Reptiles are losing more than just their tails to cats. In fact, new research shows that entire reptile populations may be dropping due to the presence of invasive felines.
Research has shown that domestic cats and their feral counterparts are capable of huge impacts to bird populations—one house cat and its offspring were purportedly all it took to push a bird that lived only on Stephens Island in New Zealand into extinction.
To get a picture of what a landscape free of cats would mean for reptiles, researchers created cat-free zones for two years in Kakadu National Park in Australia’s Northern Territory.
The researchers fenced some areas and left others of equal size open. They placed camera traps around the fenced plots to make sure cats weren’t getting in, and had to remove only one feline intruder during the experiment. Meanwhile, cats were detected in each of the unfenced areas at least once during the study period, and across the whole area at a density of about one cat per every two square miles, a number a little lower than the national average.
“These findings indicate that, even at relatively low densities, feral cats are exerting considerable predatory pressure on small reptile populations,” the authors noted. (Read about cat behavior in "What If Everything You Think About Cats Is Wrong?")
They monitored changes in the reptile populations by counting the number that fell into bucket traps.
They found that cat-free zones experienced “a significant increase in reptile abundance” over the course of the two-year experiment, with roughly two more reptiles in each fenced plot compared to the unfenced plots. Since many of the species caught were small lizards such as geckos, skinks, and agamas (and at least one snake), the authors believe that new generations in the species occur every year or less, meaning that two years without cats would be enough to see population differences.
Peter Marra, director of the Smithsonian Migratory Bird Center, says that the study was “wonderful.” While not involved in this research, he has conducted other work on the impact domestic cats have on a variety of species. (Related: Research finds that cats kill billions of birds and mammals each year.)
Long-nosed chameleons like this one in Madagascar sometimes use their snouts to joust. Their noses also help similar species identify each other. (Read our recent feature on chameleons.)
He says that he and other researchers are constantly being challenged to prove that cats aren’t just killing a few individuals, but are causing population-level impacts on species. He points to another study published in 2016 in the Proceedings of the National Academy of Sciences that found that cats have contributed to the extinction of at least 63 animals, including two reptile species, and believes that the recent Australian study adds to this body of research.
“It’s just another piece of evidence on this expanding and very large list of studies that show that cats are not just killing animals, but having population-level impacts on native wildlife,” he says.
Katie Lisnik, director of public policy on companion animals at the Humane Society, says that while cats certainly prey on reptiles, small mammals and birds, she doesn’t believe the population increase inside fenced areas can be put down entirely to a lack of cats. Other predators like dingoes were likely excluded, too, as well as herbivores that could have impacted the presence of reptiles in some areas.
“The assumption that all the impacts they saw [are] due to cats is a stretch,” she says. (Related: "Island's Feral Cats Kill Surprisingly Few Birds, Video Shows.")
Marra says this was his first concern about the new study as well, but he notes the researchers address this by pointing out that there aren’t many other predators of reptiles in the area. Native predators such as giant goanna lizards or squirrel-like northern quolls have already been decimated due to invasive poisonous cane toads.
Meanwhile, Australia has not been sitting idly by watching the kittens play. The South Australia government has helped fund some science-fiction-worthy strategies to eradicate feral cats, including robots that detect and spray poison on the felines.
Lisnik says that the Humane Society advocates other ways to reduce feral cat populations, such as neutering felines before relocating them or releasing them, and encouraging people to keep pets indoors.
But Marra, who says he likes cats on a personal level, says these solutions are difficult due to the secretive nature of feral cats, and the extent of neutering needed to make a difference. He says that removing them from the wild is a better option, either through kitten adoption, placing feral cats in sanctuaries or euthanizing them, which he says is a better solution than allowing them to kill other animals.
“This [study] is another wake-up call that we need to be managing our native ecosystems more responsibly,” he says.