Cats versus quolls

Sorry, cat lovers. More research has turned the spotlight on how our furry feline companions wreak havoc on wildlife.

Comparing feral cats (Felis catus) to Australia’s spotted-tail quoll (Dasyurus maculatus), a study published in the journal Proceedings of the Royal Society B suggests their success can be attributed to greater population densities, higher foraging intensity and more flexible habitat choices than their native counterparts.

These factors, estimate Rowena Hamer and colleagues from the University of Tasmania, mean that prey are up to 200 times more likely to bump into cats. In turn, this takes a toll on prey as they adopt behaviours to avoid the wily predators.

Invasive species have had a devastating impact on biodiversity worldwide and cats are a key contender.

“Feral cats are recognised as one of the most serious threats to our native wildlife,” says Hamer, being implicated in a whopping quarter of all bird, mammal and reptile extinctions worldwide.

Until now, the team says research has tended to focus on native prey responses to foreign species. To explore predator behaviours, they formulated a framework to compare cats to the quoll, a marsupial predator sometimes called a “native cat”.

“The spotted-tail quoll is roughly the same size as a feral cat, has a high dietary overlap and a similar hunting strategy,” says Hamer.

“We were therefore interested in why feral cat predation is such a massive threat to our native wildlife – what is it about cats that means the defence mechanisms of native prey aren’t effective against them?”

To test the framework, they captured and fitted 34 cats and 14 quolls with GPS tracking collars in four agricultural regions of Tasmania’s Midlands to estimate risk of prey encounters and the cost of each encounter.

The collars recorded the animals’ location every five minutes over a month to build a detailed picture of how they use the different habitats.

Not only were prey at considerably higher risk of encountering a cat than a quoll, results suggest the cat encounters themselves are likely to be equally or more dangerous – this includes direct costs, such as being eaten, and indirect costs such as reduced foraging time due to higher vigilance and avoidance of valuable habitats.

These vulnerabilities were driven by ecological differences between the two carnivores.

Cat populations built up to much higher densities: the average trumped quolls – of which females defend large, exclusive home ranges – more than 20-fold, with nine cats versus 0.4 quolls per square kilometre.

They also use their home range much more intensively.

“To use a farming analogy, it’s a bit like the difference between set stocking and rotational grazing,” Hamer explains.

“Cats revisit the same areas night after night after night, whereas spotted-tail quolls will typically move between denning and foraging sites every few days.”

This is compounded by more variable habitat preferences of cats, resulting in a wider spread than quolls, which tend to be restricted to areas of native woodland.

Understanding why cats are so deadly might help manage them and at-risk species, Hamer notes.

“Although we’d ideally just remove all the feral cats this is pretty tricky at a landscape scale,” she says, “particularly in areas like Tasmania where broad-scale controls like poison baits are a risk to native carnivores.”

The study also highlights the complex web created by invasive species that must be factored into their management.

“[F]eral cat control programs must consider how removing cats will affect non-native prey,” says Hamer, “and whether naive carnivores are present and able to exert enough predation pressure to keep introduced species such as black rats, house mice and rabbits under control.

“If not, any feral cat control must be coupled with control of these species to prevent adverse impacts on native wildlife.”

While working on effective broad-scale cat control methods, she proposes initiatives to help native prey survive cat encounters, such as complex understory habitat refuges.