In the U.S. alone, only 3% of free-roaming cats are neutered or spayed, leaving all the unneutered cats to continue reproducing and growing the feral cat population. One female cat has the ability to produce roughly 100 kittens in seven years. This high rate of reproduction among feral cats is why feral cats account for 80% of the cats that barrage animal shelters.
Sadly, out of the 80% of feral cats that are turned in to animal shelters, 72% of these cats are subjected to euthanasia. Once a cat is turned in at a pound or shelter, there are only 3 possible outcomes for that cat: being adopted; reunited with their owner; or, being euthanized.
Many people believe that feral cats are unhealthy and carry diseases that could potentially put humans and their pets at risk. According to Alley Cat, out of all of the feral cats that were examined to be spayed or neutered, less than half of one-percent (0.05) were euthanized due to medical issues. This means that almost the entire feral cat population is healthy and not carrying any diseases that could harm your pets. When cats are brought into shelters or vets through the TNR program they are given a full examination and then they are vaccinated for rabies and other diseases and given flea and parasite treatment. Any wounds that the cat may have are also repaired and treated. TNR programs will not release a cat that is too unhealthy, or that is unfit to survive in the wild. In actuality, by participating in the TNR program, cats are likely to be healthier than if left to their own devices.
There is a common misconception that the quality of life for free-roaming cats is extremely poor and that they live a life of suffering. This is indeed false. Just like any other wild animal, cats are born with instinctual survival skills and can live comfortably in the wild. They are able to find shelter if they need it and hunt for their food. Feral cats will adapt to their environment just like a squirrel would.
One belief that people have is that, if you take the food away, then the cats will go away. That is extremely false. Some areas of the U.S. have enforced feeding bans so that residents living in that area are legally deterred from not putting out food for feral cats. Just because a food source is taken away and not readily available does not mean the cats will go away and find food elsewhere. When food sources are scarce, feral cats tend to move closer to human habitations, as they grow hungrier in hopes of finding scraps of food. When cats are malnourished they have a greater risk of developing parasitic infestations and since cats move closer to humans when hungry, this leaves those infections closer to your home and your pets. Cats will also continue to breed despite the lack of food. So the moral of the story is that feeding bans do more harm than good when it comes to trying to control feral cat populations.
http://www.havahart.com/articles/benefits-tnr-programs-euthanasia