An open access species is available to any individual or other economic entity (informal community, commercial business [proprietorship, partnership, corporation], nonprofit organization, or government) without limit or payment because no party can restrict access to it in practical terms, or because no property right in the species has been claimed, granted, or assigned (res nullius) [14] [15] [16] . Appropriators secure property rights in the resource only upon its harvest or capture [17] . Open access was the original, natural conservation access model and remains the default [10] .

When an open access common pool species is a resource—i.e., a valuable good instead of a bad, like a disease vector pest [18] —economic entities may possess an incentive to exploit the resource before others do, leading to what [19] called the “tragedy of the commons.” Despite the incentive to unsustainably exploit (overexploit) an open access resource, it may persist indefinitely when the economic cost of extracting, transporting, and utilizing or selling the resource exceeds its market price, or the opportunity cost of its harvest by humans [7] .

Wild rats, for example, are considered pests, economic bads that exterminators must be paid to control [20] . Markets for rat fur and especially meat, however, do exist [21] , as do markets for live rats for use in scientific experiments. Rats can sustainably remain an open access resource, however, because their natural rate of reproduction exceeds human demand, in part due to superior domesticated substitutes. Rats domesticated specifically for experimentation, for instance, are more cheaply farmed (private access) than wild rats can be caught [22] .

More valuable resources can also remain sustainable under the open access model so long as the quantity harvested remains below the resource’s reproduction rate, or in other words when harvests do not exceed the sustainable yield rate [11] . Rabbits, for example, can remain open access (unregulated and unowned) when environmental conditions allow them to reproduce more quickly than low density human populations care to consume them, the marginal cost of harvesting and transporting them to higher density population centers exceeds their market price, and/or rabbit farming (i.e., a supply of private access rabbits) provides an alternative source of supply [23] .

Note that humans do not compete with prey like rabbits, which will die of disease, accident, nonhuman predation, or a dearth of water or food if not harvested by humans. Humans, though, may compete with specialized rabbit predators like lynx and the Spanish Imperial Eagle [24] . For the sake of competing predators, government or private wildlife managers might decide to restrict access to prey resources by placing them somewhere along the spectrum of the other two conservation access models [25] .

Intellectual humility, however, suggests that human tinkering should be minimized because ecological systems are too complex, in the sense of [26] , to be fully understood. Increasing striped bass populations in Chesapeake Bay, for example, unintentionally depleted blue crab populations already stressed by pollution encouraged by treating the massive estuary as a common pool environmental sink [27] . Although popular notions of a pristine wildness untouched by humans are fanciful and related notions of the existence of natural “balance” elide natural extinctions and mass fluctuations in wildlife populations and range [28] [29] , deliberate human attempts to “fix” the ecological systems that they have impacted create unsustainable or “fragile” outcomes [30] .

Information acquisition regarding open access resources is restricted only by natural limits, i.e., opportunity cost and the nature of the resource and its environment. Anyone who wishes, for example, could try to determine the quantity or quality of open access rabbits or rats. The incentive to do so, however, is weak because anyone at any time could harvest a substantial amount of the open access resource without any legal or moral obligation to give notice.

The incentive to preserve or improve habitat for an open access common pool resource is also scant because others could easily free ride on any investments by harvesting the resource before the habitat improver does [17] . Appropriators may even destroy habitats in the process of harvest. A wild bee colony may be extirpated when raided for its honey, for example, because the appropriator knows that another human or nonhuman predator might harvest any remaining honey or wax [31] .

American Indian groups that burned woods or prairie to hunt deer or bison improved habitat by initiating forest or grassland renewal and reducing the odds of larger and more destructive natural conflagrations [32] [33] [34] . They did so, however, under government (tribal) or private resource access regimes, not under open access common pool conditions [11] . In fact, as discussed in greater detail in below, many resources once thought to be open access common pool were in fact wholly or partially regulated privately, under various informal arrangements.

Reference [35] , for example, shows that communities, especially small, homogeneous ones relative to modern states, imposed informal harvest or use rules that rendered sustainable resources formerly wrongly considered to be open access common pool. While American Indians sometimes harvested open access resources in areas not claimed by any tribe, tribal territories were subject to various top-down regulations [36] [37] , including taboos against porcupine consumption except in emergencies [38] . Or, they were subdivided into smaller territories subject to private individual, family, or clan management, if not always “ownership” in the modern western sense [39] [40] .

In top-down regulatory conservation models, like the North American Wildlife Conservation Model (NAWCM), some local, national, or supra-governmental body asserts a weak form of ownership, often described as stewardship, over wildlife resources and then regulates access to them, ostensibly to achieve conservation goals [41] . Such regulations may include banning resource exploitation entirely, prescribing the methods, times of day, and seasons during which the resource may be harvested, and limiting the size, number, weight, and/or sex of the harvested resource [11] . In the NAWCM, licenses and tags are typically priced below market, with excess demand rationed via lottery [42] .

Government harvest regulations range from extremely loose to extremely tight. In New Jersey, for example, an individual may harvest for personal consumption every day of the year, night and/or day, up to a bushel of blue crabs using hand lines or collapsible traps so long as no crabs smaller than 4.5 inches in breadth or females of any size with eggs attached are retained. The regulations can be loose because commercial crabbing and pollution regulations, not regulation of personal consumption, remain key to conserving the species [27] . At the other extreme, a government wildlife manager might completely ban the commercial harvest of wild sheep and proclaim that only male mountain sheep with full curl horns may be taken, and only by licensed tagholders, with a vertical bow, in daylight, during a week-long season.

Ideally, government wildlife managers implement restrictions to carefully balance human demand for the resource with its biological characteristics and its place in the local ecology. In unpolluted habitats, blue crabs abound so long as juveniles and reproducing females are protected from harvest. Mountain sheep, by contrast, are more biologically sensitive to human harvest rates. Killing old rams, the reason for the full curl stipulation, culls the biologically least important member of the herd, leaving juveniles, reproductive-age females, and maturing males more food and cover. Happily, the horns that signal advanced age induce hunters to pay thousands of USD for the right to try to harvest one [43] . Moreover, most tagholders fail to harvest a ram, which increases the allure of the hunt by rendering each full curl ram a highly esteemed “trophy” [44] .

Government access regulations can be highly effective, but their efficacy may degrade as environmental and socioeconomic conditions change, becoming either too loose, thus endangering wildlife quantity, or too restrictive, thus endangering wildlife quality. Regulations that become too restrictive reduce resource utilization below sustainable yield while threatening, in the absence of natural predators, overpopulation and its attendant costs on humans (more collisions, livestock predation, crop depredation, etc.), other wildlife (forage or prey species), and the resource itself (smaller, less resistant to disease, less resilient in the face of weather and other shocks, etc.). Regulations that become too loose threaten resource depletion. Sometimes government wildlife managers respond quickly to changes in a resource’s population or in human harvest demand, but other times weak incentives mean that their response lags or is insufficient [41] .

Moreover, in places with strong property rights and mostly privately-owned land or water, government wildlife managers cannot easily obtain information about changes in a resource population. In South Dakota, for example, government wildlife managers contract with rural postal carriers to conduct counts of pheasant broods spotted on backcountry dirt roads because they cannot easily conduct a pheasant census on private farmland [45] .

The incentive and information problems facing government wildlife managers are important constraints on their ability to set access regulations that are neither too tight nor too loose. Moreover, government access regulations often cover large swathes of territory, rendering any mistakes widespread and potentially quite biologically costly [26] .

The incentives of top-down regulators to provide habitat also vary widely. In North America, government wildlife managers want people to enjoy camping, fishing, hunting, trapping, and wildlife watching because excise taxes on their equipment, along with use and license fees, pay the salaries of wildlife managers. Managers, therefore, want wildlife to be abundant and accessible and that may mean providing more quality habitat through prescribed burns, dredging, paying farmers to maintain wetlands, constructing wood duck nesting poles, and so forth [3] [46] .

In other places, though, government wildlife managers receive salaries regardless of habitat quality, or the quantity or quality of wildlife, so more wildlife and more wildlife users simply make more work for them, encouraging wildlife managers to allow, or even abet, habitat destruction [4] .

Public choice theory also suggests that government wildlife managers may put their own interests ahead of those of wildlife and/or the public in other ways as well [7] [11] [16] [47] . Reference [48] , for example, shows that official U.S. bison policy almost caused bison extinction while reducing the physical stature and overall health of Plains Indians. Similarly, [49] shows that U.S. wild horse management practices hurt local economies while not effectively managing wild horse populations or the local ecological systems they impact. According to [50] , NAWCM regulators continue to forbid markets for wild game meat even though legalization could help to reduce the population of locally overabundant species, as such markets long have in Europe.

In the private access model, a commercial, nonprofit, or community-based economic entity asserts ownership over the resource, controlling access to it via legal or customary rights [15] . The entity grants access to the resource to its owners or members and/or to outsiders on mutually agreeable terms, which can be in kind and/or in cash, typically at a market clearing price with price discrimination adjustments for youths, women, handicapped veterans, retirees, and so forth [51] [52] . Examples include hunting clubs, safari ranches, fee fishponds, hunting and fishing concessions and leases, and aquariums, menageries, zoos, wildlife parks, sanctuaries, and other privately-owned wildlife viewing venues [33] . In the US alone, revenue for such entities, including day access fees and long-term leases, total several billion dollars per year [14] .

Many informal arrangements are included in the private access model as defined herein, even arrangements some consider examples of a “closed access common pool” conservation model because they are not outwardly commercial in nature. Wildlife resources in those situations are closed (i.e., not open) access because access is controlled through ownership in common. Ownership in common is often conflated with common pool and hence open access, but it is really a form of joint ownership, though, unlike in a corporation, without transfer rights. Icelandic sheep ranchers, for example, limited the number of sheep that each could graze on the summer ranges that they owned in common, establishing informal grazing rights that members could exercise but not sell to outsiders [17] .

While some of the entities exerting similar access controls may have some of the characteristics of government, they are typically intimately governed, like a local government but also like a nonprofit. They should be considered private entities rather than governmental whenever membership is voluntary, and all members are impacted by access management decisions [13] . As [7] (p. 31) put it: “the appropriators themselves… find ways to organize themselves,” much like a co-op or employee-owned business. And as [10] claims (p. 52), “private property pertains to actions that people undertake because they choose to, and about which the remainder of society forbears from contesting.”

In the governmental access model discussed above, by contrast, participation is compulsory [53] and the decision-making units are often much larger, such that many members/citizens have little or no stake in access decisions, exacerbating the principal-agent costs discussed above and by [7] and others. In the private access model, entities making access decisions face relatively low agency costs, a concept often characterized by non-economists as high levels of “trust,” as in [54] . The low agency costs/high trust render the entities more likely to monitor access outcomes relatively frequently and carefully, to update their access decisions when appropriate, and more generally to behave rationally.

Rational economic entities possess strong incentives to conserve any goods, including wildlife resources, they expect to be able to assert ownership rights in the future, so that the goods are available for their own future use, or their sale or lease to another economic entity [16] [17] . That may mean creating quality information about the resource’s quality and quantity. It may also mean restricting the resource’s movement to the boundaries of the private owner’s property via the use of high fences and/or buying land or leasing hunting, fishing, and/or trapping concessions larger than the natural range of the resource [11] .

Great Britain, for example, vests the ownership of wildlife in real estate owners, who often charge extractive users for access rights. Government wildlife managers set seasons for different species, but property owners set bag limits and charge fines for careless shooting. If caught, trespassers must pay a fee or surrender harvested game, for which there are legal markets, to the property owner, not to government wildlife managers [11] .

A similar private resource approach transformed southern Africa (Botswana, Mozambique, Namibia, South Africa, and Zimbabwe) starting in the 1970s. Landowners there face few top-down restrictions and have learned ways to maximize revenue from their ranches, leading to thriving, diverse, innovative wildlife management practices and business models that include both consumptive (hunting) and non-extractive (photo safari) users. In stark contrast to much of the rest of the continent, which employs mostly open access or low-quality government access models, the region now has more wildlife than a century ago [2] .

The private resource model works best for wildlife species that do not need to migrate over long distances unless they can be relied upon to return to specific breeding or feeding areas [11] [55] . Before the advent of open ocean commercial fishing, for example, American Indian tribes were able to farm salmon by controlling access to their river migration routes and spawning streams [56] , a type of private resource conservation management still used today, primarily for sport fishing, in parts of Europe [57] . Similarly, nesting birds like eiders have been sustainably conserved by the owners of their nesting grounds so they can continue to harvest their down and eggs in perpetuity [33] .

Moreover, it is possible in some places to exercise ownership rights over large territories. Examples include the enormous private reserve called American Prairie [58] , African and Canadian trophy hunting concessions [59] , and cooperative bison ranching in western South Dakota [60] .

Property owners also possess incentives to provide habitats appropriate for the types of wildlife that they own and/or to improve the aesthetics and/or energy or agricultural efficiency of their properties. Regardless of the precise reason for their creation, shelterbelts and other human-created habitat improvements often provide suitable general habitats for a range of wildlife species [61] . Trees planted to provide winter browse and cover for deer, for example, will also succor other animals, from songbirds to squirrels and rabbits to raccoons and may eventually provide the owner with fruit, fuel, or timber.

The incentive to create suitable wildlife habitat decreases to the extent that property owners cannot control access to wildlife. That may be due to weak property rights, especially in nations too weak to discourage trespassers but strong enough to limit private enforcement through violence. Or, it may be due to state stewardship of wildlife, as in North America [3] .

A quasi-natural experiment in the highlands of southern Ethiopia showed that tropical forest cover shrank fastest in a national park and two unoccupied hunting concessions, which de facto turned the forest into an open access common pool resource. Tropical forest cover, though, increased on an active timber concession and an occupied hunting concession because the leaseholders were able to control access to their concessions [62] . Another study in Nepal showed that forests effectively controlled by private community groups halted and even reversed regional deforestation trends [63] .

Where private parties possess insufficient incentive to maintain wildlife habitat, government wildlife managers have provided subsidies with some success, though at the risk of creating a political backlash [61] . A more sustainable model would induce nonowner private parties to voluntarily pay for habitat maintenance or improvement via ecotourism or direct donation, especially where property rights are strong enough to minimize misuse of donated funds [64] [65] .

In some jurisdictions, policies create perverse incentives for landowners. Some 35 percent of the world’s modern mammal extinctions have occurred in Australia, in part due to top-down regulations that encourage native habitat destruction. Reforming those policies to allow for property rights in wildlife could stop or reverse habitat destruction, allowing native flora and fauna to expand their range and rendering native species more resilient to shocks [14] . If, after implementation of such reforms, conservation goals remain unmet, governments or nonprofits could directly pay private landowners in Australia, New Zealand, or elsewhere to increase the population of threatened or endangered species on their property, encouraging them to create suitable habitat and to discourage poaching [2] .

Table 1 summarizes this section by highlighting the conditions in which each major conservation model is most likely to succeed or fail at conserving specific wildlife resources and their respective effects on wildlife habitat.

One of the many unintended consequences of Covid-19 lockdowns across much of the globe was a documented increase in poaching in many nations, including Malaysia [70] , India [71] , Nepal [72] and several African countries [73] [74] . Top-down regulation of wildlife was particularly fragile in Africa because it was new there, largely implemented around the turn of the new millennium during a “bushmeat crisis” that threatened the livelihoods of growing numbers of poor agriculturalists and led to increased top-down regulatory attempts [75] .

Increases in the domestic dog and cat populations also threaten wildlife, especially when fragile socioeconomic conditions induce humans to allow their pets to forage for themselves [76] .

Attempts to limit livestock herds, to combat global climate change, may increase market prices enough to induce more people to harvest wild animals to meet their protein and fat needs [77] . Disruptive climate change reforms may also decrease the ability of states to enforce wildlife regulations and increase incentives to contest government access rules [47] , as in Sri Lanka during its 2022 economic depression and political revolution [78] . In fact, poaching typically increases when and where perpetrators are less likely to be caught, or to be prosecuted if caught, which is typically in low-capacity states, like Zimbabwe during its early Third Millennium hyperinflation [79] .

In places with stronger rule of law and more effective enforcement, various unanticipated shocks may stress open access resources and wildlife managed by government regulators, especially those with incentives to sell more licenses and tags. The wildlife managers may interpret increased interest in hunting, fishing, and trapping as a revenue boon before fully understanding the potential adverse effects of higher demand on resource populations.

Policymakers in some places may have incentives to talk about sustainability and to create government access conservation regimes that appear to protect wildlife but that condone unsustainable rates of exploitation. That way, they win the support of both rural inhabitants who know that despite the law they can kill predators or foragers that endanger their livelihoods and urban inhabitants who do not comprehend the de facto details of the de jure policies [33] .

Policymakers may also exploit voter indifference. In North America in the second half of the nineteenth century, bison numbers declined rapidly because policymakers kept them an open access resource even as the cost of harvesting them plummeted due to the advent of railroads and modern rifles. It is now widely understood that the U.S. government sought the extermination of bison to decrease the economic and military capacity of Plains Indian tribes reluctant to move to Reservations and to create biological space for the domesticated cattle of settlers [80] . Bison may have gone extinct if remnant herds had not been privatized, albeit extralegally, and bred with domesticated cattle [81] .

Many species are bred in captivity even when they thrive in the wild, serving as a private “ark” in the face of ecological shocks [82] , though at the risk of reducing the genetic fitness or even domesticating the captive population [83] [84] . Most species are not bred in captivity but when a species declines in population, each surviving member becomes more valuable, increasing the incentive of economic entities, like zoos, ranchers, or government wildlife managers, to privatize and breed them [85] .

When government regulators forbid privatization, or when insufficient economic incentives to privatize a species exists, extinction may result. Such is the case with many smaller animals, especially non-vertebrates, and was the case with the passenger pigeon, which was thought to be insufficiently different commercially from other pigeons and doves to bother saving. Government access regulation came too little, too late for the little birds, which remained an open access resource for too long, partly due to pressure from farmers who viewed them as agricultural pests. Although many were shot by hunters, most were taken with huge nets in their nesting grounds. Outlawing that method of harvest alone may have been sufficient to save the species if implemented before its final population collapse [41] .

Despite commercial incentives to privatize them, the Carolina parakeet also went extinct. A virus combined with the usual rate of harvest may have led to extinction quickly, before government wildlife managers or parties potentially interested in their privatization, who sold their feathers to milliners, could react. Historical accounts [86] and genomic analysis [87] strongly suggest that, for whatever reason, the population collapsed suddenly, in the western part of its range around 1914 and in the eastern part around 1940 [88] , rather than decreasing steadily over decades before collapse, as was the case with passenger pigeons and bison.

Historically, humans themselves, or their domesticates or commensals, created biological shocks large enough to extirpate species. The first arrival of humans on oceanic islands like Hawaii and New Zealand, for example, precipitated extinction events due to the tragedy of the commons problem and/or the introduction of invasive species [14] [89] .

Cetaceans (whales), especially the larger, slower-reproducing species, also suffered from the tragedy of the commons, high demand for their oils, bones, and blubber, improved technologies/lower harvest costs, and, in the twentieth century, an ineffective top-down supranational regulatory regime. International pressure reduced demand and increased harvest costs enough, however, to allow most species to stabilize and even, in some cases, to rebound. The current equilibrium, however, remains fragile [90] .

Note that indigenous whalers did not cause the whale crisis as their demand was too small and their harvest was mostly composed of smaller, faster reproducing species. Even the New Bedford sailing whalers of the nineteenth century barely dented most cetacean populations. It was the advent of steam ships, cannon harpoons, and, eventually, factory ships, combined with increased worldwide demand for whale products, which brought whale populations to the brink in the twentieth century [91] .

Although government access has led to more species extirpations and extinctions than private access has, top-down government access conservation regulation might be retained as an access model of last resort. Policymakers, though, need to face limits on their power. They should, for example, encourage the efficient movement of wildlife resources from open to private access when environmental and socioeconomic conditions warrant [11] , like during wars, pandemics, or other periods of extreme socioeconomic disruption. At a minimum, that means government wildlife regulators need to monitor the quantity and quality of open access resources and encourage their privatization when demand increases and/or the cost of harvest decreases enough to bring harvest rates up to biological reproduction rates. Inexpensive hunting or trapping by drone, for example, could pressure remote furbearer populations, many of which are currently so difficult to harvest that they remain open access or lightly regulated by government wildlife managers [41] .

Government regulators may also play important roles as coordinators between private resource owners and public resource agencies, like public lands and parks managers, to ensure that their respective programs and efforts do not work at cross purposes. Coordination efforts, however, should remain just that, and not mechanisms for government control of access [14] .

Moreover, government access may also be the least bad choice when species ownership cannot be conjoined with territorial holdings. Certain migratory species, however, like salmon, could be privately owned and managed through ownership of their breeding grounds if their harvest could be limited or stopped elsewhere over their range or lifecycle.

In addition, government wildlife managers may be needed to create and/or to help enforce transferable “catch shares,” a property right to harvest a specified number of a specific resource, so that the creatures covered by such arrangements do not devolve into de facto open access resources subject to the tragedy of the commons [92] . Catch shares aid humans as well as wildlife by decreasing risk taking [17] . They do so by substituting a share of harvest quotas for fixed date seasons that encourage risky and sometimes environmentally damaging harvest races akin to temporarily returning wildlife to the open access common pool [93] . It is important, however, not to allow government wildlife managers too much discretion, lest they collude with commercial interests to raise total quotas above sustainable levels, as has happened in some fisheries [94] .

Government wildlife managers may also encourage habitat creation and maintenance suitable for non-commercial species [95] on private lands and have increasingly done so to varying degrees in the US [96] . Again, though, their discretion must be constrained to reduce the chance of unintended consequences [97] . Generally, reducing human-produced harms, like pollution, poses less risk/is more sustainable than trying to micromanage complex ecological systems from the top [26] [98] .