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Overpopulation occurs when a population of a species exceeds the carrying capacity of its ecological niche. Overpopulation is a function of the number of individuals compared to the relevant resources, such as the water and essential nutrients they need to survive. It can result from an increase in births, a decline in mortality rates, an increase in immigration, or an unsustainable biome and depletion of resources. 
In the wilderness, the problem of overpopulation of species is often solved by growth in the population of 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 controlling the prey population and ensuring its evolution in favor of genetic characteristics that enhance escape from predation (and the predator may co-evolve, in response).
In the absence of predators, species are bound by the resources they can find in their environment, but this does not necessarily control overpopulation, at least in the short term. 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 affect a sharp reduction in population in a very short lapse (a population crash). Lemmings, as well as other less popular[why?] species of rodents, are known to have such cycles of rapid population growth and subsequent decrease.
Some 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) are 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 overpopulation caused by introduction of a foreign species abound:
Examples of overpopulation caused by natural cyclic variations include:
The human population has been growing continuously since the end of the Black Death, around the year 1400, although the most significant increase has been in the last 50 years, mainly due to medical advancements, increases in agricultural productivity and the historically-unique availability of abundant cheap energy. The rate of population growth has been declining since the 1980s. Most contemporary estimates for the carrying capacity of the Earth under existing conditions are between 4 billion and 16 billion. In 2013 the human population was 7 billion. By 2025 the world population is expected to grow by an additional 1 billion.  Depending on which estimate of overpopulation is used, human overpopulation may or may not have already occurred.
The InterAcademy Panel Statement on Population Growth, circa 1994, has stated that many environmental problems, such as rising levels of atmospheric carbon dioxide, global warming, and pollution, are aggravated by the population expansion. Other problems associated with overpopulation include the increased demand for resources such as fresh water and food, starvation and malnutrition, consumption of natural resources (such as fossil fuels) faster than the rate of regeneration, and a deterioration in living conditions. However, some believe that waste and over-consumption, especially by wealthy nations, is putting more strain on the environment than overpopulation.
Ethical issues of humaneness arise also from the unintended population growth of dogs, cats, and other domestic animals. Outcomes include euthanization of former pets and release of former pets to the wild.