TNR programs (Trap-Neuter-Release) are common in many Western countries as a way to control feral cat populations. The idea is that when there is a community of wild, unowned domesticated cats we can help reduce and eliminate this population by capturing them, testing for certain diseases, vaccinating, spaying and neutering them, then returning them to the wild. Advocates of this policy believe that simply capturing and euthanizing these cats provides a vaccuum in the population which allows new cats to enter and continue the process. They also believe that this is the most human method of handling feral cats. But does this really work?
Unfortunately a growing number of studies is casting considerable doubt on the efficacy of such programs. The stated goals of reducing the number of feral cats is rarely reduced unless there is considerable effort made. In fact, studies have shown that in these cases populations tend to remain stable rather than decreasing. The more that researchers have looked at the issue, the less support there is for the idea that TNR programs reduce feral populations.
Yet you'll find that most humane societies, animal control departments, and rescue groups support this idea, and even promote the position that it really does work. If you simply look at the logic of procedure it really should work since you're removing breeding cats from a population yet keeping them there to compete for resources. Unfortunately the reality of the situation doesn't tend to support this, and many of the studies quoted by such groups really don't provide data as the grops think they do.
Does that mean that TNR shouldn't be done? Honestly, I'm on the fence about it. You'd be hard-pressed to find anywhere in the world where an average TNR program has eliminated a population of feral cats. By this evidence it really is a waste of money. At the same time, we must be affecting populations by spaying and neutering, and hopefully reducing cats with FIV or feline leukemia by testing for and euthanizing those who are positive. Maybe it's my heart rather than my mind that wants to believe in continuing them. I still wonder if there are better ways to spend limited resources.
For more information here are a few links. Read through them, compare the data, and make your conclusions.