CHRIS UHLMANN, PRESENTER: Researchers have been documenting the decline of several native mammal species in northern Australia for years.

The northern quoll is one of them, but there has been an alarming drop in its numbers with the arrival of the cane toad.

Saving the small mammal is a challenge, but researchers haven't given up the fight.

Sara Everingham reports from the Top End.

SARAH EVERINGHAM, REPORTER: It's a small solitary predator which belongs to the same family as the Tasmanian devil. The northern quoll is a nocturnal creature, but it's also one that's full of curiosity.

STEVE WINDERLICH, KAKADU NATIONAL PARK: They are iconic, they are an animal people love to see around. They're important to Aboriginal people.

SARAH EVERINGHAM: But the northern quoll is just one small mammal that in recent decades has been disappearing from many parts of northern Australia, even from Kakadu, a World Heritage listed conservation reserve. Once the quoll was one of the most common small animals in the park.

STEVE WINDERLICH: In terms of a national park, our job is to maintain biodiversity and maintain natural and cultural values. Quolls are a part of that.

SARAH EVERINGHAM: It's 20 years since Steve Winderlich came to work in Kakadu and he doesn't need data to know what's happened to the park's hapless little resident.

STEVE WINDERLICH: Quolls and other animals were quite common, you'd see them very regularly, they'd actually be in your house sometimes, they'd live in your roof, to seeing virtually none, so it was really, the before and after was quite drastic.

SARAH EVERINGHAM: At the beginning of the decade, a monitoring program in Kakadu was trapping around 66 quolls, but by the end of the decade the number had dropped to three.

It appears the arrival of the cane toad in Kakadu was the tipping point for a population already in decline. The quoll's brief life cycle made them particularly vulnerable. Males live for just short of a year. They die after the reproductive season and for a time the population is made up of only females.

JONATHAN WEBB, RESEARCHER, SYDNEY UNIVERSITY: When toads first invaded Kakadu, quolls went rapidly extinct because the first toads they encountered were these giant toads at the invasion front and they're lethal to most predators. So the quolls that attacked those toads died and so they had no opportunity to learn.

SARAH EVERINGHAM: But researchers from the University of Sydney are working with park managers to train quolls to steer clear of the toad. Here at the Territory Wildlife Park they've been making sausages out of cane toads to feed to quolls.

DION WEDD, TERRITORY WILDLIFE PARK: We use sausages. We made up toad sausages. They have enough toxin in them to tell the quolls that that taste is not a good thing to encounter. How is this little fella going, Linda?

LINDA: He is going great.

SARAH EVERINGHAM: Quolls bred at the park stay here for up to eight months being trained before they're released into the wild.

DION WEDD: The toads are here to stay. We've got to somehow as environmental managers work out a way to enable our wildlife to exist with toads.

SARAH EVERINGHAM: Every three months in Kakadu these quolls trained in captivity are monitored to see how they're faring in the wild, and the early news is good.

JONATHAN WEBB: Some of our females that we trained to avoid cane toads have survived for two years now in the wild and they regularly encounter toads when they're out foraging.

We're going to fit a transmitter to find out how they will disperse and we'll also find out what the major sources of mortality for quolls like this.

SARAH EVERINGHAM: Dr Jonathan Webb and PhD student Tegan Cellars are trying to see if juvenile quolls learning from their mothers, and there to avoid toads and there is some evidence of success.

TEGAN CELLARS: We're listening into a frequency on a baby quoll and as we get closer, the beats will get louder and that's how we can find it, probably under a rock or something like that.

SARAH EVERINGHAM: But as the quolls forage through Kakadu at night, the cane toad isn't the only threat. The researchers are trying to find out what else is killing the quoll.

STEVE WINDERLICH: In fact, the evidence suggests that their population was declining before the toad got to Kakadu, so there is something else happening and there is also something else happening to other small mammal species that don't go near the toads.

SARAH EVERINGHAM: Researchers believe feral cats and the decline of traditional fire techniques are largely to blame for the drop in mammal populations in Northern Australia.

ALARIC FISHER, DIR. BIODIVERSITY, NT GOVT: We know very well that the declines are happening and we have a good idea of what species are declining. The really tricky thing to work out now is exactly what's causing it so that we can address the problem.

SARAH EVERINGHAM: The toad diversion research in Kakadu is just one research program being run in the Top End to protect native mammal fauna. In one case an endangered bandicoot was spread to several islands to the north to create backup populations. Another island is being used to breed northern quolls, away from the threats on the mainland.

ALARIC FISHER: So those are sort of stop-gap measures to protect particular species while we try and address the management of the landscape, particularly on the mainland, to try to improve its quality for mammals generally.

SARAH EVERINGHAM: The relocation programs have been successful, but on the mainland there is still plenty of work to do. For researchers, the toad diversion program in Kakadu is another positive sign. In recent years, there has been little good news for the northern quoll.

STEVE WINDERLICH: It's very promising, it's very exciting to see at least something positive happening, but it's still a long way to go, it's only a small part of the picture and there will be no one-size-fits-all solution to this.

CHRIS UHLMANN: The image of that cane toad going through the blender is going to stay with me for a while, one of the few things I didn't think of as a kid in Townsville.

Sara Everingham reporting.

That's the program for tonight. We'll be back at the same time tomorrow night but for now, goodnight.