This is particularly true of fast breeding animals like rabbits, cats and mice. The most effective way to control unowned cat colonies is to desex them and return them to their territory. This stops breeding and nuisance behaviour, and it reduces the population because the desexed cats keep undesexed intruders out of their territory. Australian scientists are working on ways to genetically engineer viruses and bacteria so that infected animals will become infertile. The method, called immuno-contraception, is designed to affect only one species.
Work is well under way to stop the breeding of rabbits, foxes and mice.
Fertility control is the best option for long-term control of populations, without using cruel killing methods year after year. You can get more detailed versions of sections of this fact sheet, including references.