Many wildlife populations are increasing in many areas around the United States. A major example is the populations of White Tailed Deer across the country. One of the major problems with the overpopulation of deer is that they are over-browsing the forests in which they inhabit. This chokes out ecosystem diversity by creating a nearly homogenous understory of plants that are not eaten or rarely eaten by the deer.
Overpopulation also plays a major role in disease transmission. The more individuals there are, the more direct contact there is and therefore, the more incidences of possible transmission of diseases. Another factor is that in increased population levels, there are just more individuals to get sick and to pass the sickness on. Along with this, many diseases that spread to humans but are also prevalent in wildlife (such as Lyme disease and West Nile Virus) is increasingly concerning for humans as populations of the wildlife that carry these diseases increase. A recent model has shown that when culling is used in combination with contraception use to combat rabies in a localized outbreak without killing the diseased animals, it is more effective than vaccinations alone. Another disease, known as brucellosis (primarily found in bison), is predominantly transmitted through contact with infected aborted fetuses. This means that if the pregnancies in infected bison were stopped in the first place, this disease would lose its primary method of transmission.
The major way that overpopulation was combatted previously was through hunting and trying to control population levels through culls. An alternative to this is to control the population through contraception use in the wildlife populations. Modern wildlife contraception began in 1988 when a product planned for human contraception known as porcine zona pellucide (PZP) was tested on 14 captive mares, where it stopped 13 pregnancies. The dosage was small enough to be fired from a dart rifle and was tested on wild horse populations on Assateague Island by Irwin Liu, Jay Kirkpatrick and John Turner. The results of this test showed that only 4% of vaccinated mares gave birth that year as opposed to 45% of unvaccinated mares. This process was then adapted for use on elephants in South Africa where yearly culls were outlawed but the population needed to be controlled.