Overpopulation (biology)

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.

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