Controlling feral cats in ecologically sensitive areas: Is "trap, neuter, return" effective?

Colony of feral cats. The potential negative impact of feral cat populations on ecologically sensitive places has created perhaps the biggest bone of contention between the conservation movement and animal rights groups. 

This issue is particularly critical on islands where feral cats have been implicated in the extinction of a number of native species.

Some animal rights proponents have promoted "trap, neuter, and return" programs as a potential humane solution to the problem.

Under this approach, feral cats are trapped, sterilized, and then released back to local areas where volunteer "colony caretakers" provide periodic care.

One of the assumptions underlying the approach is that neutered cats will have smaller home ranges and will stick around the areas where they are being fed, which will keep them from preying on native wildlife in natural areas.

However, a new study from Santa Catalina Island of the coast of California challenges this assumption and raises questions about the effectiveness of using sterilization programs alone.

Efforts to control feral cats on Santa Catalina include trap, neuter and return (TNR) programs at the two villages at the ends of the island.

As part of their study published in the journal Mammalogy, researchers Darcee Guttilla and Paul Statt from California State University Fullerton conducted their own TNR experiment. They captured feral cats using traps, neutered half the sample, and released both sterilized and non-sterilized individuals outfitted with GPS collars.

They found little difference in the home-ranges between sterilized and non-sterilized cats. Neutered and non-neutered individuals traveled long distances between human populated areas and the interior of the island which is comprise primarily of wildlands.

They estimated the island-wide feral cat population at 600-750 individuals and found trapability to be very low.

The authors write, "The influx of subsidized cats to natural habitats, combined with their high vagility and low trappability, makes TNR an unlikely solution for controlling feral cats on a large, rugged island like Catalina and, more generally, in other locations where human populations abut ecologically sensitive areas."

This is bad news for native prey on the island including birds reptiles, and small mammals.

This is also problematic for the highly threatened Catalina Island fox, which has little immunity from diseases brought from the mainland and nearly went extinct after an outbreak of canine distemper virus.

Scientists worry that feral cats on the island - in addition to putting the fox at risk to disease - may also compete with the foxes for food and displace them from their habitat. Based on their findings, the authors write,

"Until resources are available to implement more proactive control measures in these areas, cats trapped in the island interior should be removed and delivered to a shelter; if they are deemed adoptable, cats should be sterilized and added to the adoption pool on the mainland. If they are not adoptable or if there are insufficient resources to support relocation, they should be euthanized."

--by Rob Goldstein

Guttilla, D., & Stapp, P. (2010). Effects of sterilization on movements of feral cats at a wildland–urban interface Journal of Mammalogy, 91 (2), 482-489 DOI: 10.1644/09-MAMM-A-111.1