Shooting or trapping feral cats may increase their numbers, a new study has found.
The accidental finding, made during research into the ecological impact of feral cats, emphasises the need to monitor the effects of culling programs, say wildlife biologist Billie Lazenby of the Tasmanian department of primary industries.
"You may be inadvertently doing more damage than good," she says.
Feral cats are often culled because they threaten biodiversity. But, says Lazenby, the effectiveness of culling has only really been studied on islands rather than in mainland areas, where new cats can come in and replace those that have been removed Lazenby and colleagues were carrying out research in the tall forests of southern Tasmania.
Their study was designed to compare small mammal numbers in sites where feral cats were allowed to roam free with those where the cats were reduced through culling.
The results are still being analysed, but in the meantime, the researchers discovered something surprising.
As part of their study, they used remotely-controlled baited infrared cameras to verify a fall in cat numbers with culling.
Although the number of cats being cage-trapped and euthanised fell -- implying the culling was successful -- the camera monitoring suggested cat numbers were rising.
"In the areas that I had tried to reduce cat numbers I recorded an increase in cat numbers," says Lazenby. "I actually had more cats running around on those sites than beforehand."
"We recorded a 75 to 211 per cent increase in the minimum number of feral cats known to be alive in the culled areas."
The results are published in a recent issue of the journal Wildlife Research.
Lazenby thinks the unexpected observation may be explained by the fact that dominant cats are often bolder and more confident and so more likely to explore traps than subordinate cats.
"If you remove a dominant individual from an open population you're likely to get a few subordinates coming in to check out the territory that's been freed up," she says.
So once the "trappable" dominant cats had been removed, this allowed an influx of many more subordinate animals, she says.
"That's why we observe this spike in cat numbers following our culling operation," says Lazenby.
She says within a year the cat numbers in the culled areas stabilised to the original numbers.
While culling is only ever a short term solution unless it is ongoing, this is the first time it has been shown to cause an increase in cat numbers.
"What we really should be focusing on when we talk about managing introduced species like feral cats is reducing their impact," says Lazenby.
"But it's really important that we keep in mind that you don't always reduce impact by reducing numbers, as one individual might cause 90 per cent of the damage."
Lazenby says that using fences to exclude cats, or increasing the number of hiding places (such as log piles) for small prey may be more effective strategies to protect biodiversity in some cases.
She says the study highlights the importance of monitoring to check the outcomes of management strategies.
"It's not unusual to have these unexpected outcomes," she says.