Australasian Science: Australia's authority on science since 1938

Rewilding Australia

Joanne Draper

The proposed rewilding of Tasmanian devils on the mainland jumps a time gap of about 1750 generations, has no clear understanding of what caused devils to go extinct in the first place, and returns no lost function that hasn’t already been made up by other species. Credit: Joanne Draper

By Allen Greer

Are there ecological benefits behind proposals to return Tasmanian devils to the mainland and dingoes to south-eastern Australia, or is “rewilding” simply “biological control” rebranded?

A current controversy in Australian conservation biology is the introduction of one or more species to help “manage” an ecosystem “naturally,” continuously and cheaply instead of artificially, episodically and expensively – as with human management. In Australia, advocates of “rewilding” have proposed introducing native species, exotic species and even extinct species.

Although rewilding proponents usually talk in terms of species, their primary concern is “lost” ecological functions caused by species extinctions. Functions of the greatest interest are those delivered by species with a big role in ecosystems. These so-called “keystone” species are often large predators or herbivores.

However, any one predator or herbivore delivers only a small subset of all predation and herbivore services. It is important, therefore, to scrutinise rewilding proposals for the precise functions thought to be lost and the evidence that the species proposed for rewilding can provide them.

Rewilding proposals are least problematic when they seek to replace a function delivered by a recently extinct local population by translocating individuals of the same species from a still-extant population.

The benchmark for rewilding is the introduction of grey wolves into Yellowstone National Park in 1995 to replace the population eradicated in 1926. The primary lost function was predation on large herbivores, such as deer and elk, which had increased to such numbers they were changing the vegetation structure of the park with flow-on effects to other animals and plants.

Translocations even within one species remain “exotic” because the source population will always differ from the extinct population. For example, grey wolves vary genetically over their range, and become smaller from north to south. The Yellowstone wolves came from disparate packs in Alberta, Canada, about 1000 km north of Yellowstone.

This intractable residual human influence in even the “best” translocations is why the only form of rewilding true to the name is unaided recolonisation. All other forms are human manipulations.

In this light, the “best” proposal for a rewilding in Australia would be allowing dingoes to re-enter south-east Australia. This would only be a case of rewilding, however, if you believe that the dingo, which was introduced into Australia about 4000 years ago, is now a naturalised “native” species.

This belief, however, has two surprising implications. First, it implies that the “solution” to every other more recently introduced species is just the passage of time. After all, cats have already been in Australia for more than 200 years. Second, part of the dingo’s “functionality” is predation on truly native species as well as exotic species.

The “best” rewilding proposal for Australia using a truly native species is the release of Tasmanian devils on the mainland. However, this proposal jumps a time gap of about 3500 years (about 1750 generations), has no clear understanding of what caused devils to go extinct in the first place, and returns no lost function that hasn’t already been made up by other species.

When rewilding seeks to replace an ecological function previously provided by an extinct species, the most closely related replacement species is more likely to provide the lost functions. This means the next “best” species after the extinct one is usually another species in the same genus.

But just as populations of single species differ, species within the same genus differ even more. The functional replacement will be imprecise and other functions – some unexpected and perhaps even unwanted – may be introduced. It is important, therefore, to examine rewilding proposals for their analysis of possible unwanted “functions” that the replacement species may bring with them.

The only Australian rewilding proposal involving two species in the same genus is the “replacement” of the world’s largest-ever goanna, the 330 kg late Pleistocene Megalania, with the world’s largest living goanna, the 160 kg komodo dragon. But among the spectrum of functions performed by goannas, the komodo dragon would only partially cover the lower end of the range of Megalania’s functions and, more importantly, would extend down into the range of several of Australia’s large native goanna species. Ironically, one overlapping function with Megalania would be ambush predation on human-sized prey.

Another problem with rewilding proposals is that the longer a species has been extinct, the less understood are its ecological functions. Consider how little we know about the basic ecology of the only recently extinct thylacine (30 kg). In 2012, Dr Stephen Wroe’s lab at the University of NSW used 3D simulation technology to analyse the thylacine’s jaw structure and mechanics, and concluded that the species caught small to medium-sized mammals such as bandicoots (1.5 kg), possums (5 kg) and wallabies (18.5 kg) (AS, June 2012, pp.19–22).

But in 2013, using the latest isotope techniques on the tissues of the thylacine and its possible prey species, the same lab concluded that the species hunted medium to large mammals such as pademelons (12 kg), wallabies (18.5 kg) and wombats (35 kg) (tinyurl.com/paosy9x). Furthermore, the extent to which thylacines hunted singly or in a group is not known. What hope is there, therefore, of inferring much about the ecological functions of the late Pleistocene megafauna, which went extinct tens of thousands of years ago? Why “rewild” on the basis of someone’s current speculations about them?

Rewilding philosophy sometimes invokes a need to “re-balance” ecosystems that were supposedly put out of balance by the late Pleistocene extinctions. But how do you identify and quantify that lost balance, and how do you estimate what part of the balance has been redressed by 40,000 years of subsequent evolution and behavioural adjustment by living species?

Some rewilding proposals lead logically to some surprising conclusions. Proposals to replace the functions of extinct species that have no close living relatives have to be built around exotic species that are either already in Australia or will have to be imported from overseas.

The megafauna, for example, contained many grazers and browsers, and today many of those general browsing and grazing functions are presumably filled, at least partially, by cattle, donkeys, horses and water buffalo on the one hand and camels and goats on the other. However, allthese exotic species, bar camels, have hard hooves that cut up the soil surface, unlike the soft feet of extinct native marsupials.

Currently these exotic species are culled. With a little imagination, every exotic species in Australia could probably be found filling, at least partially, a function lost since the late Pleistocene.

An even more alarming logical consequence of rewilding’s emphasis on ecological function and balanced ecosystems instead of species replacement is that it allows, in principle, the extinction of a native species or even entire ecosystems and their subsequent replacement with functionally similar exotic species or ecosystems. The logical conclusions of this rewilding philosophy are likely to alarm the general public, which relates to nature through species and not functions.

There is also a curious logic in rewilding arguments that if an extinction occurred naturally and led to an “imbalance,” there is no reason for humans to do anything about it, yet if humans caused the extinction we “ought” to fix it. Because the issue hangs on the cause of the extinction instead of the seemingly more substantive outcome, the argument for “rebalancing” is moral rather than biological. But why should we feel a need to atone for an act that in most cases occurred long ago and in different cultural circumstances?

Some rewilding advocates hope to surmount the problem of extinction with selective breeding or cloning of the lost species. But the longer a species is extinct, the more the environment evolves and leaves it behind. A thylacine cloned tomorrow would be at least 75 years (35 generations) out of date but a megafauna species would be so 40,000 BC.

Humans are rapidly changing the natural environment, and even living species will have a hard time keeping up, let alone species that got off the evolutionary bus millennia ago. Indeed, it could even be argued that for an entire species to spend even one generation in a cryogenic freezer or a zoo – that is, in the total “management” of humans – is enough to make it a relic.

Almost all rewilding proposals acknowledge the need to conduct enclosure trials to see if they deliver the expected outcomes and to reveal any unforeseen consequences. Such experiments, however, are logistically challenging, expensive and potentially inconclusive. They require at least one large fenced enclosure – at least 100 km2 has been suggested for one devil trial – and at least one cycle of monitoring before, during and after the presence of the species of interest. Furthermore, the statistical significance level of the results would have to be high to be compelling.

Interestingly, many rewilding proposals, are just old-fashioned biological control proposals under a new name. The proposal to introduce African elephants to northern Australia to eat introduced 3-metre tall gamba grass is completely analogous to the introduction of the exotic cane toad to eat exotic cane beetles. To have any claim to being a rewilding proposal, the elephant idea would have to posit a “lost” function in an Australian species of grazing 3-metre tall grass. Australia’s largest “lost” grazer, however, weighed only 90 kg, considerably less than many now resident exotic grazing species that cannot control gamba grass.

The proposal to release Tasmanian devils onto the mainland to control cats and foxes is similarly a biological control proposal. The devils have evolved no functionality in regard to placental predators the same size (fox) or smaller (cats) than themselves because there were none in Australia until 200 years ago. Whether Tasmanian devils would provide good biological control of cats and foxes is a separate question that has already been answered in part by the fact that they didn’t stop the spread of cats throughout Tasmania.

Similarly, if the dingo is viewed simply as the first exotic species in Australia, the proposal to let the species back into many parts of its former range on the mainland is simply rebranding Australia’s first exotic species as a biological control agent for cats, foxes and some herbivores such as rabbits. Even then there is no consensus among ecologists as to whether the dingo, as a biological control agent, would actually provide a net gain to native species.

The dingo “rewilding” proposal contains three surprising consequences.

First, the dingo has long been considered at least partially responsible for the demise of the two largest native carnivores on mainland Australia, the Tasmanian devil and the thylacine, so if the only surviving species, the devil, was released on the mainland it would presumably have to be protected from its old nemesis.

Second, if dingoes returned to the more densely settled parts of the mainland they would increasingly interbreed with feral domestic dogs. Those dog genes would introgress into the more remote dingo populations, hastening the loss of the breed’s new dinky-di identity.

Third, dingoes in south-east Australia would provide protective cover to feral dogs because you could not bait or trap the latter without affecting the former.

Like all conservation initiatives, re­wilding is based on a view about nature. Throughout their lives, individual conservationists carry on an internal debate about their philosophy of nature but few ever articulate their views. Without a philosophical context, however, conservation initiatives like rewilding are just ad hoc responses to particular problems.

My own view is that nature is what exists in the absence of human influence. Human management should only involve the removal of past influences and the prevention of new ones.

Impractical? Perhaps. But it can only be judged in comparison with other philosophies.

What this view means in terms of the two exotic species most often mentioned as the target of biological control measures dressed up as rewilding proposals in Australia – cats and foxes – is a reduction in their numbers without the introduction of other human influences.

As the control of rabbits has shown, the best way to date is with targeted microbes, such as feline immunodeficiency virus, feline and canine distemper (alongside the vaccination of domestic animals) or, ideally, a sexually transmitted microbial disease. In this vein, federal Environment Minister Greg Hunt’s impending 10-year program to control cats in Australia could have no better core initiative than a research centre dedicated to the development of viral and bacterial agents (and their vaccines) targeting cats initially before moving onto foxes, dingoes (canids) and eventually the myriad other exotic species that are degrading nature in Australia.

“Rewilders” are looking for salvation at the wrong end of the size spectrum. Bring on the microbial engineersand leave the elephants in Africa.

Allen Greer is a biologist and author of Biology of Australian Lizards, Biology of Australian Snakes, and The Encyclopedia of Australian Reptiles (now discontinued). He has also written and maintains online The Tasmanian Devil: Its Biology, Facial Tumour Disease and Conservation (http://www.sugargum.wix.com/tasmaniandevil).