This article is a reprint from the Science and Conservation Center
This is an oft-asked question, but usually it is asked without much understanding. The number of animals or percent of an animal population that needs to be treated is dependent upon many factors and there is no simple answer. For example, what is the goal? Is it the absence of all animals (impossible with contraception), or a 20% reduction, or a 50% slowing of the growth rate or zero population growth? Even after the goal has been identified, data required to answer this question include at very least, the reproductive rate, mortality rate, and immigration and emigration rate for the population. Equally important, these data on reproduction and mortality must be site-specific to the particular herd in question and not generalized data or data derived from other locations or populations. One of the most serious deficiencies noted at discussions of new projects is the lack of sound biological data and vague or no goals.
Another consideration is the length of time necessary to achieve population goals. Contraception is not a good way to reduce populations of long-lived animals rapidly. It takes time for the animals to die off and, as seen earlier, one of the results of wildlife contraception may be increased longevity among treated animals. Wildlife contraception must be viewed as a long-term commitment. Also, humans should consider that most wildlife overpopulation problems were created over many years and that quick solutions are not the result of rational thought or fair to the wildlife.