Overpopulation or overabundance is a state in which the population of a species is larger than the carrying capacity of its environment. This may be caused by increased birth rates, lowered mortality rates, reduced predation or large scale migration, leading to an overabundant species and other animals in the ecosystem competing for food, space, and resources. The animals in an overpopulated area may then be forced to migrate to areas not typically inhabited, or die off without access to necessary resources.
Judgements regarding overpopulation always involve both facts and values. Animals are often judged overpopulated when their numbers cause impacts that people find dangerous, damaging, expensive, or otherwise harmful. Societies may be judged overpopulated when their human numbers cause impacts that degrade ecosystem services, decrease human health and well-being, or crowd other species out of existence.
Background In ecology, overpopulation is a concept used primarily in wildlife management. Typically, an overpopulation causes the entire population of the species in question to become weaker, as no single individual is able to find enough food or shelter. As such, overpopulation is thus characterized by an increase in the diseases and parasite-load which live upon the species in question, as the entire population is weaker. Other characteristics of overpopulation are lower fecundity, adverse effects on the environment (soil, vegetation or fauna) and lower average body weights. Especially the worldwide increase of deer populations, which usually show irruptive growth, is proving to be of ecological concern. Ironically, where ecologists were preoccupied with conserving or augmenting deer populations only a century ago, the focus has now shifted in the direct opposite, and ecologists are now more concerned with limiting the populations of such animals.
Supplemental feeding of charismatic species or interesting game species is a major problem in causing overpopulation, as is too little hunting or trapping of such species. Management solutions are increasing hunting by making it easier or cheaper for (foreign) hunters to hunt, banning supplemental feeding, awarding bounties, forcing landowners to hunt or contract professional hunters, using immunocontraception, promoting the harvest of venison or other wild meats, introducing large predators (rewilding), poisonings or introducing diseases.
A useful tool in wildlife culling is the use of mobile freezer trailers in which to store carcasses. The harvest of meat from wild animals is a sustainable method of creating a circular economy.
Immunocontraception is a non-lethal method of regulating wild-animal population growth. Immunocontraception has been successfully used or tested in a variety of wild-animal populations including those of bison, deer, elephants, gray squirrels, pigeons, rats and wild horses. Among the limitations of injectable immunocontraceptives are a relatively long time between vaccine administration and a reduction in population size (although stabilization of population size occurs faster) and the need to be in close proximity with animals for injection. Oral vaccines do not have the latter limitation, but they are still not as well developed as injectable vaccines.
Judgements about overpopulation of wildlife or domestic animals typically are made in terms of human purposes and interests; since these vary, such judgements may vary, too. Judgements about human overpopulation are even more contentious, since the purposes and interests involved may be very important, even rising to the level of existence itself.Hardee, K., Newman, K., Bakamjian, L., Kumar, J., Harris, S., Rodríguez, M., & Willson, K. (2013). Voluntary family planning programs that respect, protect and fulfill human rights: a conceptual framework. Futures Group. Nevertheless, all people and every society have an interest in preserving a habitable biosphere, which may be compromised or degraded by too may people. In the context of rapid climate change, mass species extinction and other global environmental problems, discussions regarding human overpopulation are inevitable.Shragg, K. (2015). Move upstream: a call to solve overpopulation. Freethought House.Tucker, C. (2019). A Planet of 3 Billion. Atlas Observatory Press.
Recent scientific evidence from many sources suggests Earth may be overpopulated currently. Evidence of rapidly declining ecosystem services was presented in detail in the Millennium Ecosystem Assessment of 2005, a collaborative effort involving more than 1,360 experts worldwide. More recent scientific accounts are provided by ecological footprint accounting and interdisciplinary research on planetary boundaries for safe human use of biosphere.Higgs, K. (2017). Limits to growth: Human economy and planetary boundaries. Journal of Population and Sustainability, 2, 15–36. The Sixth Assessment Report on Climate Change from the IPCC and the First Assessment Report on Biodiversity and Ecosystem Services by the IPBES, large international summaries of the state of scientific knowledge regarding climate disruption and biodiversity loss, also support the view that unprecedented human numbers are contributing to global ecological decline.IPCC. (2014). Summary for policymakers. Climate change 2014: Mitigation of climate change. Cambridge. Intergovernmental Panel on Climate Change.IPBES. (2019). Summary for policymakers. Global assessment report on biodiversity and ecosystem services. Intergovernmental Panel on Biodiversity and Ecosystem Services Secretariat. Recent estimates of a sustainable global human population run between two and four billion people.Examples include Theodore Lianos and Anastasia Pseiridis, "Sustainable Welfare and Optimum Population Size," 2016; Christopher Tucker, A Planet of 3 Billion: Mapping Humanity's Long History of Ecological Destruction and Finding Our Way to a Resilient Future, 2019; Partha Dasgupta, Time and the Generations: Population Ethics for a Diminishing Planet, 2019; Lucia Tamburino and Giangiacomo Bravo, "Reconciling a Positive Ecological Balance with Human Development: A Quantitative Assessment," 2021.
Judgements about human or animal overpopulation hinge partly on whether people feel a moral obligation to leave sufficient habitat and resources to preserve viable populations of other species. Recent biodiversity losses show that humanity's success in supporting larger human populations over the past century has depended on reducing the populations of many of Earth's other species.Foreman, D., & Carroll, L. (2014). Man swarm: How overpopulation is killing the wild world. Live True Books. This is a special example of the competitive exclusion principle in ecology, which states that two species which compete for the same limited resource cannot coexist at constant population values. Today humanity essentially competes with other species everywhere on Earth.Rolston III, Holmes (1994). Conserving Natural Value. Columbia University Press. We thus face choices regarding whether to preserve populations of other species and limit our own, or not.Wilson, E. O. (2016). Half Earth: Our planet's fight for life. WW Norton & Company. These essentially ethical choicesDonaldson, S., & Kymlicka, W. (2011). Zoopolis: A Political Theory of Animal Rights. Oxford University Press.Aldo Leopold, 1966. A Sand County Almanac with Essays on Conservation from Round River. New York: Ballantine Books. will make a difference in future judgements about overpopulation.
Well-studied species
Deer In the Scottish Highlands, the arrangement in which landowners privately cull the overpopulation of red deer has proved an abject failure. Scotland's deer are stunted, emaciated, and frequently starve in the Spring. As of 2016, the population is now so high that 100,000 deer would need to be culled each year just to maintain the current population. A number of landowners have proven unwilling to accede to the law, requiring government intervention anyway. It has been necessary to contract professional hunters in order to satisfy landowner legislation regarding the annual cull. Millions of pounds of taxpayers' cash is spent on the annual cull. As of 2020, 100,000 deer are shot each year. Compounding the problem, some landowners have used supplemental feeding at certain shooting blinds in order to facilitate sport hunting.
Overpopulation can affect forage plants, eventually causing a species to alter the greater environment. Natural ecosystems are extremely complex. The overpopulation of deer in Britain has been caused by legislation making hunting more difficult, but another reason may be the proliferation of forests, used by different deer species to breed and shelter. Forests and parks have caused Britain to be much more forested than it was in recent history, and may thus perversely be causing biodiversity loss, conversion of heath habitat to grassland, extirpation of grassland and woodland plants due to overgrazing and the changing of the habitat structure. Examples are bluebells and primroses. Deer open up the forest and reduce the amount of brambles, which then has knock-on effects on dormice and certain birds which nest near the ground, such as the capercaillie, dunnock, nightingale, song thrush, willow warbler, marsh tit, willow tit and bullfinch. Populations of the nightingale and the European turtle dove are believed to be primarily impacted by muntjac. Grouse populations suffer due to smashing into the fencing needed to protect against deer.
A significant amount of the environmental destruction in Britain is caused by an overabundance of deer. Besides ecological effects, overpopulation of deer causes economic effects due to browsing on crops, expensive fencing needed to combat this and protect new afforestation planting and coppice growth, and increasing numbers of road traffic incidents. High populations cause stripping of the bark of trees, eventually destroying forests. Protecting forests from deer costs on average three times as much as planting the forest in the first place. The NGO Trees for Life spent weeks planting native trees in Scotland, aiming to rebuild the ancient Caledonian Forest. After winter snowdrifts in 2014/2015 flattened the deer fences, more than a decade's growth was lost in a matter of weeks. In 2009 – 2010 the cost of forest protection in Scotland ran to £10.5m.
Some animals, such as muntjac, are too small and boring for most hunters to shoot, which poses additional management problems.
In the United States the exact same problem is seen with white-tailed deer, where populations have exploded and become invasive species in some areas. The state of Wisconsin has an estimated population of 1.9 million White tail deer, measured in 2020. In continental Europe roe deer pose a similar problem: although the populations were formerly much less, they have swelled in the 20th century so that although two and a half million are shot each year by hunters in Western Europe alone, as of 1998, the population still appears to be increasing, causing problems for forestry and traffic. In an experiment where roe deer on a Norwegian island were freed from human harvest and predators, the deer doubled in population each year or two. In the Netherlands and southern England roe deer were extirpated from the entirety of the country except for a few small areas around 1875. In the 1970s the species was still completely absent from Wales, but as of 2013, it has colonized the entire country.Algemeen Dagblad; Hoera, daar ligt weer een dode otter As new forests were planted in the Netherlands in the 20th century, the population began to expand rapidly. As of 2016 there are some 110,000 deer in the country.
Birds