- New research in Indonesia shows that small islands, often considered extinction hotspots, can act as crucial refuges for rare mammals like the anoa and babirusa, which face severe threats from deforestation and poaching on larger land masses.
- Genetic analyses of more than 110 individuals revealed that while small-island populations have lower genetic diversity and higher inbreeding, they also carry fewer harmful mutations — likely because long-term isolation allowed natural selection to purge them.
- Smaller islands were also found to host higher-quality, better-protected forest habitats, suggesting that conserving these areas may be more effective than attempting “genetic rescue” by moving animals from mainland populations, which could introduce harmful mutations.
- The study highlights the need to refine taxonomy, prioritize protection of small-island habitats, and integrate these overlooked areas into conservation planning, as they may hold the key to the long-term survival of iconic and endemic small-island mammals.
Animals living on small islands are often thought to be more susceptible to extinction compared to those distributed across mainland land masses. Small population sizes, limited habitat availability, and genetic isolation can propel species into a downward spiral. However, new research from Indonesia’s biodiverse Wallacea region suggests that in areas facing intense anthropogenic disturbance, small islands can in fact provide crucial genetic and ecological refuges for rare mammals.
The new study focuses on two forest-specialist mammals endemic to the island of Sulawesi and its smaller offshore islands: the anoa (Bubalus spp.), a type of dwarf buffalo; and the spectacularly tusked babirusa pig (Babyrousa spp.).
Both mammals have suffered steep population declines in recent decades as logging, mining and agricultural expansion have replaced their forest habitats. Populations of anoa are classified as endangered on the IUCN Red List, while babirusa populations range from vulnerable to endangered. With their numbers thin on the ground, researchers often have trouble studying them.
“With elusive species like anoa and babirusa, you don’t get to meet them a lot,” said study lead author Sabhrina Gita Aninta, a researcher at Queen Mary University of London. “This is where genomics can help — you can amplify their DNA from scraps of material [like scat and hairs] to get a lot of information.”
By combining DNA analyses of more than 110 individuals, including museum specimens, with habitat suitability modeling, Aninta and her colleagues compared the genetic health of anoa and babirusa from the large main island of Sulawesi with those living on smaller offshore islands, such as Buton and Togean.
They found that although small-island populations had less genetic diversity and more inbreeding than their larger main island counterparts, they carry fewer harmful genetic mutations. This could be due to long-term isolation allowing natural selection to purge these negative mutations from the small island populations over time, the authors say.
Aninta said the genetic insights can help inform conservation efforts for anoa and babirusa. “If you understand how species evolved through time, how they respond to their environment … you can predict how they might react to conservation actions in the future,” she told Mongabay.
The team also found that smaller islands provide higher-quality and better-protected forest habitats for both species, whereas Sulawesi’s main island populations are exposed to more deforestation and poaching, which have caused massive population declines.
“In the smaller islands, they actually have really high habitat quality,” Aninta said, adding that the availability of such suitable habitat has likely been a factor in enabling island populations to “purge” harmful mutations.
The study “highlights the potential of smaller islands as refuges for the conservation of these Wallacean iconic endemic mammals,” said Roberto Rozzi, curator of palaeontology at Martin Luther University Halle-Wittenberg in Germany, who wasn’t involved in the study. The team amassed an “impressive dataset,” he noted, given the challenges of obtaining more than 100 genetic samples of anoa and babirusa, which represent “rare and threatened marvels of island evolution.”
Rozzi said the taxonomy of both anoa and babirusa is complex and the subject of ongoing scientific debate. There are two recognized species of anoa in Sulawesi: the lowland anoa (Bubalus depressicornis) and the highland anoa (B. quarlesi); and three species of babirusa there: the Sulawesi babirusa (Babyrousa celebensis), the hairy babirusa (B. babyrussa) and the Togean Island babirusa (B. togeanensis).
Conservationists should be refining this taxonomy and each species’ distribution as a matter of urgency, Rozzi said, to ensure the maximum amount of diversity can be preserved through conservation actions. “Populations of anoa and babirusa from different regions of mainland Sulawesi and offshore islands likely represent distinct taxonomic units, all of which merit protection,” he said.
The authors caution against genetic rescue strategies that involve translocating individuals from larger mainland populations to smaller island ones, since this could risk introducing harmful genetic mutations that could undermine the health of the island populations. Rather, safeguarding the high-quality, smaller island habitats should be the priority, they say.
Ultimately, the findings indicate that small islands aren’t always riskier places for wildlife survival. As Aninta put it: “It goes against what we’re taught as conservationists.” Small islands can in fact be safe havens for species suffering catastrophic declines from human impacts in other parts of their range.
Yet these small islands are often overlooked and are rarely the focus of conservation planning. Protecting small island forests from development pressure and establishing monitoring programs to keep track of important wildlife will be vital to ensuring the long-term survival of these rare mammals, according to Aninta.
“Small islands should not be sidelined,” she said. “Some have great potential for conservation actions [and] monitoring is important to make sure [wildlife] is okay,” she said. If populations are found to be stable, then the priority should be on keeping high-quality island habitats as they are.
“Basic habitat protection is still very underrated,” Aninta said. But it’s what these rare and endemic mammals require: “They just need a place to live, to breed happily and to not be stressed.”
Banner image: An adult male babirusa photographed in Gorontalo in Sulawesi. Image courtesy of Simon Mitchell.
Carolyn Cowan is a staff writer for Mongabay.
Citation:
Aninta, S. G., Drinkwater, R., Carmagnini, A., Deere, N. J., Priyono, D. S., Andayani, N., … Frantz, L. (2025). The importance of small island populations for the long-term survival of endangered large-bodied insular mammals. Proceedings of the National Academy of Sciences, 122(26), e2422690122. doi:10.1073/pnas.2422690122
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