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Rebalancing Australia’s ecology?

By Chris Johnson - posted Tuesday, 2 October 2007

Australia holds the world record for mammal extinctions. We have lost 20 species in the last 200 years, almost half the global total in that time. But an impending ecological tragedy in Tasmania could mean this dismal record is about to get worse.

Mammal extinctions on the mainland were caused primarily by the red fox and feral cat. Foxes never became established in Tasmania, and although Tasmania has feral cats they seem to have had little impact. Aside from the extinction of the thylacine (the “Tasmanian tiger”), Tasmania has kept a healthy mammal fauna.

The fox now has a foothold in Tasmania, thanks to a recent series of accidental introductions and possibly malicious releases. Tasmanian wildlife and agricultural authorities are striving mightily to suppress this small population before it explodes and wrecks their island’s biodiversity and damages farming. We must hope that they succeed, but there is a deeper problem.


The Tasmanian devil population is collapsing as the horrible Devil Facial Tumour Disease sweeps over the island. In a few years, devils will have practically disappeared from the wild. The species should survive: there are healthy animals in captivity and plans are being made to set up more “insurance” populations on small islands and in large free-range enclosures. These will provide colonists for return to the wild when the disease itself is safely extinct. But in the meantime Tasmania’s ecology may have been irreparably damaged.

The devil is the largest surviving marsupial predator. Ecological research worldwide is telling us that large predators play important roles in ecosystems, one of which is to hold populations of smaller predators in check.

Species like wolves and big cats suppress smaller predators by preying on them, but more importantly by maliciously attacking and killing them. In places that still have large predators, small predators like foxes and feral cats live in fear, avoiding the territories of their larger enemies and holding out in small numbers in habitats that give them protection. Foxes and cats are versatile predators and can breed up to high numbers very quickly - give them a free hand by removing top predators and they become exceptionally destructive.

As I argue in a recent book, mainland Australia is the signal example of this. With the extinction of the marsupial lion following first arrival of people, and of the thylacine and devil within the last few thousand years, the mainland lost all its native large predators. The dingo stepped into the role of top predator, but was then suppressed over large areas by Europeans. Much of southern mainland Australia was thereby left completely open to invasion by foxes and cats, which proceeded to wipe out medium-sized ground-dwelling mammals.

On the other hand, there is growing evidence that where dingo populations remain intact, native mammals gain some protection from fox and cat predation. For example, the presence of the dingo across large areas of central and northwest Australia helps to explain why the bilby persists in those areas, having been hunted to death everywhere else.

Have devils played the same protective role in Tasmania? Maybe the reason that cats have so far done little damage to Tasmanian wildlife is that devils have been keeping them under control. If this were true, we should be seeing cats increase as devils decline. The early signs from Tasmanian wildlife authorities’ surveys are that this is happening. Feral cats may be about to emerge as the same savage threat in Tasmania that they have proven themselves to be on the mainland.


It could also be that it was because of devils that previous introductions of foxes to Tasmania failed. Tasmanian wildlife experts such as Dr Eric Guiler and Nick Mooney suggest that devils could have harassed invading foxes, competed with them for food, and killed their young. The impact of this would depend partly on devils having a numerical advantage over foxes. If fox numbers now increase as devils decline, and the tables are turned, it may be far more difficult for devils to eventually stage a recovery.

So, we may be watching the destruction of a crucial ecological balance in Tasmania. If so, the place will soon resemble much of the mainland, with introduced species (cats, maybe also foxes) the most abundant predators, native wildlife hunted to low numbers, and much of the mammal fauna gone. Experience on the mainland tells us that four species will be at especially high risk: the Tasmanian bettong, eastern quoll, red-bellied pademelon and eastern barred bandicoot. All were widespread in southeast Australia but are now extinct (or in the case of the bandicoot, nearly so) everywhere but Tasmania.

We should be planning to preserve these species, but how? One way would be to take some animals into captivity or set up small populations on islands where they would be safe from foxes and cats, for later release back into the wild along with devils. This might work, but the insurance populations would be small and precarious, and perhaps genetically unviable.

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About the Author

Chris Johnson is a Professor of Ecology in the School of Marine and Tropical Biology at James Cook University. His book Australia’s Mammal Extinctions: a 50,000 year history won the Whitley Medal for 2007.

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