Our study of one of Australia's most secretive marsupials sheds light on how changes to forests have enriched biodiversity
Have you ever wondered what the Australian continent looked like 2 million years ago, or even 6 million years ago? Our recent study of the long-nosed potoroo has used genetic tools to peer back in time, examining how changes in the wet coastal forests of eastern Australia have influenced marsupial biodiversity. We found that the distribution of genetic diversity within long-nosed potoroo populations appear to have tracked the expansion and contraction of the wet forests they inhabit along the east coast.
While we currently think of Australia as an arid continent, it was once covered in wet forests, until about 33 million years ago (Eocene) when these forests started to be replaced with more arid vegetation. These changes were more pronounced during the climatic oscillations (cycles) that characterised the Pleistocene (starting 2.6 million years ago) when wet forest habitats contracted and expanded repeatedly as the climate oscillated between dry and wet phases.
So how does studying the genetics of a marsupial help us understand these changes in vegetation? Some species are very picky about where they live. The long-nosed potoroo, for example, likes wet forests with lots of thick ground cover and avoids open or dry habitats. So when major habitat contractions occurred as the result of increased aridity, populations of these ‘habitat specialists’ became isolated in wet forest refugia and gene flow between these isolated patches stopped, resulting in genetic differentiation, the signatures of which can be seen in the genomes of potoroos today.
Our study examined potoroo populations from southern Queensland, New South Wales, Victoria and Tasmania and found the genetic signatures of these past events. However, while some of the patterns we saw were expected, others were a surprise.
We found Tasmanian potoroos were genetically different from those on the mainland. This makes sense since Tasmania is an island and this genetic split is seen in many species. But around 14,000 years ago (and at various other times during the Pleistocene) there was a land bridge between Tasmania and the mainland, so potoroos may have been able to move across what is now Bass Strait. However, dating the genetic differences indicated that the last time there was gene flow between Tasmanian and the mainland Potoroos was around 2 million years ago. This suggests that the land bridge which formed several times during the Pleistocene was most likely too dry and didn’t have the right forest habitat to allow dispersal for potoroos.
Some other patterns we detected were more surprising. We found a very deep genetic split on the mainland right across the Sydney basin. This region hadn’t been documented as a biogeographic barrier before, but we dated the split to around 6 million years ago, suggesting that a major habitat break formed here that prevented gene flow in the potoroos across this region. The genetic split that is still evident in mainland potoroos is so great that long-nosed potoroos north and south of Sydney may actually be different species, however further research is required to determine this.
We also identified areas where forest must have remained intact and allowed gene flow for long periods of time, such as in southern NSW and across much of Victoria, as well as across much of Tasmania (dissected only by the midlands in central Tasmanian). This was different to patterns seen in the reptiles of this region, highlighting how different taxa were able to exploit, or not, past habitat changes.
While traditionally it is the fields of palaeontology and palaeobiology that are used to look into the past, modern genetic techniques are also able to tell us much about how species have moved across the planet in response to past climate and habitat changes. These findings are also likely to provide hints at how these species will react to future changes to their environment.
Dr Greta Frankham, Australian Centre for Wildlife Genomics, AMRI
Dr Mark Eldridge, Principal Research Scientist, Australian Museum Research Institute
Frankham, GJ., Handasyde, KA, and Eldridge MDB (2015) Evolutionary and contemporary responses to habitat fragmentation detected in a mesic zone marsupial, the long-nosed potoroo (Potorous tridactylus) in south-eastern Australia, Journal of Biogeography