Besides the issue of overpopulation in wild species, human beings have introduced a different problem: that of artificially constructed populations (domestic pets, cattle, agriculture, etc.)
In the wilderness, the problem of animal overpopulation is solved by predators. Predators tend to look for signs of weakness in their prey, and therefore usually first eat the old or sick animals. This has the side effects of ensuring a strong stock among the survivors, and controlling the population.
In the absence of predators, animal species are bound by the resources they can find in their environment, but this does not necessarily control overpopulation. In fact, an abundant supply of resources can produce a population boom that ends up with more individuals than the environment can support. In this case, starvation, thirst and sometimes violent competition for scarce resources may effect a sharp reduction in population in a very short lapse (a population crash). Lemmings, as well as other less popular species of rodents, are known to have such cycles of rapid population growth and subsequent decrease.
Some animal species seem to have a measure of self-control, by which individuals refrain from mating when they find themselves in a crowded environment. This voluntary abstinence may be induced by stress or by pheromones.
In an ideal setting, when animal populations grow, so do the number of predators that feed on that particular animal. Animals that have birth defects or weak genes (such as the runt of the litter) also die off, unable to compete over food with stronger, healthier animals.
In reality, an animal that is not native to an environment may have advantages over the native ones, such being unsuitable for the local predators. If left uncontrolled, such an animal can quickly overpopulate and ultimately destroy its environment.
Examples of animal overpopulation caused by introduction of a foreign species abound:
Examples of animal overpopulation caused by natural cyclic variations include:
Pet overpopulation can be an ecological concern as well as a concern over animal welfare, with overpopulation occurring when there are more domestic cats and dogs than there are people wanting them as pets, independent of ecological carrying capacity. It is also a financial problem: Capturing, impounding and eventual euthanasia costs taxpayers and private agencies millions of dollars each year, according to the American Veterinary Medical Association.
Although the term overpopulation is a generic term, true overpopulation means that there are actually more pets available than there are homes available. If one examines the numbers of homes which are seeking pets (as an example) and finds that 100 of those homes want pets which are, say, 1yr old and of a pure breed under 25 pounds, then to say that overpopulation exists simply because not all of the animals in a shelter will meet the desired criteria (1yr old/purebred under 25lb) is not exactly overpopulation. The fact that some shelters elect to kill animals is not necessarily indicative of overpopulation, as there is no standard for shelters reporting, tabulating, counting or categorizing their numbers/stats nationwide; in fact, it is not even known exactly how many animal shelters exist in the United States.
Overpopulation, then, is often used generically to refer to those pets that are not finding homes for reasons such as breed, temperament, age, health, or size. It does not necessarily mean there is no owner who desires a pet, but that there is no owner found which necessarily wants that particular animal at that time.
The welfare organization IFAW claims that where there's pet over population, particularly in developing countries, dogs and cats suffer from neglect and abandonment, deplorable living conditions, insufficient or nonexistent veterinary care, and substandard practices. Such animals are victims of inhumane treatment, not only because of intentionally inflicted cruelty, but also because of poverty, lack of pet care knowledge, the absence of animal welfare legislation and enforcement, apathy, and unsubstantiated beliefs.