General Info:
There are approximately 60 million feral cats in the United States. The goal of TNR is to reduce the number of feral cats while increasing their quality of life. After a feral cat has been spayed or neutered, screened for FeLV and FIV, and has received a rabies vaccine, it is returned to its original colony where it can live out a full, healthy, non-reproductive life. TNR is endorsed by many animal oriented organizations including the American Veterinary Medical Association. TNR has been shown to be the least costly as well as the most efficient and humane way of stabilizing feral cat populations.
The standing alternative to TNR is extermination, which is not only inhumane, but costly as well. Continued attempts to catch feral cats, house, euthanize, and dispose of them costs much more than to simply TNR them.
A side product of extermination is the Vacuum Effect. Cats are relatively territorial animals. When they are removed from a colony, new ones will move in and then breed to capacity. By keeping non-reproductive cats in the colony, new cats will be much less likely to join.