Credit: Simon King/Nature Picture Library

Dingoes can wreak havoc on Australia’s sheep population, so the canines have been fenced off from a large section of the country. But new research suggests that excluding dingoes can lead to a population boom in their preferred prey: kangaroos. And this may, in turn, change the plant composition of the landscape. The finding is the latest addition in a long and contentious dispute about the ecological effects of dingoes on Australia’s ecosystems.

That debate is itself part of a wider discussion on ‘trophic cascades’ — the idea that top predators change the behaviour and numbers of herbivores, which then affect the vegetation and even the soil chemistry of a habitat. The latest research, published on 10 May in Proceedings of the Royal Society B1, doesn’t resolve the debate, but it does bring a more rigorous approach to a field criticized for relying on “weak inference” instead of controlled experiments2

The study was made possible by Australia’s dingo-proof fence, one of the world’s longest and most remote fences. It was built decades ago to keep the predators away from livestock. But the presence of the fence has also resulted in an inadvertent experiment, because “two completely different ecological universes” have emerged, according to ecologist Michael Letnic at the University of New South Wales in Sydney. Either side of the fence, there are obvious differences in the numbers of kangaroos, and in the kinds and amounts of plants. There is more grass on the dingo side, and more woody vegetation, such as trees and shrubs, on the kangaroo side.

Spotlighting the issue

To gauge how the dingoes are affecting the environment, Letnic and his colleague Timothy Morris, who conducted the work while at the University of New South Wales, estimated kangaroo and dingo numbers on either side of the fence. They did this by conducting night-time surveys of animal numbers using spotlights. The team also established 16 plots, each 11 metres square, with 8 on either side of the fence, and half of these being closed off to kangaroos, to probe how the herbivores affect the vegetation.

On the dingo side of the fence, researchers spotted 85 of the canids and 8 kangaroos. On the kangaroo side, the team found a single dingo and more than 3,200 kangaroos.

The plots on the dingo side of the fence showed no real differences in vegetation. But on the other side of the fence, the kangaroo-exclusion areas had about 12% more vegetation cover, implying that high numbers of the herbivores reduce the plant cover in a landscape.

Fenced-off plots on the kangaroo side of the fence also had more soil carbon, phosphorus and nitrogen, suggesting that intense grazing outside the plots was changing the soil chemistry of the area. Letnic thinks that the kangaroos tend to move nutrients away from the grasslands — the animals rest under trees during the day, and so nutrients are deposited in wooded parts of the area when the kangaroos defecate.

Ongoing debate

Ben Allen, a wildlife ecologist at the University of Southern Queensland in Toowoomba, is the lead author of a review published in March that questions the rigour of research on trophic cascades2. And despite the fact that Morris and Letnic have a control component in the kangaroo-proof enclosures — unlike many trophic-cascade studies — Allen doesn’t find the new work convincing.

He says that some of the methods used, such as the spotlight counts, are too crude to provide accurate numbers. And he feels that alternative hypotheses, for example the presence of sheep in the area and differing water availability, haven’t been adequately considered as explanations for the landscape-scale vegetation differences.

Ultimately, the 12% difference in vegetation cover seems too small to affect the structure of the whole ecosystem, Allen says. Dingoes will certainly suppress kangaroo numbers, he adds. “But I don’t agree that this is the driver of the whole system.”

Ecologists Matt Kauffman at the University of Wyoming in Laramie and Oswald Schmitz at Yale University in New Haven, Connecticut, agree that some researchers in the trophic-cascade field have become overenthusiastic. But both think that the new dingo work provides a clear signal that there’s a link between the canines and the broader ecosystem. “This is the kind of experimental evidence that Allen and his group are calling for,” says Schmitz. And they applaud the use of the kangaroo-exclusion plots.

“That’s one of the strengths of this study — that it is experimental,” says Kauffman. “We are desperately trying to understand what it means when we lose large carnivores, and what it might mean in places where we restore them or they are re-establishing.”

Credit: Mitsuaki Iwago/Minden Pictures/FLPA