Overpopulation occurs when a species' population exceeds the carrying capacity of its ecological niche. Overpopulation is a compared to the resources they need to survive, like water and essential nutrients. 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.
In the wilderness, the problem of overpopulation is often solved by a growth in the population of predators. Predators tend to look for signs of weakness in their prey, and therefore usually eat the old or sick animals first. 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 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 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 situation, 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 for 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:
- In the Patagonia of Argentina, European species such as deer and trout were introduced into the local streams and forests, respectively, and quickly became a plague, competing with and sometimes driving away the local species of fish and ruminants.
- In Australia, when rabbits were introduced by European immigrants, they bred out of control and ate the farm crops and food that both native and farm animals needed. Farmers hunted the rabbits, and also brought cats in to guard against rabbits and rats. These introduced cats created another problem, becoming predators of local species.
Examples of locust overpopulation caused by natural cyclic variations include:
- the 2004 Locust Outbreak in West and North Africa.
- the Australian locust plagues.
- Palestine's eight-month-long 1915 locust plague.
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, probably due to medical advancements and an understanding of health and longevity, increases in agricultural productivity, an already sizeable fertile population and the historically unique availability of abundant cheap energy. The rate of population growth has been declining since the 1980s. As of 2011, the United Nations Population Division projected that global population growth will nearly halt by 2050, stabilizing at around 9.15 billion. However, some scholars contend that this projection is off, and another estimate by the UN puts the population growing to possibly 15 billion by 2100. 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 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 and its effect on global warming, and pollution, are aggravated by the population growth. 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.
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