Overpopulation refers to a population which exceeds its sustainable size within a particular environment or habitat. Overpopulation results from an increased birth rate, decreased death rate, the immigration to a new ecological niche with fewer predators, or the sudden decline in available resources. Therefore, overpopulation describes a situation in which a population in a given ecosystem limit the resources available for survival.
Overpopulation can have several effects on the environment, as well as other species within an ecological system. Indeed, human overpopulation has resulted in technological advances which have increased human lifespan and fertility, and consequently placed pressure on global resources. Such effects are such that the planet is currently in a novel geological epoch called the Anthropocene. In general, overpopulation results in an ecological disruption as resources are depleted. This disruption can lead to the decline of other populations which compete for the same resources. Typically, such effects result in the cycling between periods of population growth and periods of population decline until it can reach homeostasis within a particular ecological niche. Some examples of naturally regulated population growth are rodents, rabbits, and various insect populations (e.g., army worms and locusts).
In situations of overpopulation caused by the introduction of a foreign species for which they have no natural predators, they can become an invasive species. An example is the inadvertent introduction of zebra mussels to the North American water systems. Since zebra mussels are natively from the Black Sea and Caspian Sea, they have no natural predators in the foreign ecosystems of North America and parts of Europe. As such, zebra mussels quickly became an invasive species, clogging water treatment pipes, affecting power plants, and impacting the local freshwater fish populations. It is estimated that the overpopulation of zebra mussels has cost approximately $5 billion USD since their introduction. The image below illustrates an infestation of zebra mussels on a North American lock due to the overpopulation of zebra mussels in the North American waterways. Other economic effects of overpopulation include those caused by crop destruction, as seen with the overpopulation of rabbits in Australia. While the overpopulation of rabbits destroyed farmers crops, leading to poor yields, the continent also experienced a loss of native plant species, as well as the removal of precious topsoil due to erosion.
Another effect of the overpopulation of one species, is the increased population growth of the natural predators of such species. This effect is generally considered to be positive, as the predator population serves to control the overpopulated prey species. Such effects also serve to drive evolutionary changes as the prey species evolves to avoid increased predation.