Inbreeding or chlamydia? Its an impossible situation. But its a situation that koalas are facing in the suburbs of Sydney, Australia, where low levels of genetic diversity are threatening one of the only chlamydia-free populations of koalas thats left.According to a new analysis in Conservation Genetics, the koalas in the southwestern suburbs of Sydney are some of the only koalas in New South Wales that are still free of chlamydia, a contagious disease thats devastated the other koala populations in the state (and in Australia as a whole). But the koalas in Sydney still arent safe, thanks to their low levels of genetic diversity. Without diversity, endangered species risk succumbing to disease outbreaks and environmental threats, said Elspeth McLennan, one of the authors of the analysis and a researcher at the University of Sydneys School of Environmental and Life Sciences, according to a press release. Read More: Why Do So Many Weird Animals Live in Australia?The Threats Against Sydneys Suburban Koalas Chlamydia, a contagious disease thats spread amongst koalas and humans alike, is a major threat for todays koalas. Brought on by two separate species of bacteria in the animals, Chlamydia pecorum and Chlamydia pneumoniae, the disease (which isnt thought to transfer between koalas and humans) can cause blindness and infertility and is tied to dramatic declines in koala populations.Only a few areas are still safe from the disease for koalas, including the southwestern suburbs of Sydney, where koala populations sit at around 600 to 1,000 individuals.Taking tissue samples from 111 koalas in this area and beyond and subjecting the samples to genetic analysis, McLennan and her colleagues found that the animals had high levels of inbreeding and low levels of genetic diversity. On average, koalas in the Sydney populations have cousin or half-sibling relationships, McLennan said in the release. These dynamics could cause trouble for the suburban koalas, leaving them susceptible to disease. But any possible solutions to this issue pose their own problems, as any attempt to bring koalas in from other areas could cause chlamydia to spread, with potentially catastrophic consequences. (Indeed, the koalas high levels of inbreeding and low levels of genetic diversity mean they are likely to lack the genetic defenses against chlamydia, leaving them vulnerable to the disease if the disease did spread.)Its a classic Catch-22 situation, McLennan said in the release. If koalas from other areas of New South Wales come into Sydney, they could bring genetic diversity. But they could also bring chlamydia. If the latter happens, individual koalas are unlikely to have enough genetic variation to adapt to the threat, McLennan added in the release. Isolation, Inbreeding, and the Looming Danger of DiseaseHow did Sydneys koalas become so inbred in the first place? McLennan and her colleagues say that urbanization has isolated the animals, preventing them from breeding with koalas from outside of the city. This isolation protects them from chlamydia but leaves them at risk if they were ever to encounter the disease. Instead of some individuals being able to naturally clear chlamydia without it progressing to blindness and infertility, it is possible all individuals will contract the infection whereby it progresses to the later stages of the disease, McLennan said in the release.According to the new analysis, it is possible that koalas from beyond Sydneys southwestern suburbs will wander closer to Sydney. Indeed, McLennan and her colleagues found that koalas from Wollondilly Shire, where chlamydia is present, are already moving to Campbelltown, where it is not. Koalas are endangered in New South Wales and in several other areas of Australia, having seen a 24-percent decline in population in the past two decades. While the threats in southwest Sydney seem particularly dire, they hint at the troubles that are threatening koala populations elsewhere. Beyond southwest Sydney, the results show the importance of managing koala populations and their surrounding landscapes. We need to ensure ongoing connectivity between all koala populations to maintain their health and resilience to threats, McLennan said in the release. Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.