Animals are often judged overpopulated when their numbers cause impacts that people find dangerous, damaging, expensive, or otherwise harmful.
[1][2] 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.
Immunocontraception has been successfully used or tested in a variety of wild-animal populations including those of bison,[14] deer,[15] elephants,[16] gray squirrels,[17] pigeons,[16] rats and wild horses.
[25][26] In the context of rapid climate change, mass species extinction and other global environmental problems, discussions regarding human overpopulation are inevitable.
[31] 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.
[26][34] 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.
[33][36] 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.
[36][43] In the Scottish Highlands, the arrangement in which landowners privately cull the overpopulation of red deer has proved an abject failure.
[10] Compounding the problem, some landowners have used supplemental feeding at certain shooting blinds in order to facilitate sport hunting.
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.
[57] 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.
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.
Grubbing and overgrazing by geese completely denudes the tundra and marshland, in combination with abiotic processes, this creates large desert expanses of hypersaline, anoxic mud which continue to increase each year.
Biodiversity drops to only one or two species which are inedible for geese, such as Senecio congestus, Salicornia borealis and Atriplex hastata.
The yellow rail (Coturnicops noveboracensis) appears to be extirpated from areas of Manitoba due habitat loss caused by the geese, whereas on the other hand the semipalmated plover (Charadrius semipalmatus) appears to be taking advantage of the large areas of dead willows as a breeding ground.
Experimentally excluding geese by means of fencing in North Carolina has found heavily affected areas can regenerate after only two years.
[57] Management strategies in the USA include increasing the bag limit and the number of open hunting days, goose egg addling, trapping and relocation, and egg and nest destruction, managing habitat to make it less attractive to geese, harassment and direct culling.
[58] In Denver, Colorado, during moulting season biologists rounded up 300 Canada geese (of 5,000 in the city), ironically on Canada Day, killing them and distributing the meat to needy families (as opposed to sending it to a landfill), to try to curb the number of geese, following such programs in New York, Pennsylvania, Oregon and Maryland.
[59] In Russia, the problem does not seem to exist, likely due to human harvest and local long-term cooling climate trends in the Russian Far East and Wrangel Island.
[57] For organisations such as Ducks Unlimited, the resurgence of goose populations in North America can be called one of the greatest success stories in wildlife management.
This could be considered a perfect time for wildlife managers to allow hunters or trappers to harvest as much of these animals as necessary, for example lynx in Canada, although on the other hand this may impact the ability of the predator to rebound when the prey population begins to exponentially increase again.
[71] 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.
[76] This is done typically in four ways: Similar methods can be used to determine the population of fish; however some key differences arise in the extrapolation of data.
Factors such as migration may not be relevant when determining population in a specific locales while more important for others such as the many species of salmon or trout.
[78] Monitoring of waterways and isolated bodies of water provide more frequently updated information on the populations in specific areas.
In the case of the Mute swan, Cygnus olor, their population has rapidly spread across much of North America as well as parts of Canada and western Europe.
[81] This species of swan has caused much concern for wildlife management as they damage aquatic vegetation, and harass other waterfowl, displacing them.
Because such cattle populations begin to starve and die in the winter as available forage drops, this has caused animal rights activists to advocate supplemental feeding, which has the effect of exacerbating the ecological effects, causing nitrification and eutrophication due to excess faeces, deforestation as trees are destroyed, and biodiversity loss.
[85][86] Despite the ecological effects of overpopulation, wildlife managers may want such high populations in order to satisfy public enjoyment of seeing wild animals.
[45] Others contend that introducing large predators such as lynx and wolves may have similar economic benefits, even if tourists rarely actually catch glimpses of such creatures.