With 11% of its mammalian fauna extinct within just the last few centuries, it has the highest rate of mammal extinctions in the world. On top of that, 15% of the remaining species are also endangered, with more yet more on the verge of being so. All in all, things are not going well for Australia's mammals. The typical explanation for this is that, as an isolated island continent, Australia's endemics are especially vulnerable to introduced species. It was the foxes, goats, and horses that did it, or so the story goes. In this post I will be arguing against that prevailing idea, and continuing on my previous post on Australia in arguing that invasive species are merely a symptom of a wider problem, and that a few of the introductions may in fact be part of the solution.
Australia: Land of marsupials, especially extinct ones.
I will spell out my main thesis from the get go. The reason why Australia's animals are being driven extinct at such a rapid rate is that the most important members of the fauna, the megafauna and the keystone species, have both already been mostly wiped out, destabilizing the entire continent's ecosystems and rendering them unable to adapt. In recent years we have learned that many ecosystems function from the top-down instead of bottom-up, meaning that instead of the smaller species functioning as the basis for all larger animals as previously thought, the larger creatures actually exert a greater impact upon the rest of the ecosystem. Megafauna is crucial to biodiversity and ecological processes, and without them the entire rest of the ecosystem becomes fragile, in some cases, like in Europe, collapsing entirely. Australia's animals are often presented in the same way as island endemics - inherently vulnerable creatures that evolved in an isolated environment. This is undeniably the case for most actual island endemics, but the thing is, Australia is not an island. Every single species native to mainland Australia has at some point had to compete with a vast fauna of other animals, completely comparable to that found on all of the other continents. Thus, saying that the natives of Australia were not used to the level of competition exerted by the introduced newcomers such as foxes and cats is just not true, they absolutely are, in fact they evolved in such an environment. But it is also true that foxes and cats are having a huge and negative impact on the native species of Australia. So, how can one reconcile this apparent paradox? Animals are a very diverse bunch, and even if they fill the same niches, their general behavior may still be very different. As an example, compare the thylacine with the grey wolf. Both are(were) medium sized predators with similar bauplans, both fill(ed) essentially the same roles in their given ecosystems as the principle medium sized predators, yet they act(ed) differently. Thylacines were mostly incapable of tackling larger prey, as indicated by their weak jaws, and as such were probably mostly restricted to smaller game, while wolves are major predators of animals several times their weight. In the pristine Australian ecosystems, it seems that the only animals predating on large herbivores were equally large carnivores, while the smaller predators were mostly restricted to equally or smaller prey.
So, how does this relate to the introduced predators such as foxes? Well, in pristine Australia, there were fox-sided predators aplenty, but they had different behaviors and diets than actual foxes. An example of such a predator is the misleadingly named Tasmanian devil, which once used to range widely across mainland Australia. It shares the same general prey as foxes, but having co-evolved with the other native Australian species, does not cause any major decline in them. In fact, there is strong evidence to suggest that it actively benefits them, and its presence on Tasmania has often been cited as the reason as to why foxes and cats never managed to become established on the island. And this is the crux of the issue. In a complete ecosystem, every niche is already taken. Small predators, medium sized predators, large predators, and all the ranks in between, they're all present. They don't wipe out the other native animals, as otherwise they obviously would not be able to share ecosystems, but what they do seriously affect is introduced competitors. The chance of a large predator being successfully introduced to an ecosystem with all of its trophic levels intact is pretty much nul. This is why Africa is essentially much completely devoid of major invasive species, while areas like Australia are riddled by them. In Australia, pretty much all of the medium sized and large predators have been wiped out. When the Aboriginals first arrived, they exterminated most of the major predators, such as the giant monitors (Megalania), terrestrial crocodiles (Quinkana), and marsupial lions (Thylacoleo). Then came another wave of immigrants several thousand years later, who proceeded to wipe out the thylacine and tasmanian devils on the mainland, but brought with them the dingo. When the Europeans arrived, the dingo was subsequently heavily persecuted to the point of near extinction, and thus we were left with an entire continent virtually without large predators. Nature is adaptive, and when a niche is open, it will be taken. When pretty much every single niche on an entire continent is open, you can be certain you will get invaders. We set the board on which the Foxes, cats, and other introduced species are now playing. Had we not destroyed most of Australia's original fauna, the opportunity for such large scale invasions would never have existed to begin with.
Australia's native predators, like this Thylacoleo, have been slowly and progressively wiped out over the years since humans first arrived on the continent. Art by Peter Schouten
So what to do? Since we have wiped out most of the native species, and since the introduced ones are already firmly established, how do we reverse this trend? Well, I won't pretend to have the complete answer to that broad a question, and I very much doubt it is possible to completely reverse the downwards trend, at least for all species. I do however see clear ways to mitigate the damage, and to make some actual quantifiable improvements. For one, not all of Australia's predators are extinct. Most are, but some relatively large critters remain, most notably the various monitor lizards, the tiger quoll, and the Tasmanian devil. Many of the monitors have become rare and locally endangered due to the increase of cane toads (Rhinella marina), but experiments are underway to see if they can be taught not to eat the frogs. If they are successful, it could halt their decline, and perhaps even allow them to recolonize previous parts of their range. Tiger quolls are heavily declining throughout most of their range, save for Tasmania, which is also the only place where the Tasmanian devil remains. The latter are not safe there however, as the confined population is being slowly wiped out by devil facial tumor disease, a form of transmissible cancer that is rapidly spreading through the population. The Tasmanian government does not want the devils reintroduced to the mainland, for fear that it might harm tourism, but the moment conservationists care more about a goverment's income than the extinction of a keystone species is the moment they have lost their way. Reintroducing the devil to the mainland is a cause that all Australian conservationists should be fighting for. A large part of the reason the tiger quolls are declining is due to habitat destruction, as they are native to the western coast of the continent, one of Australia's most heavily cultivated and managed regions.
Tiger quolls and devils are both in trouble, but they are also a potential key to solving many other species problems. Reintroducing the devils to parts of their range from which they were extirpated, and introducing the quolls to parts of Australia we do not currently know if they have ever inhabited, could quite probably help save them, and the many species that depend on native predators. Many parts of northern Australia still have healthy populations of dingos, and if they were supplemented by quolls and devils, that would go a long way towards reestablishing a guild of native predators. That term is of course rather arbitrary, as dingos are not strictly native. They were introduced by humans some 4000 years ago, and may have been part of the reason why the thylacine went extinct on the mainland in the first place. Today however, they are one of the good guys, as evidence suggests that their presence is a major boon to native marsupials, simply for the reason that they control the numbers of foxes, cats, and multiple introduced herbivores. A healthy ecosystem needs larger predators, and while dingos are not themselves that big, like their wolf ancestors, they fill the niches of animals significantly bigger than themselves. Australia will never be completely rid of animals such as foxes, they are here to stay, but the damage that they do could be minimized. In fact, I would argue, if their populations were controlled by healthy numbers of predators and competitors, they could be fully integrated into the continent's ecosystems. Predators of their size are, as mentioned before, not a new thing in Australia, but without anything to hold them back, they overwhelmed the continent's fauna. If incorporated into a larger predatory guild, they may actually be able to coexist with native animals in a sustainable way, though this is obviously something of a pipe dream at the moment.
Tiger quolls could be part of the solution to Australia's introduced species problem
I will discuss the introduced herbivores too, but I already covered them quite extensively in the previous post, so I will make it relatively quick here. While the introduced predators are the most famous of Australia's "invasives", the majority of introductions have been of herbivores. Horses and donkeys, known locally as Brumbies and Burros respectively, are now common throughout much of the continent. Goats are abundant in the southwestern regions, mostly where dingoes have been exterminated or suppressed, but do not seem to fare particularly well where predation is high. Both rabbits and hares have been introduced, with rabbits having the greater distribution, and many species of deer have been brought over, mainly from Europe, though most are confined to relatively small areas. Perhaps most famous of the introduced herbivores are the camels, which are both numerous and incredibly widely distributed throughout the central deserts and savannahs of the continent. Forming enormous congregations and strongly affecting the floras of the ecosystems they inhabit, they are the target of a serious culling effort. Another species with a large bounty on their heads is the water buffalo, which is present in parts of the northern swamplands. It is undeniable that these species have a major influence on the ecosystems they inhabit, but this is where I make perhaps my largest contention. I don't think the effects of most of these species are negative, at least not inherently. I am not talking from a cultural or ethical perspective here, but rather a completely ecological one. We are not used to Australia having large populations of megafauna, but this is a case of shifting baseline syndrome, where we forget what came before, perceiving recent conditions as normal even if they are anything but. As mentioned earlier in the article, Australia used to be populated by a wide variety of megafauna, and most of these were herbivores. With giant kangaroos (Procoptodon), rhino-sized wombats (Diprotodonts), and completely bizarre animals like the marsupial tapirs (Palorchestes), the continent was home to a rich fauna of large browsers and grazers. These were all wiped out when humans first arrived, 40,000 years ago, but the fauna and flora of the continent has coevolved with them, and it was the absence, not the presence, of large grazers that was abnormal.
But of course, if a population of herbivores gets too large, it can be damaging. This is the case even with native species, as demonstrated perfectly by the great environmental damage dealt by the overpopulated deer in Britain. Predators are necessary to manage herbivore populations, and this brings us full circle. In order to have a truly healthy ecosystem, all trophic levels need to be present. That means reintroducing locally extinct species, or, if the species is globally extinct, as is the case with most Australian megafauna, bringing in ecological surrogates that fill much the same role as the missing animals. The camels and horses introduced to Australia fit this bill perfectly, and have the potential to reinvigorate what were previously stale and depauperate ecosystems. This is, of course, on the condition that they are controlled by predators. Mega herbivores and apex predators, those are two key roles that have now been brought back to Australia. Instead of wasting money attempting to exterminate the introduced herbivores while simultaneously fighting a futile war against the cats and foxes, we should accept the continent's new megafauna while trying to reintroduce some of the extinct predators. Only by doing this, by fixing the ecosystems top down, can we hope to stabilize the free fall that Australia's wildlife is currently in. Much like in Europe, conservationists in Australia have been have been fighting the wrong enemies and neglecting the right allies. Unlike much of Europe however, Australia has the space to really pull of ecological restoration at an epic scale, if only they want, and that is of course perhaps the most important part of the problem.