t appear to be the case in the fragmented wastes of the Kinabatangan floodplain. According to the findings of a new study, led by scientists from Cardiff University, U.K., in collaboration with the Sabah Wildlife Department, the Kinabatangan lizards are heavily dependent on patches of natural forest near oil palm plantations for key resources, such as breeding habitat and shelter. The researchers, who published their results in PLOS One, found far greater numbers of lizards in natural forest patches near oil palm plantations than in the plantations themselves.

The Lower Kinabatangan River in Sabah, Malaysian Borneo. Image by Rhett A. Butler/Mongabay

“It was very surprising,” Sergio Guerrero-Sánchez, a researcher at Cardiff University and lead author of the new study, told Mongabay in an interview. “Usually we expect monitor lizards to be more abundant in anthropogenic habitats, but in the Kinabatangan floodplain, we found more lizards in the forest than in plantations.”

The results add to a growing body of evidence that demonstrates the crucial role of natural forest patches in and around oil palm plantations for populations of iconic species, such as orangutans, and for commonplace yet nonetheless essential generalist species, such as monitor lizards.

The health of monitor lizard populations is an important benchmark when managing highly fragmented landscapes, such as the Kinabatangan floodplain, where nearly half the land has been converted to oil palm plantations, said Benoît Goossens, conservation biologist at Cardiff University and study co-author. As generalists, they cycle nutrients and control levels of carrion, consequently regulating the incidence of pathogens in the environment.

A palm oil estate in Sabah, Malaysian Borneo. The lack of cover and higher temperatures in plantations make them less suitable for breeding and hatchling lizards. Image by Rhett A. Butler/Mongabay

To study the monitor lizards, Guerrero-Sánchez and his colleagues hauled meter-long (3-foot) wire mesh traps into the oil palm plantations and the forest reserve along the Kinabatangan River. Over three years, they trapped more than 400 individual lizards, each of which they microchipped, weighed and measured. The microchips helped them track how often each lizard returned to the traps, in turn enabling them to estimate the population size.

Overall, the Kinabatangan monitor lizard population was “highly abundant and stable,” ranging between 27 lizards per square kilometer (70 lizards per square mile) in oil palm plantation to 212 lizards per km2 (549 lizards per mi2) in natural forest.

Some of the lizards were enormous, tipping the scales at 25 kilograms (55 pounds), and all were armed with sharp weaponry from teeth to tail. Acquiring the skills to handle them safely was a steep learning curve: “I had to learn which part of them is most dangerous — which part of the animal I had to be most scared of,” said Guerrero-Sánchez. A further challenge was coping with the stench of chicken entrails that the team used to bait their traps: “Every time we came back from the forest, our presence was known immediately,” he added.

Researchers give a captured Asian water monitor lizard a quick pre-release inspection. Lizards under 15 kg weight can be held safely by one person. Image courtesy of Ashley Deanna Kunesh
Researchers help a small monitor lizard out of a trap, being careful to secure the head and tail, which are the most dangerous parts of the animal. In total, the weighing and measuring process lasted no more than 15 minutes. Image ©Rudi Delvaux

The researchers suggest the success of monitor lizards in Kinabatangan’s riparian forests is due to the relative scarcity of their main carnivore competitor, the Sunda clouded leopard (Neofelis diardi). This paucity secures them the pole position as the habitat’s top predator.

Since smaller lizards weighing less than 5 kg (11 lb) were particularly abundant in forests, the researchers concluded that patches of natural forest serve as important breeding sites and provide shelter for hatchlings. Conversely, the lack of cover and higher temperatures in exposed oil palm plantations could harm egg development and lead to confrontations between hatchlings and larger, more aggressive lizards, the study says.

“Yes, [monitor lizards] are successful in plantations, but they still need the forest to breed, and it’s also a safety realm for smaller individuals,” Goossens said, adding that species such as orangutans and elephants will also come into plantations, but they similarly depend on nearby areas of forest for crucial resources, without which they could not survive.

“All these high conservation value forest patches are extremely important for wildlife,” he said. “If the whole forest is converted to oil palm plantation, then that is it, that is the end of the area.”

The researchers are now examining the mobility of monitor lizards and other species within the fragmented landscape to design buffer zones and habitat corridors that will enable the widest possible range of animals to negotiate the sea of oil palms.

A key priority is protecting and restoring the sliver of riparian forest along the Kinabatangan River: “Since it links the mangroves on the east coast with the central forest in Sabah, it is a very important corridor and the bigger it is, the better,” Goossens said. “We can’t fight against oil palm plantations … [They are] there for the long term, so we need to find the best management recommendations for the species that live there.”

A monitor lizard taking a brief moment to recover from capture before running off into the undergrowth of the Lower Kinabatangan Wildlife Sanctuary. Image courtesy of Sergio Guerrero-Sánchez

Banner image: An Asian water monitor lizard (Varanus salvator) walking along the banks of the Kinabatangan river, Sabah, Malaysian Borneo. Image ©Alexandre Schaal


Guerrero-Sánchez, S., Goossens, B., Saimin, S., & Orozco-terWengel, P. (2021). The critical role of natural forest as refugium for generalist species in oil palm-dominated landscapes. PLOS ONE, 16(10). doi:10.1371/journal.pone.0257814


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