The rise of the giant lizards

What happens if you destroy a tropical rain forest and replant it with a monotonous monoculture of palms for the production of palm oil? We turn our attention to the northern part of Borneo, which is part of Malaysia. Rain forests have been cut down, turning some 20 percent of the land into plantations.

Not many animals can survive on these giant plantations, except scavengers, like snakes, rats, mice and lizards. They provide vital services including the removal of carrion, which is a crucial step in recycling nutrients and preventing disease.

Scientists captured 118 individual animals, with the most abundant being the South-east Asian water monitor (Varanus salvator macromaculatus), a large lizard and the Malay civet cat (Viverra tangalunga)[1]. There was a consistent trend: the more disturbed the land, the more lizards they found and the less civet cats they found.

Water monitors are primarily adapted for life in and around water. These 'dragons' have a phenomenal ability to eat almost anything that can fit inside their stomachs. Their diet includes small invertebrates, crustaceans and amphibians through to larger mammals, birds and their eggs. They’ll even eat other monitors.

This unfussy eating is what enables lizards to survive in the wasteland of oil palm. In the natural forests that surround the plantations, they face competition from mammal scavengers and predators such as sun bears, otters, civets and mongoose. There, water monitors are found only in relatively low numbers and at significantly smaller sizes.

But those mammals struggle to survive in the plantations, where a lack of shade boosts the temperature and lower plant diversity filters up through the food chain. Lizards can handle the heat and the presence of extra food from human refuse means water monitors appear to thrive there, reaching large sizes (in excess of two meters) and high numbers.

This 'survival of the toughest' is what drove the males of the species to reach such formidable sizes. But it is also one of the reasons why degraded habitats such as oil palm may be an ecological trap. The easy availability of food from human garbage sites or domestic animals draws in extra male monitors, resulting in increased competition for prime spots in the plantations then means they use up lots of energy, and risk serious injury just holding onto their territory and fighting off other males.

So, what we have is a monoculture of palms and a monoculture of lizards. This cannot be a positive issue, but it's even worse. This type of habitat is also a prime breeding ground for mosquitoes. On Borneo they carry the parasite Plasmodium knowlesi, which has recently been found to be a major cause of human malaria in Malaysian Borneo[2].

[1] Twining et all: Increasing land-use intensity reverses the relative occupancy of two quadrupedal scavengers in PLoS One - 2017
[2] William et al: Severe Plasmodium knowlesi Malaria in a Tertiary Care Hospital, Sabah, Malaysia in Emerging Infectious Diseases - 2011