When is a native species also invasive, and how can we tell? This may seem a strange question, but it highlights the difficulty we sometimes face determining the boundaries of the area in which a species naturally occurs. Especially when detection is imperfect and those boundaries may change over time. Animals move. Plants move. Sometimes a species will naturally move into a new area, and we recognise this as a range expansion. At other times, a species may only be able to move to a new area with human help (deliberate or unintentional), and this may create a new, invasive population.
Think about Australia. If I asked you to name an invasive mammal, you might choose a fox, cat, rabbit or pig. These are clearly not native to Australia. What if I ask you to name a native mammal? Maybe you chose a red kangaroo or a wombat? They are both native to Australia, but they are not native to ALL of Australia. If we were to move a native species to a new part of the country where it had never previously occurred, it may not find the resources it needs to survive, but if it did, we may have created a new invasive population – an invasive native.
An interesting thought exercise perhaps, but why does it matter? In some cases, it may not really matter at all. But in other cases, where one species has a detrimental effect on other wildlife, understanding its status as native or invasive can be important for wildlife management.
So, let’s discuss sugar gliders. The sugar glider (Petaurus breviceps) may be one of the better-known marsupials globally, thanks to its popularity as a pet in some parts of the world. These cute little gliding possums are native to Australia, Papua New Guinea and Indonesia, and are currently listed as Least Concern by the IUCN red list. However, as above, although the sugar glider is native to Australia, it is not native to ALL of Australia.
In particular, there has long been uncertainty about the status of sugar gliders in Tasmania, a large island State south of the Australian mainland. Sugar gliders are thought to have been introduced to the island from mainland Australia by early European colonists, but there has always been some uncertainty around this, so for legislative purposes sugar gliders are still considered to be a native species in Tasmania, and as such have legal protection. This hasn’t really been a problem, until…
As their name suggests, sugar gliders are known for a diet of nectar and sap, supplemented with invertebrates. But more recently, researchers have discovered that, in Tasmania at least, they’ve developed a taste for endangered parrots, eating eggs, chicks and adults on the nest!
In fact this may represent a relatively new feeding strategy for the sugar gliders, exacerbated by deforestation, which has forced hollow-nesting birds to compete with sugar gliders for real estate. Now, a combination of habitat loss and predation by sugar gliders threatens the continued survival of the critically endangered swift parrot and also poses risks for other birds in Tasmania, including the endangered forty-spotted pardalote and, potentially, the critically endangered orange bellied parrot.
Fortunately, the Difficult Bird Research Group is on the case, and they have come up with some innovative solutions to the sugar glider problem, including glider-proof nest boxes (AKA “Possum-Keeper-Outers”), now immortalised in cartoon form by First Dog on the Moon. However, nest boxes alone will not save the parrots, and resolving the status of sugar gliders in Tasmania would help conservationists to develop the most appropriate management strategies.
This is where our new paper (online today at Diversity and Distributions) fits in. With Catriona Campbell as lead author (this is the first publication from her PhD – congratulations Cat!), our University of Canberra team worked with Difficult Bird researcher Dejan Stojanovich, Clare Holleley from CSIRO’s Australian National Wildlife Collection, and Kathryn Medlock from the Tasmanian Museum and Art Gallery, to use two different lines of evidence to understand how the sugar glider got to Tasmania.
First, we looked at historical sources. The colonisation of Australia by Europeans was documented by numerous accounts of the wildlife encountered around new towns and settlements. In Tasmania, there are records of sugar gliders being introduced to the Launceston area from Port Phillip (in Victoria, mainland Australia) in the 1830s, including the below account from Ronald C. Gunn at the 1845 meeting of the Tasmanian Society:
Further, we were unable to find any records of sugar gliders in Tasmania before 1845, despite the passion for natural history evident in the early 1800s, which resulted in the collection and preservation of specimens of many Australian mammals.
Second, we compared DNA sequences of sugar gliders from across Australia and Papua New Guinea, to look for genetic evidence of the provenance of Tasmanian sugar gliders. I’ll explain. Tasmania was once connected to mainland Australia by a land bridge. Rising sea levels flooded this land, which became the Bass Strait, and turned Tasmania into an island somewhere between 9,000 and 25,000 years ago. If sugar gliders are native to Tasmania, that means that they reached Tasmania under their own steam, before the land bridge disappeared. It also means that Tasmanian sugar glider populations would have been separated from their closest mainland relatives for thousands of years. We would expect such a population history to be reflected by:
- Genetic diversity within the Tasmanian sugar glider population, because the original founders may have reached Tasmania from several source populations, and because thousands of years should be plenty of time for new genetic variants to arise in Tasmania. Relatively high genetic diversity within Tasmania has been seen in other Tasmanian animals that reached the island via the land bridge, for example the pademelon.
- Genetic differentiation between Tasmanian and mainland populations of the sugar glider, because new genetic variants would also be expected in mainland populations, and the same new variants would be unlikely to independently occur in both places. This has been observed for other marsupials that have both mainland and Tasmanian populations, for example the long-nosed potoroo and the eastern grey kangaroo.
Instead, we found that all of the Tasmanian animals that we sampled had identical mitochondrial DNA sequences. Further, the DNA sequence variant we found in Tasmania was 98% identical to one of the DNA variants we found in southern mainland Australia. This suggests that the Tasmanian sugar gliders are all derived from a single source population, likely in Victoria or South Australia, and that they have been separated from that source population for hundreds, rather than thousands of years. This is all remarkably consistent with historical records of sugar gliders being taken to Tasmania from Victoria in the 1830s.
From our perspective, this is fairly convincing evidence that sugar gliders are not native to Tasmania. This confirms that, unlike many other Tasmanian marsupial populations (for example southern brown bandicoots), the Tasmanian sugar glider population does not represent a distinct evolutionary lineage and as such has no need for separate conservation management. Instead, our results provide support for listing the sugar glider as an introduced pest species in Tasmania.
There is now an urgent need to enable the effective conservation of endangered birds like the swift parrot. The impact of sugar gliders on swift parrots has been compounded by the loss of forest habitat (upon which both species depend): some to illegal firewood collectors, some to legal forestry operations. Preservation of remaining habitat for Tasmania’s endangered forest species will be crucial, and perhaps new management strategies can be developed that will consider how forestry activities and habitat availability influence sugar glider behaviour. If only we could somehow persuade the gliders to give up their taste for poultry, that would surely be a winning solution!
[Main photo: sugar glider in the Atherton Tablelands, Queensland. Photo credit Joe McKenna, sourced from Flickr, CC BY 2.0]