Trap, neuter and release (TNR) is touted as an idea whose time has come. The website Neighborhood cats is one example of many advocating the technique (www.neighborhoodcats.org/whatistnr.htm) and going as far as claiming TNR as the "only method proven to be humane and effective at controlling feral cat population growth". At the recent International Animal Welfare Conference on the Gold Coast, several delegates questioned me about TNR as a method of feral cat control in Australia.
I can't see TNR as a viable option in Australia.
Too many cats and too few people is my view. The United States has a human population density over 10 times greater than that of Australia (30 people per square kilometre compared to 2.6). Cats are also the most widely and sparsely distributed feral animal in Australia. So I don't think it would make the slightest impact on Australia's feral cat population using TNR.
Of course that's different to city-wide or suburb-wide control of cat colonies where the technique is used in the United States. Its success is quite variable. Castillo and Clarke (2003) concluded that advocates would be better off directing their efforts to the underlying problem of managing irresponsible pet owners. Nutter (2005) found that TNR can be effective and provide a viable option for feral cat management.
Dr. Nutter's study is interesting. Three of her findings are particularly important. Firstly, neutered cats live a lot longer than intact cats. Secondly, at least 75 to 80% of cats in the colony need to be neutered to cause population decline and thirdly, there needs to be virtually no immigration into a colony if the population is to decline.
These factors combined mean that TNR colonies take about 13 years to run to extinction. So if the aim is to get rid of a colony, there needs to be a dedicated effort over a long period as well as little likelihood of new animals entering the population. I think it is going to be extremely rare to see this set of circumstances occurring. In these American programs, the colony is almost always fed by people that care for the cats. I'd be interested in the emigration rate of cats leaving the colony, especially in Australia where I imagine few colonies are regularly fed by humans - these cats cat be doing enormous damage to wildlife in very low numbers. The attached picture is one from New Zealand by Dr. Elaine Murphy and shows the diet of one feral cat for one day.
A major factor that influences those advocating TNR is that it is an alternative to lethal control. In her study, Dr. Nutter pointed out harvesting (this could be removal and rehoming or lethal control), even at very high levels every one or two years will not lead to long-term reduction in the numbers of cats because of re-establishment through immigration.
So if one is to argue that TNR is likely to be ineffective in Australia, one has to always acknowledge that lethal control is also probably going to be ineffective. My understanding of the conference delegates that asked me about TNR was that they were under the impression that significant lethal control of feral cats was going on in Australia - they were leaning towards TNR as an alternative to killing the cats.
The fact is, we lack the tools to do widespread cat control at the moment. To my knowledge, no authorities in Australia are undertaking widespread control except where the population is confined in some way, so the "TNR as an alternative to killing" argument is a moot point. (There is active research to develop better tools, which could change the situation). For colony cats, or strays around cities, I don't think TNR stands up in this country because our rehoming rates from shelters are relatively high. In Canberra, I believe the RSPCA re homes almost all the cats turned in - so in that case maintaining cat colonies by TNR would be a huge waste of resources. It may be different elsewhere.
The views of the cat gurus are most welcome.
Castillo, D. and Clarke, A.L. (2003) Trap/Neuter/Release methods ineffective in controlling domestic cat "colonies" on public lands. Natural Areas Journal 23:247-253.
Nutter, F.B. (2005). Evaluation of a trap-neuter-release program for feral cat colonies: population dynamics, home ranges and potentially zoonotic diseases. Ph.D. Thesis, North Carolina State University, pp 224.