Koala refuge is becoming a genetic dead end

Kangaroo Island off South Australia is home to one of the largest populations of disease-free koalas. At first glance, this looks like a great conservation victory.

Tourists and locals alike view the island as a sanctuary, especially compared to mainland habitats where koalas face constant threats. Yet beneath the surface, the story is not as simple as numbers alone.

Koala haven with a dark side

Flinders University scientists recently examined the seemingly thriving koala population on Kangaroo Island and uncovered an unexpected weakness.

The island appears to be a sanctuary, with an abundant and largely disease-free population. Yet genetic testing revealed a very different reality.

The koalas, despite their large numbers, have far less genetic variation than their mainland relatives. In fact, levels of inbreeding on Kangaroo Island are significantly higher than those seen in koala populations from Victoria and Queensland.

Koalas face genetic limits

The explanation lies in the unusual origins of this group. During the 1920s, conservationists moved fewer than two dozen koalas from Victoria to the island, hoping to secure the species’ survival after decades of hunting and habitat destruction had nearly wiped them out.

Although this small group managed to establish a population, their limited genetic makeup was passed on to every generation that followed.

Over time, the lack of new genetic material compounded, leaving today’s koalas plentiful in number but carrying the legacy of a very narrow genetic foundation.

Mainland koalas decline

As the koala population on Kangaroo Island steadily increased, the story on the mainland unfolded quite differently. Across Australia’s east coast, koala numbers have plummeted under the combined weight of human activity and natural disasters.

Hunting once devastated populations, while road construction cut through vital habitats and isolated groups of animals. To make matters worse, diseases such as chlamydia continue to spread, and recurring bushfires have destroyed vast stretches of forest.

These pressures have left mainland populations fragmented, smaller, and officially classified as endangered.

A population without a strong foundation

In contrast, Kangaroo Island seemed to embody resilience. Even after the catastrophic bushfires of 2019–20, which burned more than half the island and decimated habitats, the koalas there endured and remained plentiful.

Their survival gave the impression of a stronghold, a beacon of hope for a species in decline elsewhere. Yet the truth is more complicated.

When scientists looked at the genetic data, the picture that emerged was sobering. The very foundation of this population may not be strong enough to secure its future.

Genetic warning for koalas

“Kangaroo Island koalas are a successful conservation story in terms of population size. But their genetic health tells a different story. Their limited diversity and high inbreeding means they might struggle to adapt to future threats like disease or climate change,” noted study lead author Dr. Katie Gates.

The researchers found long stretches of identical DNA inherited from both parents. Scientists call these runs of homozygosity, and they are a hallmark of inbreeding.

Harmful genetic variants appeared more frequently in a double dose in individuals, increasing the likelihood of health issues. This pattern raises risks of fertility problems and developmental abnormalities. Some captured koalas already show such conditions.

Genetic rescue for koalas

For now, the population has mostly resisted chlamydia and koala retrovirus, but its limited genetic toolkit may not withstand new outbreaks.

Conservationists fear that diseases or environmental changes could strike hard, especially if multiple threats emerge at once.

Without stronger genetic diversity, even minor health challenges could spread quickly, reducing survival rates and weakening the long-term stability of this seemingly secure population.

To counter these risks, researchers suggest a strategy known as genetic rescue. This involves carefully introducing koalas from diverse mainland populations.

“Kangaroo Island has the potential to remain an important conservation refuge for koalas. But without genetic management, this ‘ark’ could become a trap. Our work highlights the importance of ongoing genomic monitoring for species managed in isolation,” said Professor Luciano Beheregaray, senior author of the study.

Lessons for conservation

The findings extend beyond koalas. Conservation on islands and within fenced reserves offers safety but not long-term security unless genetic diversity is actively maintained.

True sanctuaries require not just protection from external threats but also strong genetic foundations supported through careful planning, ongoing monitoring and, when needed, the deliberate introduction of new individuals.

Without these efforts, isolated populations risk becoming vulnerable to hidden genetic weaknesses that may only surface when facing disease, environmental change, or other unforeseen challenges.

The research, published in the journal Molecular Ecology, shows that even apparent success stories must be examined closely to ensure lasting survival.

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