ALERT's John Terborgh is one of the world's most eminent ecologists. A fellow of the U.S. National Academy of Sciences and winner of the prestigious MacArthur 'Genius' Award, he is truly one of the most important thinkers we have about ecology and the environment. Having just spent six months visiting James Cook University in north Queensland while traveling across much of Australia, John has intriguing ideas he'd like to share about the conservation of Australia's imperiled biodiversity.
Today, Australia appears to be entering its third great wave of mammal extinctions.
The first occurred roughly 40,000 years ago, following the arrival of humans when Australia’s megafauna fell prey to Aboriginal hunters. Some 54 species vanished, most of which were larger than the Red Kangaroo, Australia’s largest surviving mammal today. Some of the disappearing species, like the marsupial herbivore Diprotodon, were the size of a rhinoceros.
Among these casualties of megafaunal collapse (let’s call them 'collateral damage') were all of Australia’s top carnivores, the giant monitor lizard, Megalania, the “marsupial lion,” Thylacoleo, and the "marsupial wolf", the Thylacine.
Along with the Tasmanian Devil, the Thylacine vanished from mainland Australia after the arrival of the dingo, although both survived until recent times in Tasmania in the absence of Dingoes.
No Big Predators Anymore
After this wave of megafaunal extinctions, Australia -- unlike any other continent -- was left without predators of mammals except relatively small ones: the Dingo, the Wedge-tailed Eagle, and an assortment of pythons, goannas (monitor lizards), and quolls ("marsupial 'cats"). Other continents retained so-called apex predators including the lion, tiger, jaguar, and wolf, but nothing so formidable survived in Australia.
As a consequence, Australia’s larger surviving mammals -- such as Red and Grey Kangaroos, Wallaroos, and some wallabies -- were left with only the Dingo and human hunters as agents of population control. Systematic persecution of Dingoes and the forcing of Aboriginals off their land led to outbreaks of kangaroos, with devastating consequences for both vegetation and livestock.
To mitigate economic losses to its livestock industry, Australia supports two contradictory policies.
On the one hand, it supports an active program of persecution of Dingoes through trapping, poisoning, and building of fences.
And on the other hand, it sponsors a commercial harvest of kangaroos and wallabies to counteract the lack of Dingo predation.
In effect, current government policies are costing citizens twice over -- with a surcharge, as explained below, in the form of declining and endangered mammal species. Does this make sense? There are better options.
The Rise of Superpredators
The extinction of Australia’s apex predators left Australia extraordinary vulnerable to superpredators. I am not talking about monsters such as Tyrannosaurus rex, but rather about novel, introduced predators that fail to evoke a fear response in native prey.
Such a predator can wreak havoc on naïve prey, and in some cases can drive species to extinction.
Humans are the most destructive superpredator ever, having precipitated wholesale megafaunal extinctions around the world and having transported other superpredators to countless islands, lakes, and rivers with hundreds of resulting extinctions.
Humans were the agent that brought two devastating superpredators to Australia -- the red fox and the domestic cat.
Foxes began to spread around 1830 from release points in southeastern Australia and quickly expanded westward into the heart of the continent. By 1850, once-common small mammals had begun to disappear.
Fueled by a rabbit outbreak (which kept the foxes well-fed) and their capacity to feed on fruits, lizards, and insects in addition to mammals, foxes became extraordinarily abundant and overwhelmed slow-reproducing native mammals, including both rodents and marsupials.
From 1850 to about 1960, 27 small-bodied mammal species became extinct in the southern half of the continent in a process that ran far ahead of biological investigation, so that many extinctions took place in remote and largely unexplored regions of the outback.
Experts on Australian mammals today broadly agree that foxes were the principal, if not the only, cause of these extinctions.
The Third Extinction Wave
Now Australians are faced with the looming prospect of a third wave of mammalian extinctions, this one focused on the northern half of the continent.
A colleague who was personally involved in monitoring mammal populations across northern Australia related his experience in a recent conference. In the 1980s, small mammals abounded in the savanna zones of Western Australia, Northern Territory, and western Queensland. Live-trapping of small mammals was highly productive. Some mornings every trap contained an animal.
But trapping success today has fallen rapidly to the opposite extreme. Now, in many spots where small mammals once abounded, 100 traps catch nothing. What has happened to precipitate such a drastic decline?
At this point the precise cause of mammal declines is unclear, although several hypotheses seem to have merit. Most conservationists point at feral cats as the principal culprit. Cats avidly pursue and consume small vertebrates, and are known to have devastating impacts on islands, having driven many island vertebrates to extinction.
In an experiment gone awry, the Australian Wildlife Conservancy constructed a large enclosure in Queensland for the purpose of re-establishing the Bilby decades after its disappearance from the state. At first, bilbies thrived inside the fencing but a cyclone precipitated flash floods that breached the fence, allowing cats to enter. Within two years, the Bilbies were gone.
Some scientists familiar with the situation are unconvinced that cats pose the only threat to small mammals in the northern savannas, expressing doubt that cats could drive any species to extinction over such a vast area -- and emphasizing the awkward fact that cats have been present across the north for decades. So, why are species disappearing only now? (Note: foxes have not colonized tropical north Australia, so cats are the only superpredator in this region.)
Other Possible Extinction Drivers
In the face of these objections, other plausible hypotheses for mammal decline gain credibility, including the intensification of livestock grazing, the increased size and frequency of savanna fires, the arrival of toxic cane toads, and the possibility of epidemic disease.
Let’s consider the last two of these first because there is little evidence to support either. Many of the threatened mammals, both rodents and marsupials, subsist on plant matter and would have no occasion to bite cane toads -- something they would have to do to be poisoned by them.
However, cane toads, which can attain a body mass of one kilogram (2.2 pounds), are capable of swallowing the smallest mammals whole, making them another potential superpredator. But one would not expect cane toads to be responsible for declines in larger herbivorous mammals.
As for epidemic disease, there is no smoking gun in the form of mass die-offs, and it seems implausible that a broad spectrum of rodents and marsupials would be vulnerable to a single disease agent.
That said, there are two recent counterexamples: the susceptibility of multiple species of bats to white-nose disease in North America, and the complete extinction of scores of different frog species to chitrid fungus in the Americas, Australia, and elsewhere -- so we must remain alert to the possibility of a pandemic.
Grazing and Burning
In my opinion, the intensification of grazing regimes and comprehensive annual burning across the north are more plausible drivers of large-scale species declines. Both grazing, especially overgrazing, and fire eliminate ground cover, requiring small mammals to forage in the open where they are highly vulnerable to cat predation.
These circumstances lead to another hypothesis: that altered grazing and fire regimes interact synergistically with cat predation to the severe detriment of small mammals. This is a rather messy hypothesis, not the simple kind favored by scientists, but it may hold more of the truth than any other.
Foxes and Cats
Foxes and cats (both wild and/or feral) are present on all the other ice-free continents, but do not cause extinctions -- except in Australia, where, as superpredators, they have outsize effects.
Foxes, and cats especially, can have devastating local impacts on small mammals, birds, and lizards, and do so in North America but only locally around farms and villages. Farther out in the countryside and in protected areas, foxes and cats are suppressed by larger predators, including species such as coyotes, wolves, bears, bobcats, and mountain lions.
Coyotes, the most ubiquitous of these larger predators, have a particular dislike of cats, pursuing and killing them wherever they come in contact. Thus, wherever there are established populations of larger predators, medium-sized predators like cats and foxes are controlled and impose only low-level predation on the creatures they prey on.
Implications for Conservation Policy
Could this model work for Australia as a strategy for rescuing endangered mammals as well as some threatened birds (such as the Night Parrot) and lizards?
The current policy of the Australian Wildlife Conservancy, along with federal and state agencies, is predator control. Millions of dollars are being spent to fence out, trap, shoot, or poison cats, foxes, and Dingoes, both within and outside of protected areas.
Concerted predator-control efforts can be effective, at least in the short run, but they must be sustained whether budgets are fat or lean. Predator control does not offer a permanent solution or anything resembling natural regulation of the ecosystem. As a policy, it commits society to the nearly impossible task of sustaining predator-control efforts in perpetuity.
The Australian Wildlife Conservancy is building hundreds of kilometers of predator-proof fences to create enclosures where endangered species can prosper in the absence of cats and foxes. But does this approach offer a permanent solution?
No. Enclosures are expensive, must be rigorously maintained at high cost, and are subject to being breached by storm water or fallen trees. And as a largely untested method of species recovery, it is possible (I would venture to say likely) that unexpected problems will arise among enclosed populations.
Even in the most optimistic scenario, mammal populations could be restored to only a tiny fraction -- much less than 1 percent -- of the vast expanse of Australia’s great outback.
A Better Way to Conserve Nature?
Is there a better way? I think there is. Given the many drawbacks of a policy of fenced enclosures and permanent predator control, other strategies should be explored.
A number of prominent Australian biologists are urging Dingo recovery in parts of the north where livestock production is not threatened. I am simply echoing this plea.
Let the Dingo be given a fair chance to demonstrate its effectiveness in controlling cats. Australia is replete with national parks, public lands, and private reserves that could serve as testing grounds. With management control over large areas, cattle can be removed to reduce grazing and fire-breaks built to halt most wildfires, thereby removing two potential contributory factors in mammal declines -- leaving only predation as the sole remaining driver.
Would Dingoes do an adequate job of “rewilding” the northern savannas -- allowing native wildlife species to thrive once again? This is the big question that must be answered.
All of the currently endangered small mammals of the north have coexisted with the Dingo for the last 4,000 years or so. Dingoes certainly take small mammals, but the bulk of their diet is made up of kangaroos and wallabies, and these are not threatened.
Surveys of mammals taken on both sides of the massive “Dingo Fence” in South Australia reveal that foxes are decidedly less common where dingoes are present.
The influence of Dingoes in reducing cat numbers appears to be variable and perhaps context dependent, calling for further study. It would be unrealistic to suppose that Dingoes could eliminate cats altogether, but it might be sufficient for them to create a “landscape of fear” that limits where cats hunt -- thereby ensuring the survival of viable populations of small mammals and other vulnerable vertebrates.
It’s definitely worth a try, because the current alternatives are, at best, only stop-gap measures. To save Australia's ecology -- especially in the north where wildlife populations are currently collapsing -- bringing back the Dingo may be our best hope.