Feral cats have long been suspects in killing native species reintroduced into conservation areas. DNA testing of the carcasses puts felines firmly in the frame.
Conservation scientists from UNSW Sydney have used DNA technology to identify feral cats as the primary predators responsible for the deaths of reintroduced native animals at two conservation sites.
The finding fits in with research data that suggests feral cats have killed more native animals than any other feral predators in Australia, and are believed to be responsible for two thirds of mammal extinctions since European settlement.
But in a study published recently in the Australian Mammalogy journal, the researchers from UNSW and the University of Adelaide say the effect of feral cats on native animals is likely larger than previously thought.
Study co-author Professor Katherine Moseby says correctly determining cause of death of native animals and attributing it to the right predator has always been difficult.
"In past releases, we'd be radio tracking animals after release, and if we found dead animals, it would be difficult to determine what caused their death. We would use field evidence like animal spoor, carcass remains, or bite marks on collars to guess whether it was feral cats, birds of prey, fox, or if they just died a natural death.
"And because we're working in really remote areas, it is difficult to access vets for necropsy. So taking a DNA swab of the dead animal was a really good way to identify if predation was the cause of death. And then we decided to compare the DNA outcomes with evidence in the field to determine if the latter was a reliable method of determining cause of death."
The main finding of the study was that field evidence was not a reliable indicator of predation by feral cats, and that DNA and necropsy were needed to confirm cat predation.
Whodunnit
Most feral cat DNA was found either on radio tracking collars fitted to some of the animals post-release, or on wounds to the body. The study focused on two South Australian sites where researchers had released native animals in previous translocations of four different species.
At one site the researchers released 148 brush-tailed possums and 110 western quolls between 2014 and 2016, while at the other, the scientists released 42 greater bilbies and 89 bettongs in 2017.
Of the 389 animals released at both sites, a total of 74 animals were confirmed killed by cats, with 96% of these – or 71 – determined by DNA analysis. Six animals were confirmed killed by cats after veterinarians performed post-mortem analysis, while feral cats were witnessed at three freshly killed carcasses. Three of the four species released still managed to survive in reduced numbers, but sadly the bettongs were no match for feral cats.
Interestingly, of the six animals confirmed killed using vet necropsy, five of these were confirmed by DNA. Similarly, with the three carcasses that were witnessed as killed by cats, two returned positive cat DNA. So even DNA underestimated the number of animals killed by cats.
"The DNA is good, but it's not infallible, and a lot of that is because we're trying to get DNA from cat saliva found on the carcass, and that's quite difficult because DNA degrades quickly in the environment," says Prof. Moseby.
"But ultimately it highlighted that there are a lot more cat killings than previously thought."
Herding cats
As senior author Ned Ryan-Schofield explains, one of the main reasons for the study was to determine if conservationists working in the field could use field data to provide more immediate clues about the extent of cat predation.
"This could trigger prompt management action like increased cat control rather than waiting until DNA analysis occurred. However, we found that field evidence of cat predation are not obvious and even DNA testing is not infallible," says Mr Ryan, a postgraduate researcher with University of Adelaide.
But according to the researchers, we're still a long way off effectively controlling feral cats in the Australian outback and cats will remain a threat to wildlife for years to come.
"Until we develop genetic tools or other broadscale methods targeted at feral cats, we can only rely on intensely managing them as best we can. We hope that this research might encourage more conservationists to use DNA and necropsy to identify the cause of death of animals in wildlife reintroductions, and to increase cat control even if no obvious evidence of cat predation are present," Prof. Moseby says.
