Australasian Science: Australia's authority on science since 1938

Conservation Needs More Bite

Credit: Angus McNab

Recent science suggests that the dingo holds the key to protecting mainland Australia's unique biodiversity. Credit: Angus McNab

By Euan Ritchie

What role can devils and dingoes play in curbing Australia’s rate of species extinctions?

It is apparent to anyone walking through the Australian bush that there is little to be concerned about – other than deadly snakes and spiders. In Australia, unlike in Africa’s savannas or North America’s forests, you won’t be eaten by a large predator.

However, less than 50,000 years ago Australia had a diverse group of large predators, including the thylacine (Tasmanian tiger), a mainland devil species, marsupial lion (Thylacoleo) and crocodile-sized varanid lizards (Megalania). While it is a relief that we don’t have to worry about becoming another species’ meal, the absence of these and other large predators is still impacting our ecosystems.

What is even more concerning is that this is representative of a global trend – the dramatic loss of top predators from environments. For example, large mammalian carnivores (e.g. wolves, bears and many large cats) and sharks have declined by more than 90% in many parts of the world. The gray wolf was estimated to have a population size of over 200,000 prior to European arrival in the United States, but now probably numbers less than 5000.

Why does it matter if some species disappear, and aren’t we better off without them? The answer is a resounding no, as demonstrated by a recent and considerable body of work dedicated to understanding the ecological roles of predators and how these affect humans in ways that many of us fail to appreciate.

A quote from the ecologist Daniel Janzen perhaps best describes the problem of species extinctions: “What escapes the eye... is a much more insidious kind of extinction: the extinction of ecological interactions”.

What Janzen is referring to here is the way in which species are linked to each other within a complex ecological web. By breaking key links within it, such a web can unravel with profound and often disastrous effects.

The most well-known example of this is the loss and subsequent reintroduction of wolves into Yellowstone National Park. When wolves were removed, elk over-browsed vegetation, erosion increased, flow regimes of rivers changed, beavers disappeared and songbirds also declined. When wolves were subsequently reintroduced in the mid-1990s, many of these effects began to be reversed.

In short, predators have two key roles in the landscapes in which they occur. First in regulating the number of herbivores and their effects (grazing impacts) in a given habitat, and second in controlling the impacts of smaller subordinate predators (mesopredators). The control of herbivores by predators has long been established, but apex predator control of mesopredators is only beginning to be understood now.

So what relevance does all of this have for Australia? Australia’s native fauna has been ravaged – we have lost more mammal species than anywhere else on Earth in the past 200 years. One could be forgiven for thinking this is a tragic event of the past, but what is alarming is that mammal species and populations are again disappearing from the north to the south of this continent. Dramatic losses of native species are occurring in both Kakadu National Park and Tasmania, which were once considered safe havens for our native mammals.

Why has Australia suffered so much extinction? Somewhat ironically and perhaps at first confusingly, it is due to the impacts of predators, namely the red fox and feral cat, and the absence of larger predators that could control these species.

The critical point here is that not all predators are the same in the way they affect ecosystems. Which brings us to the question: what predators does Australia have that could potentially control cats and foxes? The answer is the Tasmanian devil and dingo, two species with vastly different histories and public identities.

The devil, which is now confined to Tasmania, occurred as a widespread and larger species in mainland Australia less than 5000 years ago. Devils in Tasmania now face a very serious challenge to their own survival due to the lethal impacts of a contagious cancer – devil facial tumour disease (DFTD) – that has decimated populations.

It now appears that red foxes have been introduced and are possibly in the process of becoming established in Tasmania. There is an interesting story here as it has been suggested that there have been many attempts to introduce foxes into Tasmania, but devil predation of fox cubs may have prevented their establishment. With devil populations now reduced, the time is right for foxes and cats, another species likely to be controlled by devils, to increase their numbers and distributions considerably.

One might assume that if devil populations recover from DFTD they will once again control foxes and cats. But if fox populations were to become large, through weight of numbers the roles could be reversed, with fox predation of young devils likely to prevent devil recovery. This serves to highlight just how complex and context-dependent predator interactions can be.

And what of dingoes? The dingo arrived in Australia around 3500–4000 years ago, and as such is considered native by some people and introduced by others. Confusingly, dingoes are afforded protection as native species in some areas while in others they are eradicated under the guise of wild dog control.

I would argue (perhaps controversially) that it is irrelevant whether dingoes are native or introduced, and indeed for that matter how genetically pure they may be. What is important is the function they are performing in the absence of any other large, land-living predator. Like wolves and many other predators, it is now clear that dingoes are critical in shaping our Australian environments, primarily by reducing herbivores (particularly goats, pigs and kangaroos) and reducing or modifying the behaviour of other predators like cats and foxes.

Understanding the behavioural effects of predators is something we are only just coming to terms with, but it is apparent that the effects of dingoes on smaller predators goes well beyond just reducing their numbers. Work from northern Australia provides evidence that dingoes cause cats to become less active in the early part of the evening, which coincides with the peak activity of small mammals and nocturnal reptiles such as geckoes. But when dingoes are removed from habitats by poisoning and shooting, cats are released from dingo-induced fear and respond by increasing their activity in the early evening, presumably to capitalise on optimum hunting opportunities.

The effects of controlling dingoes are also more complicated than one might expect. Dingoes, like many other canids, have complex social hierarchies. When individuals are killed (particularly older, more experienced individuals), pack structures can break down. When this occurs, young animals lose the opportunity to learn from older animals how to hunt for prey, and are therefore more likely to attack calves. In addition, these dogs are able to breed because they are not being stopped from doing so by older, more dominant individuals. So, counterintuitively, controlling dingo and dog populations may actually lead to more of these animals, and more attacks on livestock, locking us in to a vicious cycle of continued baiting and increased stock loss.

With all of this in mind, what should we be doing now? Desperate times demand bold measures, so the reintroduction of devils and dingoes must be placed at the top of the agenda for conservation and management initiatives in Australia. We cannot continue with the status quo, which is clearly not working or providing long-term solutions.

Of course, it is naïve to think that this will be simple. Significant cultural, economic and environmental obstacles lie in the path of predator reintroductions. For example, sheep and dingoes will never mix, but perhaps rather than respond to this by reverting to traditional dingo control methods that may themselves contribute to other problems (e.g. more foxes and kangaroos), we should encourage and support graziers to adopt novel solutions, such as the use of guardian protection animals like donkeys, alpacas and maremma dogs.

In the face of unprecedented environmental change our best bet is to promote the natural stabilising forces of ecosystems, which include predators. We must also view conservation and land management holistically, rather than constantly intervening and treating issues in isolation as if they are somehow all disconnected.

Euan Ritchie lectures in ecology at Deakin University.