Bold claim: a deadly virus reshaped an entire island’s wildlife, turning a shy, tiny deer into a forest phenomenon. But here’s where the story gets more intriguing—and potentially controversial—as it challenges how ecosystems respond to disease outbreaks.
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 realized this event could rewrite the island’s ecological narrative. He suspected the sudden collapse of the wild pig population (Sus scrofa) due to African swine fever would create a game-changing opportunity for the greater mouse-deer (Tragulus napu), one of the world’s smallest hoofed mammals.
The pint-sized 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 gap in local records. Weighing at most about 4 kg, these tiny ungulates likely competed with wild boars for space and food—falling flora and low-lying vegetation that form a key part of their diet.
“While predation of mouse-deer by wild pigs has not been recorded, wild pigs are also known to eat deer and their fawns,” Dr. Chua notes, as the mammal curator at the Lee Kong Chian Natural History Museum, National University of Singapore (NUS). With such competition and predation potential, there was a clear need to understand the ecological consequences of this major disruption.
The results were striking. In the year following the outbreak, the mouse-deer population quintupled from a baseline to 293 individuals per square kilometer between 2019 and 2024. “It became impossible to avoid seeing a mouse-deer during our surveys,” Dr. Chua remarks. This density is the highest recorded anywhere for the species and at least three times higher than other known populations globally.
Pulau Ubin effectively became the best place on Earth to spot these shy creatures, which are typically found in forests across Southeast Asia. By comparison, Cambodia’s Southern Cardamom National Park reported about 80.7 lesser mouse-deer (Tragulus kanchil) per square kilometer, illustrating just how exceptional Pulau Ubin’s spike was.
The findings were published in the scientific journal Biological Conservation on Oct 25. The researchers attribute the rapid population surge to the deer’s ability to breed within five months of birth, coupled with the wild boar decline that reduced competition for food and allowed forest undergrowth to regenerate.
Dr. Chua suggests this may be the first concrete evidence of an ecological impact arising from African swine fever in Asia. With over 15 years of island-wide surveys conducted by his team in collaboration with the National Parks Board (NParks), they could quantify how the disease reshaped forest dynamics. This work, he says, should serve as a clarion call for other areas affected by African swine fever to investigate short- and long-term ecological consequences.
For forest managers, the takeaway is clear: adopt an ecosystem-wide perspective, because disease management in one species can cascade across others. Yet the current high deer density may be temporary. Signs of wild boar breeding emerged a year after the outbreak, and it remains unclear whether the mouse-deer population has hit environmental carrying capacity.
As Dr. Chua notes, once wild pig populations recover, the carrying capacity for mouse-deer will likely shift, stabilizing as interspecies interactions re-balance. An NUS master’s student continues to monitor both species’ populations two years post-outbreak.
The broader context comes from wildlife ecologist Matthew Luskin (University of Queensland), who notes that this study aligns with a long tradition in ecology: changes to one species can trigger cascading, sometimes surprising, ecological effects. Distinguished Research Professor William Laurance (James Cook University) adds that similar patterns likely occur in other forests, such as Malaysia’s Pasoh Forest Reserve, where wild pig fluctuations coincided with dramatic ecological changes.
For example, in 2022 Pasoh’s wild boar numbers plummeted due to African swine fever, and younger trees began thriving as pigs no longer damaged saplings with their nesting behavior. On Pulau Ubin, however, the deer surge is not expected to drastically alter the forest structure because mouse-deer are small and relatively low-impact compared with boars.
The study also offers practical methods for wildlife counting. Line transect sampling—walking predefined paths at night and recording every mouse-deer spotted along a fixed distance—proved more precise for abundant wildlife, enabling researchers to estimate total populations. Camera traps, meanwhile, remained valuable for rarer species like wild boars.
Beyond natural factors, Chua credits NParks’ reforestation work and its decision to let natural vegetation mature for decades as a key driver of the deer’s recovery. Co-author Robert Teo, NParks’ director for Pulau Ubin, calls the population increase encouraging for island wildlife conservation, noting more than 40,000 trees have been planted as part of ongoing reforestation and forest enrichment efforts.
Historically, reintroductions of the greater mouse-deer by the Singapore Zoo and NParks to central Singapore had failed. The rust-colored mouse-deer would not be seen on the mainland again until 2012, when Chua located a population in the Western Catchment forests. The lesser mouse-deer, native to Singapore, has persisted in Bukit Timah and Central Catchment Nature Reserve for years.
Discussion and questions: This case highlights how a pathogen can ripple through an ecosystem in surprising ways. If similar dynamics occur elsewhere, what would be the most effective, ethically sound ways to balance disease control with conservation? Do you think current forest management strategies adequately account for such cascading effects, or should they routinely incorporate cross-species ecosystem analysis even when a disease targets a single species?
