Bronwyn Fancourt, University of Tasmania
Eastern quolls – small, fleet-footed and ferocious – are one of Australia’s few surviving marsupial predators. They were once so common in southeast Australia that when Europeans arrived the quolls were reportedly hyperabundant.
But by the 1960s they were extinct on the mainland, driven down by a combination of disease, poisoning, persecution and predation.
Despite their mainland demise, eastern quolls continued to thrive in Tasmania – until recently. Across Tasmania, quoll numbers declined by more than 50% in the 10 years to 2009 and show no sign of recovery.
Recognising this worrying decline, the quolls have recently been listed as endangered internationally and in Australia. This is a stark reminder of how quickly a common species can plunge towards extinction.
But the quolls can still recover, as long as we act now while we still have an opportunity. In research published in Wildlife Research, I looked at what caused the decline, and how we can help.
Change in the weather
Several factors coincided with the decline, but after five years of investigation I found that a period of unfavourable weather was the most likely explanation.
Eastern quolls prefer areas with low rainfall and cold winters. But an 18-month period of warm winters and higher seasonal rainfall during 2002-03 resulted in most of Tasmania becoming unsuitable for eastern quolls. This rapidly drove their numbers down. In fact, the amount of environmentally suitable habitat in this period was lower than at any other time during the previous 60 years.
With the frequency of extreme weather events predicted to increase over coming decades, the future for eastern quolls looks uncertain.
The predator pit
Interestingly, while weather conditions have since improved, eastern quolls have not recovered. With their numbers pushed so low, the remaining small populations can no longer breed faster than other threats kill them off. Historically, when quoll numbers were higher, they could cope with these threats.
Quolls are now trapped in what ecologists call a “predator pit”. Predators, cars, poison and a range of other threats are killing quolls as quickly as they can reproduce.
So population growth is in limbo – not because any threats have increased, but because small populations don’t have the capacity to outpace those same threats anymore.
Contrary to earlier predictions, feral cat numbers in Tasmania have not increased following declines in the Tasmanian devil population. Quoll populations could previously cope with the loss of a few quolls (mainly juveniles) to cats. However, that same number of quolls killed by cats is now potentially enough to wipe out any population growth, preventing the species’ recovery.
The key factor preventing quoll recovery is their current small population. Quoll numbers need a boost, increasing reproductive capacity so that they can once again outpace the threats they are facing. This could be done by supplementing small, surviving populations in Tasmanian with quolls from captive-breeding colonies, insurance populations or the wild population on Bruny Island (which is doing better than mainland Tasmania).
Reducing feral cat numbers at key sites in early summer could also help reduce predation as juvenile quolls enter the population. That would potentially increase juvenile survival and allow quoll populations to grow and recover.
Should quolls be reintroduced to the mainland?
Since word of the eastern quolls’ plight has spread, there has been increasing talk of reintroducing them to Australia’s mainland, where they disappeared more than 50 years ago. Such proposals are often well-intentioned and could potentially help restore some mainland ecosystems.
However, this could actually serve to drive wild populations in Tasmania closer to extinction, making the species’ recovery more difficult.
With only small populations persisting in the wild, removing only one or two individuals from a population could be enough to render that population functionally extinct – and once a population is functionally extinct it is on the path to total extinction.
Similarly, using quolls from captive colonies and insurance populations for mainland reintroductions further removes valuable quolls that could be used to repopulate and recover wild populations in Tasmania.
The eastern quoll’s persistence in Tasmania decades after it disappeared from the mainland suggests Tasmania is a far safer place for eastern quolls and offers them the best chance to recover. Removing them from a relatively safe place and reintroducing them to high-risk mainland sites filled with dingoes, foxes and toxic fox baits could actually hinder, not help, their recovery. For example, while baiting foxes may reduce the threat from foxes, it takes less than half of one fox bait to kill an adult female eastern quoll.
Mainland reintroductions should definitely be a goal in the longer term. But given the dangerously low numbers in Tasmania, we shouldn’t take Tasmanian quolls for high-risk mainland reintroductions until the Tasmanian population is safe. Once numbers in the wild have recovered, wild-sourced Tasmanian quolls could be reintroduced to mainland sites without putting wild populations at risk.
It’s time to act
Australia’s declining species face a slippery slope towards extinction. The key to recovery is understanding why the species declined, then acting while there is still time.
Australia’s history is littered with examples where delays and inaction prevented small populations from recovering, with some species now lost forever. The eastern quolls’ fate is not yet sealed. But we have to act now.
Bronwyn Fancourt, Honorary Research Associate, University of Tasmania
This article was originally published on The Conversation. Read the original article.