This is a good study which supports the benefits of TNR in a community. It took place in Florida and was published in 2014. It is published in The Veterinary Journal. The researchers wanted to find out the impact that TNR has on shelter impoundment. I’ll restate that in a different way. They wanted to know whether trap-neuter-release programs run by volunteers reduces the number of unwanted cats that are taken to shelters where they might be euthanised or possibly adopted. If TNR does work to reduce shelter impoundment it is going to save lives.

This was a two-year programme. They worked in a single ZIP Code area of Florida which covered 11.9 km². Their target was to capture and neuter at least half of the estimated community cats in that area. A total of 2,366 cats participated. This represented 54% of the projected community cat population in the target area.
They measured the cat impoundment rate at shelters in comparison to the human population of the area. After two years the “per capita shelter intake” (the number of cats taken to shelters in relation to the number of people living in the area) was 3.5 times higher and the per capita shelter euthanasia rate was 17.5 times higher in the NON-TARGET area compared to the target area. Comment: in other words, the euthanasia rate in the target area for shelter cats was 17.5 times lower compared to cats in the non-target area.
They also found that in the target area where 60 cats out of a thousand were neutered annually, shelter cat impoundment was decreased by 66% over the period of the two-year study compared to 12% in the non-target area where only 12 cats out of a thousand resident cats were neutered annually.
Once again, the increased rate of neutering in the target area substantially decreased the number of cats that ended up in shelters.
As a consequence, they say that “hi-impact TNR combined with the adoption of adoptable i.e. socialise cats, had a positive benefit as stated. The authors also said that there should be nuisance resolution counselling for residents which they say is an added tool to reduce shelter cat intake. This last point appears to be a reference to convincing the community to go down the TNR route rather than fight against it.
There are, understandably, a reasonable proportion of any community in America which disagrees with TNR because they think it simply retains feral and stray cats on the ground where they are a nuisance and prey on wildlife. One last point: I wrote a recent article about the diseases spread by feral cats in a community. Based on a study they found that feral cats managed in a colony by TNR volunteers are healthier or at least as healthy as domestic cats and therefore present no more health risk than domestic cats to people in the community.
The study: Effect of high-impact targeted trap-neuter-return and adoption of community cats on cat intake to a shelter. Authors: J.K.Levy, N.M.Isaza, K.C.Scott