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

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