There are, effectively, only two choices for actively managing the size of animal populations: reducing the birth rate and increasing the death rate. (Local population size may also be controlled by movement of individuals in and out; but when the size of animal populations concerns us, movement of individuals merely relocates the concerns. We are not absolved of our responsibility for animals simply because they go somewhere else.) Killing certainly can reduce and even destroy wildlife populations if enough animals of the right description are removed from the population. Until the last decade of the twentieth century, however, fertility control for wildlife was not seen as a feasible option.
Everything changed between 1988 and 1989. The successful use of a remotely deliverable immunocontraceptive on free-ranging wild horses at Assateague Island National Seashore, in Maryland, opened a new universe of possibilities for the humane, nonlethal control of wildlife populations.
Kirkpatrick, J.F., & Rutberg, A.T. (2001). Fertility control in animals. In D.J. Salem & A.N. Rowan (Eds.), The state of the animals 2001 (pp.183-198). Washington, DC: Humane Society Press.