How a fatal virus sparked Pulau Ubin’s mouse-deer boom

SINGAPORE – When a deadly virus wiped out nearly all of Pulau Ubin’s wild boars in 2023, mammal scientist Marcus Chua knew he had to document how the ecosystem would rewrite itself.

He suspected that the sudden collapse of the wild pig population (Sus scrofa) due to African swine fever would be a game changer for the greater mouse-deer (Tragulus napu), one of the world’s smallest hooved mammals.

The pint-size mouse-deer,

celebrated in Malay folklore

for its bravery and cunning wit, was once thought extinct in Singapore until Dr Chua helped rediscover it on the island in 2008, ending an 80-year absence from local records.

The species, with individuals weighing no more than around 4kg, likely competed with wild boars for space as well as food, comprising fallen flora and low-hanging vegetation, his research found.

“While predation of mouse-deer by wild pigs has not been recorded, wild pigs are also known to eat deer and their fawns,” said Dr Chua, mammal curator at the Lee Kong Chian Natural History Museum at the National University of Singapore (NUS).

“With such competition and predation, there was an urgency to understand the ecological consequence of such a major event.”

The results were staggering. In the year after the outbreak, the mouse-deer population quintupled to 293 individuals per sq km between 2019 and 2024.

“It became impossible to avoid seeing a mouse-deer during our surveys,” said Dr Chua.

This is the highest known density recorded of mouse-deer anywhere in the world, and at least three times higher than known densities elsewhere.

It effectively makes Pulau Ubin the best place on earth to spot these shy creatures, which are commonly found in forests across South-east Asia.

Cambodia’s Southern Cardamom National Park logged an estimated 80.7 lesser mouse-deer (Tragulus kanchil) – the greater mouse-deer’s smaller relative – per sq km.

The findings were published in scientific journal Biological Conservation on Oct 25.

The researchers noted that the rapid population spike was possible because mouse-deer can breed within five months of birth.

The drastic decline of wild boars likely fuelled this boom by reducing competition for food and allowing the forest undergrowth to regenerate.

Dr Chua said this is likely the first evidence of ecological impacts arising from African swine fever in Asia.

Because his team, together with the National Parks Board (NParks), had surveyed the island for more than 15 years, they could precisely quantify how the disease reshaped forest dynamics.

“It would be a clarion call for other areas affected by African swine fever to determine what short- and long-term impacts the virus has on forest ecology,” he added.

“For forest managers, it underscores the need to adopt an ecosystem-wide perspective, as managing or responding to a disease in one species can have profound consequences for others.”

However, the current high is unlikely to last, given that signs of wild boars breeding have been detected a year after the outbreak.

It is also unclear whether the population of mouse-deer has reached the limits of the environment, otherwise known as carrying capacity.

Said Dr Chua: “With the recovery of wild pig populations now, the carrying capacity of mouse-deer will likely shift until interactions of the two species in the habitat stabilise.”

An NUS master’s student is monitoring the populations of both species two years after the outbreak, he added.

Wildlife ecologist Matthew Luskin from the University of Queensland, who has mapped the impacts of African swine fever in South-east Asia, said the research joins “a long history in ecology of documenting how changes to a single species can trigger far-reaching cascading impacts in unexpected ways”.

Distinguished Research Professor William Laurance from James Cook University said similar outcomes likely occur in forests elsewhere.

“Other forests, such as those in Malaysia’s Pasoh Forest Reserve, have also seen dramatic fluctuations in wild pig numbers and significant ecological changes as a result,” added the environmental scientist, who studies conservation challenges across the globe.

In 2022, for instance, when the population of Pasoh’s wild boars plummeted due to an African swine fever epidemic, young trees were found to thrive in higher numbers, as the pigs could no longer damage saplings with their nesting habits.

On Pulau Ubin, the spike in mouse-deer is unlikely to have a huge impact on the forest, concluded both Dr Luskin and Dr Laurance, as the herbivores are delicate and relatively innocuous as compared to wild boars.

The study also recommended cost-effective ways to count wildlife.

The researchers found that it was more precise to gauge abundant wildlife populations through line transect sampling, which involves walking pre-determined paths at night and recording every mouse-deer spotted along a fixed distance.

This allowed them to mathematically estimate the total population hidden in the forest.

Meanwhile, camera traps were useful for elusive species at lower abundance, like wild boars.

Beyond natural reasons, Dr Chua credited the mouse-deer’s recovery to NParks’ reforestation efforts and its decision to allow natural vegetation to mature over decades.

His co-author Robert Teo, NParks’ director for Pulau Ubin, called the population increase “encouraging progress for wildlife conservation” on the island, where more than 40,000 trees have been planted so far.

He added that the board will continue its reforestation and forest enrichment efforts to enhance homes for wildlife on Pulau Ubin.

The Singapore Zoo and NParks had previously attempted to reintroduce the greater mouse-deer twice in central Singapore, to no avail.

No sightings of the rust-coloured mammal were reported on mainland Singapore until 2012, when Dr Chua found the species in the Western Catchment forests.

Meanwhile,

the lesser mouse-deer,

the other species of mouse-deer native to Singapore, has persisted in Bukit Timah and Central Catchment Nature Reserve throughout the years.