Australasian Science: Australia's authority on science since 1938

Rewilding the Devil

By Allen Greer

What evidence is there that reintroducing Tasmanian devils to mainland Australia will affect the number of feral cats, rabbits and foxes?

Conservation biologists have recently floated the idea of introducing Tasmanian devils onto mainland Australia from where they disappeared a few thousand years ago. They think devils might help control the number of exotic species such as cats, rabbits and foxes. Proposed release sites are Wilson’s Promontory and the Otways in Victoria and far south-western Western Australia.

The introduction of devils onto the mainland is a feature initiative in a $40 million bid by a consortium of 40 Australian and New Zealand institutions for a “rewilding” Cooperative Research Centre.

What evidence is there, however, that devils might affect the number of cats, rabbits and foxes?

In Tasmania, two natural “experiments” are running that may be informative. One involves the removal of devils by devil facial tumour disease (DFTD) from places they have been “forever", and the other involves the introduction of devils to places they have “never been".

The removal experiment is based on annual spotlight surveys on 170-odd 10 km sections of highway throughout Tasmania from 1985 to the present. These surveys allow a comparison of the number of sightings of devils and other species over time.

A recent analysis of the data between 1985 and 2008 reveal a detectable increase in cats in only one area of the state, the north-east. Here, an approximately 80% decline in devils is associated with an approximately 100% increase in cats (the numbers are rough). This area has had DFTD the longest, so the devil/cat interaction is well established. The area is also relatively undeveloped, reducing confounding human influences in more developed areas.

The spotlight surveys show no measurable effect on rabbits but other factors, such as the introduction of rabbit calicivirus, complicate interpretation of the trends.

The surveys are uninformative for foxes due to the still low numbers of this recently introduced species.

In support of the spotlight survey results for cats, baited camera stations in southern Tasmania have recorded relatively low numbers of cats where devils are present, and vice versa. Video evidence shows that approaching devils cause cats to leave carcasses.

This study does not indicate, however, if devils affect actual cat numbers in an area. Devils may just keep cats away from the baited stations.

The “introduction experiment” started in November 2012 when devils were released onto Maria Island off the east coast of Tasmanian (a second and final release occurred in November this year). Prior to the release, the authorities surveyed the resident fauna, including the cats.

This introduction could provide the most rigorous evidence to date of the devil’s effect on a local cat population, although it may take some time for a trend to emerge. There are no foxes or rabbits on Maria.

The devils’ impact on all three species is most likely to be via predation on the young and competition for shelter sites. As to predation, devils are unlikely to catch many adults of any of the three species of interest because adult cats and foxes are too large and all three species are too agile.

The young of all three species would be another matter. Devils could physically access at least some of the dens of cats and foxes and would probably eat any unattended, dependent young.

As for rabbits, devils are good diggers when it comes to buried carrion, but there is only one second-hand observation of a devil digging out a warren.

As for competition, devils are likely to compete for dens with cats and foxes, but not rabbits. But it is difficult to speculate meaningfully about the dynamics of den competition.

On the available evidence, devils would probably suppress significantly the number of cats in some, but not all, areas. It is unclear if they would suppress rabbits to any meaningful degree. And how they will interact with foxes is simply unknown.

Devils are now kept in several large “free range” DFDT-free enclosures on Tasmania and the mainland. If animal welfare concerns could be met, one of these enclosures might provide an opportunity to observe critical interactions between devils and the species of greatest concern, the fox. Which species displaces the other at carcasses? Can each species access the other’s dens? At what size (age) can each species physically defend itself against the other?

It seems prudent to exploit fully the existing opportunities for observing the devil’s effect on cats, rabbits and foxes in Tasmania, and perhaps in a mainland enclosure, before initiating a landscape-sized, possibly irretrievable, devil-introduction “experiment” on the mainland.

Allen Greer is a biologist and author of Biology of Australian Lizards; the Biology of Australian Snakes, and the Encyclopedia of Australian Reptiles (now discontinued). He has also written and currently maintains the online book The Tasmanian Devil. Its Biology, Facial Tumour Disease and Conservation (http://www.sugargum.wix.com/tasmaniandevil). He has no institutional affiliation or conflicting financial interest.