When a population of a species exceeds the carrying capacity of its environment
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. 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.
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. 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. Recent estimates of a sustainable global human population run between two and four billion people.
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. 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. We thus face choices regarding whether to preserve populations of other species and limit our own, or not. These essentially ethical choices  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. 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.
Aquaculture operations, recreation angling and populations of endangered fish such as the schelly are impacted by cormorant populations. Open aquaculture ponds provide winter or year-round homes and food for cormorants. Cormorants' effect on the aquaculture industry is significant, with a dense flock capable of consuming an entire harvest. Cormorants are estimated to cost the catfish industry in Mississippi alone between $10 million and $25 million annually. Cormorant culling is commonly achieved by sharp-shooting, nest destruction, roost dispersal and oiling the eggs.
Geese numbers have also been called overpopulated. In the Canadian Arctic region, snow geese, Ross's geese, greater white-fronted geese and some populations of Canada geese have been increasing significantly over the past decades. Lesser snow geese populations have increased to over three million, and continue to increase by some 5% per year. Giant Canada geese have grown from near extinction to nuisance levels, in some areas. Average body sizes have decreased and parasite loads are higher. Before the 1980s, Arctic geese populations had boom and bust cycles (see above) thought to be based on food availability, although there are still some bust years, this no longer seems the case.
It is difficult to know what the numbers of geese were before the 20th century, before human impact presumably altered them. There are a few anecdotal claims from that time of two or three million, but these are likely exaggerations, as that would imply a massive die-off or vast amounts harvested, for which there is no evidence. More likely estimates from the period of 1500 to 1900 are a few hundred thousand animals, which implies that with the exception of Ross's geese, modern populations of geese are many millions more than in pre-industrial levels.
Humans are blamed as the ultimate cause for the increase, directly and indirectly, due to management legislation limiting hunting introduced specifically in order to protect bird populations, but most importantly due to the increase in agriculture and large parks, which has had the effect of creating vast amounts of unintentional sanctuaries filled with food. Urban geese flocks have increased enormously. City ordinances generally prohibit discharging firearms, keeping such flocks safe, and there is abundant food. Geese profit from agricultural grain crops, and seem to be shifting their habitat preferences to such farmlands. Reduction of goose hunting in the US since the 1970s seems to have further had the effect of protecting populations. In Canada hunting has also decreased dramatically, from 43.384% harvest rates in the 1960s to 8% in the 1990s. Nonetheless, when kill rates were compared to populations, hunting alone does not seems to be solely responsible for the increase -weather or a not yet completed shift in habitat preference to agricultural land may also be factors. Although hunting may have formerly been the main factor in maintaining stable populations, ecologists no longer consider it a practical management solution, as public interest in the practice has continued to wane, and the population is now so large that the massive culls needed are unrealistic to ask from the public. Climate change in the Arctic would appear to be an obvious cause for the increase, but when subpopulations are correlated with local climatic increases, this does not seem to hold true, and furthermore, breeding regions seem to be shifting southwards anyway, irrespective of climate change.
The nutrient subsidy provided by foraging in agricultural land may have made