Overpopulation occurs when a species' population exceeds the carrying capacity of its ecological niche. It can result from an increase in births (fertility rate), a decline in the mortality rate, an increase in immigration, or an unsustainable biome and depletion of resources. When overpopulation occurs, individuals limit available resources to survive.
The change in number of individuals per unit area in a given locality is an important variable that has a significant impact on the entire ecosystem.
In the wild, overpopulation often causes growth in the populations of predators. This has the effect of controlling the prey population and ensuring its evolution in favor of genetic characteristics that render it less vulnerable to 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 followed by a population crash. Rodents such as lemmings and voles have such cycles of rapid population growth and subsequent decrease. Snowshoe hares populations similarly cycled dramatically, as did those of one of their predators, the lynx.
The introduction of a foreign species has often caused ecological disturbance, as when deer and trout were introduced into Argentina when rabbits were introduced to Australia, and indeed when predators such as cats were introduced in turn to attempt to control the rabbits.
Human overpopulation occurs when the ecological footprint of a human population in a specific geographical location exceeds the carrying capacity of the place occupied by that group. Overpopulation can further be viewed, in a long term perspective, as existing when a population cannot be maintained given the rapid depletion of non-renewable resources or given the degradation of the capacity of the environment to give support to the population.
The term human overpopulation also refers to the relationship between the entire human population and its environment: the Earth, or to smaller geographical areas such as countries. Overpopulation can result from an increase in births, a decline in mortality rates against the background of high fertility rates, an increase in immigration, or an unsustainable biome and depletion of resources. It is possible for very sparsely populated areas to be overpopulated if the area has a meagre or non-existent capability to sustain life (e.g. a desert). Advocates of population moderation cite issues like quality of life, carrying capacity and risk of starvation as a basis to argue against continuing high human population growth and for population decline. Scientists suggest that the human impact on the environment as a result of overpopulation, profligate consumption and proliferation of technology has pushed the planet into a new geological epoch known as the Anthropocene.
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When is an area overpopulated? When its population can not be maintained without rapidly depleting non-renewable resources  (or converting renewable resources into non-renewable ones) and without decreasing the capacity of the environment to support the population. In short, if the long-term carrying capacity of an area is clearly being degraded by its current human occupants, that area is overpopulated.
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