Australasian Science Magazine

During droughts, dingoes limit the abundance of the red fox and feral cat. Credit: Bobby Tamayo

By Aaron Greenville

Dingoes are considered a pest by land managers in Central Australia, but it turns out they are effective pest managers of feral cats and foxes – until the rains come.

Nothing quite gets your blood flowing like an encounter with a large predator, whether you’re locking eyes with a lion, even from the safety of a safari vehicle, or listening to the haunting howl of a dingo in remote central Australia. Our instincts tell us to defend ourselves, be ready to get out of their way, to fight or flee. Not surprisingly, when we consider the deep-seated nature of these instincts, humans are responsible for the persecution of large predators all around the world – and their populations are declining.

However, recent research shows that large predators such as lions, sharks and wolves play important roles in their ecosystems. Sitting at the top of the food chain allows them to cast their influence widely.

The subtle but pervasive influence of top predators has become increasingly acknowledged through the study of ecosystem interactions, with important understanding emerging that top predators can limit the populations of smaller “mesopredators”. For example, the grey wolf in North America affects populations of the smaller coyote. In Africa, lions and leopards can restrict populations of the olive baboon.

When populations of these top predators are reduced, populations of the smaller predators increase. In turn, smaller prey species come under increased predation pressure and can suffer.

Like wolves in North America, Australia’s largest terrestrial predator, the dingo, is also the top dog. In a recent study published in Oecologia, we found that while dingoes in the Simpson Desert limit the number of feral cats and foxes in dry times, an explosion in the number of native rodent prey after flooding rains weakens this relationship.

On the basis of this, we suggest it might make sense for graziers and conservationists to concentrate their control efforts on wet seasons. At other times, simply deterring dingo attacks without killing the dingoes themselves should keep the smaller predators in their place.

Cascading Effects of Top Predators

Sitting at the top of the food chain, top predators can reduce populations of herbivores. These effects can then cascade through the ecosystem. Such cascades have an important influence on maintaining the diversity of many species.

For example, sea otters experienced large population declines after overhunting for the fur trade in the 18th and 19th centuries. The removal of this predator produced an irruption in the sea urchin population, as this was a favoured meal for the otter. As a result, the urchin overgrazed the kelp forests of coastal ecosystems. When the fur industry ended, sea otter populations started to recover and they reduced urchin numbers. Consequently, kelp increased in abundance again, and fish species that use the kelp also returned.

After the grey wolf was reintroduced into Yellowstone National Park, it started to prey on elk. The reduction in the elk population led to an increase in woody plants like aspen, which the elk browsed upon. The surprising take-home message is that wolves protect trees!

Such interactions up and down the food chain are among the most fascinating and complex areas in ecology, and challenge our thinking about the world’s ecosystems.

The Accusatory Finger

Australians like breaking records, but we hold one record for which we should not be proud: we lead the world in mammalian extinctions. A total of 29 species, or almost 10% of our mammals, have become extinct within the past 226 years.

This is not simply a problem of the past: in 2009 we lost a small bat, the Christmas Island pipistrelle. Even more recently a native rodent, the Bramble Cay melomys, may have squeaked goodbye. Given the high rate of endemism of our mammals, with so many species found nowhere else on Earth, this is a tragic loss for our natural heritage.

The accusatory finger apportioning blame for this shameful record points squarely at two of our introduced predators: the red fox and feral cat. The red fox is considered a threat to an estimated 76 native species, while the feral cat places at least 34 threatened native species at high risk of extinction.

Past and predicted extinctions are not confined to the more populated areas of Australia. In fact, the remote corners of Central Australia have suffered the greatest loss of mammals.

This begs the question: given that large predators have a regulatory role in suppressing the excesses of smaller predators elsewhere in the world, does the dingo play a similar role in limiting populations of the red fox and feral cat in Central Australia?

Predators Under Surveillance

There has been a long debate in ecology about the importance of the “top-down” effects of predators compared with “bottom-up” effects such as booms in resources and prey abundance. More recently, these two schools of thought acknowledged as important influences on species populations.

The deserts of the world are a great place to study this interaction between bottom-up and top-down effects. Desert populations generally tick along slowly, particularly in the climatically variable environments in the heart of Australia, but every now and then, perhaps every 10 years or more, flooding rains deluge the desert and it blooms – and booms! In boom times the desert environment can experience 60-fold increases in the abundance of native rodent prey. We set out to discover if this massive increase in food changes or modifies the interactions between top and smaller predators.

To answer questions about the interplay between desert predators we needed to use a method to monitor populations of dingoes, cats, foxes and their prey. We cannot be in the field all the time, but a remote camera can.

Remote camera traps are activated by movement. Once switched on, they lie in wait 24 hours per day. When an animal walks past, the trap is triggered. The result is a photograph of the individual, complete with a record of the time, date, temperature and moon phase recorded. This information can be used to establish what time, and where, animals are most active. All this information is gathered without any human presence or interference with the animals.

We set our cameras up to capture images along the dirt tracks in an 8000 km2 study area of the Simpson Desert. The cameras have so far collected photographs continuously for more than 2 years.

What we have discovered is that the interaction between dingoes and the smaller introduced predators is more complex than first thought. During droughts, dingoes apply top-down pressure to limit the abundance of the smaller introduced red fox and feral cat. Fewer photographs of these smaller predators were recorded when the number of photographs of dingoes increase.

In contrast, when populations of rodent prey boomed, this relationship broke down for dingoes and foxes. Instead, all predator populations increased due to the bottom-up influence of increased prey availability.

However, dingoes were still able to limit cat numbers in both drought and boom conditions, making this study one of the few examples of dingoes limiting feral cat abundance. The familiar dog chasing the neighbourhood cat seems to hold for wild populations too.

As prey populations declined and fell back to drought levels, the top-down interaction between dingoes and both of the smaller introduced predators resumed. In contrast, there was no evidence of an interaction between the red fox and feral cat.

By looking at when the photographs were taken, we could also work out when dingoes, cats, foxes and their rodent prey were most active. The red fox and feral cat showed peak activity around the middle of the night – 2 hours before the peak activity of the dingo. Although all three predator species showed some activity during all hours of darkness, this staggered activity time between the smaller predators and the dingo would potentially reduce the likelihood of cross-species encounters.

The red fox and feral cat were also most active when their rodent prey was active, highlighting again the threat that these introduced species can pose to our native wildlife. The smaller predators were most active at the same time, again suggesting there was no interference between the two species.

Implications of Our Study

Our study provides support for the concept that the dingo acts as a top predator in Central Australia, limiting the activity or abundance of the introduced red fox and feral cat, and perhaps also changing the time when the smaller predators are most active. In turn, the presence of the dingo may help to protect populations of native species, such as rodents. A predator protecting prey!

These findings have implications for how we manage introduced cats and foxes – and also dingoes. Dingoes are still persecuted across Australia due to their impact on livestock. Land managers spend a great amount of time and money implementing control programs to limit the populations of all three predators.

However, our study suggests that the dingo is an unpaid pest species manager that is at work every day. By leaving them alone, dingoes can help to control populations of cats and foxes for free.

Land managers should therefore concentrate their pest control programs during the period after large rainfall events. Imagine pooling resources that are spent every year on controlling cats and foxes in Central Australia and instead investing them every 10 years or more? If the objective is to cap cat and fox numbers, then the dingo may be able to do the work for us during dry times.

There is no denying that dingoes will attack livestock, especially sheep, and industry-funded reports suggest this costs up to $60 million annually. Currently, lethal control methods are used to deter attacks on livestock from dingoes, but alternatives to lethal control do exist. Guardian dogs efficiently protect stock from dog attack, and returns on the cost of purchasing maintaining guardians can be made within just 1–3 years. However, if guardian animals do not work in some locations, compensation to graziers for their losses may be appropriate.

We need to get smarter about how we regulate predators on the land given what we know now about the role that they themselves play in ecosystem regulation. In addition to the economic benefits to land managers from fewer cats and foxes, dingoes also play a role in regulating kangaroos, goats and pigs, which reduces competition with stock for grass.

Last, but not least, by erring from the age-old course of persecuting the top predator, we get to keep one of Australia’s iconic animals and all of the ecological benefits that are being presently uncovered.

Aaron Greenville is a PhD student with The University of Sydney’s Desert Ecology Research Group. The research paper in Oecologia is published online at http://link.springer.com/article/10.1007/s00442-014-2977-8