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.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.
Human overpopulation (or population overshoot) is when there are too many people for the environment to sustain (with food, drinkable water, breathable air, etc.). In more scientific terms, there is overshoot when the ecological footprint of a human population in a geographical area exceeds that place’s carrying capacity, damaging the environment faster than it can be repaired by nature, potentially leading to an ecological and societal collapse. Overpopulation could apply to the population of a specific region, or to world population as a whole.