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An introduced black rat scratches through leaf litter, looking for food. Nearby, a native water rat watches on, its beady eyes shining. The native rat pounces out from the shadows, sending the invader fleeing.

The encounter in Sydney bushland, captured on video, is the first documented evidence of an aggressive interaction in nature between a native water rat, also known as rakali, and a black rat.

The footage, discussed in , provides proof that rakali (Hydromys chrysogaster) actively hunt introduced black rats (Rattus rattus). This behaviour may offer a promising natural form of pest control.

Rakali are carnivorous rodents, and the largest of Australia’s 60 native rat species. Our findings suggest efforts to conserve the rakali should include Australia’s urban environments, where introduced rats cause a host of problems.

The problem of black rats

Rats have . In Australia, invasive rats are an .

Anecdotal reports suggest Sydney, for example, has a . Public concern was fuelled late last year when footage emerged of  at a popular Sydney shopping centre (see video below).

 are the two main pest rat species in Australia. Both were introduced by Europeans. They compete aggressively against other species for food and can breed quickly.

Black rats are particularly abundant in urban areas and nearby bushland. They  to urban environments, if there are no competitors around.

. Black rats , skinks and invertebrates and also eat seeds.

Black rats also pose serious health risks to humans, pets and wildlife.

They are the primary host of rat lungworm, a parasite . Rats also spread leptospirosis, a bacterial infection that has  in recent years, and infected scores of humans.

Managing introduced rats is becoming increasingly difficult. Some rodenticides have become less effective as rats . And rat poisons have been known to .

Clearly, better ways of managing introduced rats are needed. That’s where our new paper comes in.

Enter the rakali

The rakali, or water rat, is found across much of Australia. It is �����Ի��� near fresh or brackish (slightly salty) water such as creeks and estuaries. It is often described as Australia’s “otter”.

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While surveying rakali around Sydney Harbour in June 2011, we captured  of one lying in wait before ambushing a black rat.

The observation took place in bushland on the foreshore of Sydney Harbour, near Collins Beach at North Head. We had set up a motion-sensing wildlife camera as part of a pilot study to understand relationships between rakali and black rats.

At 10.22pm, the camera recorded a rakali next to a rock and hidden by vegetation. A black rat approached, and the rakali leapt out and chased it off.

But do rakali kill black rats, or just chase them? Captive rakali have been known to  in captivity. And given the larger size and carnivorous diet of the rakali, they may in fact prey on black rats in the wild.

Or rakali may reduce black rat numbers the same way  – by both preying on some and scaring others away.

Our paper also canvasses growing evidence that native rodents can resist and suppress their invasive counterparts.

For example, native bush rats (Rattus fuscipes) were  by black rats. But an experiment at Jervis Bay in New South Wales removed black rats, allowing bush rats to reclaim their territory. After the experiment ended, .

At North Head on Sydney Harbour,  to areas where they once lived led to a dramatic decline in black rat numbers.

Recent research reported on footage captured in a Perth backyard of , a small marsupial species found only in southwest Australia. However, the quenda appeared to fend off the attack. This means it’s possible rakali, which are much larger than quenda, would be even more aggressive towards black rats.

Native rats to the rescue?

Evidence is growing that native rodents can help control pest rodents.

This is especially true of rakali, which live in  where black rats are common. More research is needed to better understand the potential of rakali to manage invasive black rat populations.

Troublingly, however, native rats are vulnerable to rodenticides. To support their role in pest management, the use of poisons to control pest rats should be reconsidered.

By allowing native rodents to thrive, we may be able to harness their natural behaviours to control invasive pests safely, sustainably and effectively.

This article was first published by .