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Overexploitation, also calledoverharvesting orecological overshoot, refers to harvesting arenewable resource to thepoint of diminishing returns.[2] Continued overexploitation can lead to the destruction of the resource, as it will be unable to replenish itself. The term applies to variousnatural resources such aswater aquifers,grazing pastures andforests, wildmedicinal plants,fish stocks, and otherwildlife.
Inecology, overexploitation describes one of the five main activities threatening globalbiodiversity.[3] Ecologists use the term to describe populations that are harvested at an unsustainable rate, given their natural rates of mortality and capacities for reproduction. Such practices can result in extinction at the population level and even extinction of whole species. Inconservation biology, the term is usually used in the context of human economic activity that involves the taking of biological resources, or organisms, in larger numbers than their populations can withstand.[4] The term is also used and defined somewhat differently infisheries,hydrology, andnatural resource management.
Overexploitation can lead to resource destruction, includingextinctions. However, it is also possible for overexploitation to be sustainable, asdiscussed below in the section on fisheries. In the context of fishing, the term "overfishing" can be used instead of overexploitation, as can "overgrazing" in stock management, "overlogging" inforest management, "overdrafting" inaquifer management, and "endangered species" in species monitoring. Overexploitation is not an activity limited to humans. Introduced predators and herbivores, for example, can overexploit nativeflora andfauna.
The concern about overexploitation, although relatively recent in the annals of modern environmental awareness, has its roots in ancient practices deeply embedded in human history. Contrary to the notion that overexploitation is an exclusively contemporary issue, the phenomenon has been documented for millennia and is not limited to human activities alone. Historical evidence reveals that various cultures and societies have engaged in practices that have led to the overuse of natural resources, sometimes with drastic consequences.
One poignant example can be found in the ceremonial cloaks ofHawaiian kings, which were adorned with the feathers of the now-extinctmamo bird. Crafting a single cloak required the feathers of approximately 70,000 adult mamo birds, illustrating a staggering scale of resource extraction that ultimately contributed to the extinction of these birds. This instance highlights how cultural traditions and their associated demands can sometimes lead to the overexploitation of a species to the brink of extinction.[7][8]
The story of thedodo bird fromMauritius is another clear example of overexploitation. The dodo, a flightless bird, exhibited a lack of fear toward predators, including humans, making it exceptionally vulnerable to hunting. The dodo quickly became extinct because it was naive and lacked natural defenses against both human hunters and newly introduced species. This case offers insight into how certain species, particularly those isolated on islands, can be disproportionately affected by human activities due to their evolutionary adaptations.[9]
Hunting has long been a vital activity for survival, providing food, clothing, and tools. However, the history of hunting also includes episodes of overexploitation, particularly in the form of overhunting. Theoverkill hypothesis, which addresses the Quaternary extinction events, explains the relatively rapid extinction ofmegafauna. This hypothesis posits that these extinctions were intricately associated with human migration and population expansion. One strong piece of evidence for this theory is that about 80% ofNorth America's large mammal species became extinct within about a thousand years of humans arriving in theWestern Hemisphere. This rapid disappearance indicates a significant impact of human activity on these species, which emphasizes the profound effect humans have had on their environment throughout history.[10] A 2018 study says that large mammals started to become extinct inAfrica at least 125,000 years ago because of human hunting. This extinction occurred 90,000 years earlier than previously estimated. As humans migrated, size-biased extinctions occurred in regions that aligned with human migration patterns, resulting in a significant global decrease in average mammal body size.[11]
The fastest-ever recorded extinction of megafauna occurred inNew Zealand. By 1500 AD, a mere 200 years after the first human settlements, ten species of the giantmoa birds were driven to extinction by theMāori. This swift extinction highlights the significant influence humans can have on native wildlife, especially in remote ecosystems like New Zealand. The Māori, relying on the moa as a primary food source and for resources such as feathers and bones, hunted these birds extensively. The moa's inability to fly and their size, which made them easier targets, contributed to their rapid decline. This event serves as a cautionary tale about the delicate balance between human activity and biodiversity, highlighting the potential consequences of overhunting and habitat destruction.[5] A second wave of extinctions occurred later with European settlement. This period marked significant ecological disruption, primarily due to the introduction of new species and changes in land use. European settlers introduced animals such as rats, cats, and stoats, which preyed upon native birds and other wildlife. Additionally, deforestation for agricultural purposes has significantly altered the habitats of many endemic species. These combined factors accelerated the decline of New Zealand's unique biodiversity, resulting in the extinction of several additional species. The European settlement period serves as a poignant example of how human activities can have a profoundly negative impact on natural ecosystems.
In more recent times, overexploitation has led to the gradual emergence of the concepts ofsustainability andsustainable development, which have built upon other concepts, such assustainable yield,[12]eco-development,[13][14] anddeep ecology.[15][16]
Overexploitation does not necessarily lead to the destruction of the resource, nor is it necessarily unsustainable. However, reducing either the population or the quantity of a resource can negatively affect its quality. For example,footstool palm is a wild palm tree found in Southeast Asia. Its leaves are used for thatching and food wrapping, and overharvesting has resulted in its leaf size becoming smaller.
In 1968, the journalScience published an article byGarrett Hardin entitled "The Tragedy of the Commons."[17] It was based on a parable thatWilliam Forster Lloyd published in 1833 to explain how individuals innocently acting in their own self-interest can overexploit and destroy a resource that they all share.[18][pages needed] Lloyd described a simplified hypothetical situation based on medievalland tenure in Europe.Herders sharecommon land on which they are each entitled tograze their cows. In Hardin's article, it is in each herder's individual interest to graze each new cow that the herder acquires on the common land, even if thecarrying capacity of the common is exceeded, which damages the common for all the herders. The self-interested herder receives all of the benefits of having the additional cow, while all the herders share the damage to the common. However, all herders reach the same rational decision to buy additional cows and graze them on the common, which eventually destroys it. Hardin concludes:
Therein is the tragedy. Each man is locked into a system that compels him to increase his herd without limit—in a world that is limited. Ruin is the destination toward which all men rush, each pursuing his own interest in a society that believes in the freedom of the commons. Freedom in a commons brings ruin to all.[17]: 1244
In the course of his essay, Hardin develops the theme, drawing in many examples of latter-day commons, such asnational parks, the atmosphere, oceans, rivers, andfish stocks. The example of fish stocks had led some to call this the "tragedy of the fishers."[19] A major theme running through the essay is thegrowth of human populations, along with the idea thatEarth's finite resources are commonly shared.
The tragedy of the commons has intellectual roots tracing back toAristotle, who noted that "what is common to the greatest number has the least care bestowed upon it",[20] as well as toHobbes and hisLeviathan.[21] The opposite situation to a tragedy of the commons is sometimes referred to as atragedy of the anticommons: a situation in which rational individuals, acting separately, collectively waste a given resource by underutilizing it.
The tragedy of the commons can be avoided if it is appropriately regulated. Hardin's use of "commons" has frequently been misunderstood, leading Hardin to later remark that he should have titled his work "The tragedy of the unregulated commons."[22]
Inwild fisheries,overfishing occurs when afish stock has been fished down "below the size that, on average, would support the long-termmaximum sustainable yield of the fishery".[23]
When a fishery starts harvesting fish from a previously unexploited stock, thebiomass of the fish stock will decrease, since harvesting means fish are being removed. For sustainability, the rate at which the fish replenish biomass through reproduction must balance the rate at which the fish are being harvested. Increasing the harvest rate will lead to a further decrease in the stock biomass. At a certain point, the maximum harvest yield that can be sustained will be reached, and further attempts to increase the harvest rate will result in the collapse of the fishery. This point is called themaximum sustainable yield and, in practice, usually occurs when the fishery has been fished down to about 30% of the biomass it had before harvesting started.[24]
Fish stocks are said to "collapse" if their biomass declines by more than 95 percent of the maximum historical biomass recorded.Atlantic cod stocks were severely overexploited in the 1970s and 1980s, leading to their abrupt collapse in 1992.[1] Even though fishing has ceased, the cod stocks have failed to recover.[1] The absence of cod as theapex predator in many areas has led totrophic cascades.[1]
About 25% of world fisheries are now overexploited to the point where their current biomass is less than the level that maximizes their sustainable yield.[25] Reducing fishing pressure often allows these depleted fisheries to recover until the stock biomass reaches the optimal level. At this point, harvesting can be resumed near the maximum sustainable yield.[26]
Thetragedy of the commons can be avoided within the context of fisheries iffishing effort and practices are regulated appropriately byfisheries management. One effective approach may involve assigningindividual transferable quotas (ITQs) as a form of ownership to fishermen. In 2008, a large-scale study of fisheries that used ITQs and ones that did not provided strong evidence that ITQs help prevent collapses and restore fisheries that appear to be in decline.[27][28]
Water resources, such aslakes andaquifers, are usually renewable resources that naturally recharge (the term"fossil water" is sometimes used to describe aquifers that do not recharge). Overexploitation occurs if a water resource, such as theOgallala Aquifer, is mined or extracted at a rate that exceeds the recharge rate, that is, at a rate that exceeds the practical sustained yield. Recharge usually comes from local streams, rivers, and lakes. An aquifer that has been overexploited is said to beoverdrafted or depleted. Forests enhance the recharge ofaquifers in some locales, although generally forests are a major source of aquifer depletion.[29][30] Depleted aquifers can become polluted with contaminants likenitrates, or they can be permanently damaged by subsidence or saline intrusion from the ocean.
This turns much of the world's underground water and lakes into finite resources with peak usage debates similar tooil.[31][32] These debates usually focus on agriculture and suburban water usage, but the generation of electricity from nuclear energy or coal and tar sands mining is also water resource intensive.[33] A modifiedHubbert curve applies to any resource that can be harvested faster than it can be replaced.[34] Though Hubbert's original analysis did not apply to renewable resources, their overexploitation can result in aHubbert-like peak. This conclusion has led to the concept ofpeak water.
Forests are considered overexploited whenlogging occurs at a rate that exceeds the rate ofreforestation. Reforestation competes with other land uses such as food production, livestock grazing, and living space for further economic growth. Historically, the utilization of forest products, including timber and fuel wood, has played a key role in human societies, comparable to the roles of water and cultivable land. Today, developed countries continue to utilize timber for building houses and wood pulp forpaper. In developing countries almost three billion people rely on wood for heating and cooking.[35] Short-term economic gains from converting forests to agriculture or overexploiting wood products typically result in a loss of long-term income and biological productivity.Madagascar,Southeast Asia, and many other regions have experienced lower revenue because of overexploitation and the consequent declining timber harvests.[36]
Overexploitation is one of the main threats to globalbiodiversity.[3] Other threats includepollution,introduced andinvasive species,habitat fragmentation,habitat destruction,[3]uncontrolled hybridization,[37]climate change,[38]ocean acidification[39] and the driver behind many of these,human overpopulation.[40]
One of the key health issues associated with biodiversity is drug discovery and the availability of medicinal resources.[41] A significant proportion of drugs are derived fromnatural products that come directly or indirectly from biological sources. Marine ecosystems are of particular interest in this regard.[42] However, unregulated and inappropriatebioprospecting could potentially lead to overexploitation, ecosystem degradation, andloss of biodiversity.[43][44][45]
Species from all groups of fauna and flora are affected by overexploitation. This phenomenon is not bound by taxonomy; it spans across mammals, birds, fish, insects, and plants alike. Animals are hunted for their fur, tusks, or meat, while plants are harvested for medicinal purposes, timber, or ornamental uses. This unsustainable practice disrupts ecosystems, threatening biodiversity and leading to the potential extinction of vulnerable species.
All living organisms require resources to survive. Overexploitation of these resources for protracted periods can deplete natural stocks to the point where they are unable to recover within a short time frame. Humans have always harvested food and other resources they need to survive. Human populations, historically, were small, and methods of collection were limited to small quantities. With an exponential increase inhuman population, expanding markets, and increasing demand, combined with improved access and techniques for capture, theexploitation of many species is beyond sustainable levels.[46] In practical terms, if continued, it reduces valuable resources to such low levels that their exploitation is no longer sustainable and can lead to theextinction of a species, in addition to having dramatic, unforeseeneffects on theecosystem.[47] Overexploitation often occurs rapidly as markets open, utilising previously untapped resources or locally used species.
Overexploitation and misuse of natural resources is an ever-present threat forspecies richness. This is more common inisland ecology, where islands are like miniature worlds. Islandendemic populations are more prone toextinction from overexploitation, as they often exist at low densities with reduced reproductive rates.[48] A good example of such populations are island snails, such as the HawaiianAchatinella and the French PolynesianPartula. Achatinelline snails have 15 species listed as extinct and 24 as critically endangered[49] while 60 species ofPartulidae are considered extinct, with 14 listed as critically endangered.[50] TheWCMC has attributed over-collecting and very low lifetime fecundity to the extreme vulnerability exhibited among these species.[51]
As another example, when the humblehedgehog was introduced to the Scottish island ofUist, the population greatly expanded and took to consuming and overexploiting shorebird eggs, with drastic consequences for their breeding success. Twelve species ofavifauna are affected, with some species numbers being reduced by 39%.[52]
Where there is substantial human migration, civil unrest, or war, controls may no longer exist. With civil unrest, for example in theCongo andRwanda, firearms have become common, and the breakdown of food distribution networks in such countries leaves the resources of the natural environment vulnerable.[53] Animals are even killed as target practice or simply to spite the government. Populations of large primates, such asgorillas andchimpanzees,ungulates, and other mammals, may be reduced by 80% or more by hunting, and certain species may be eliminated.[54] This decline has been called thebushmeat crisis.
Overexploitation threatens one-third of endangeredvertebrates, as well as other groups. Excluding edible fish, the illegaltrade in wildlife is valued at $10 billion per year. Industries responsible for this include the trade inbushmeat, the trade inChinese medicine, and thefur trade.[55] The Convention for International Trade in Endangered Species of Wild Fauna and Flora, orCITES, was set up in order to control and regulate the trade in endangered animals. It currently protects, to a varying degree, some 33,000 species of animals and plants. It is estimated that a quarter of the endangered vertebrates in the United States of America and half of the endangered mammals are attributed to overexploitation.[3][56]
Overall, 50 bird species that have become extinct since 1500 (approximately 40% of the total) have been subject to overexploitation,[57] including:
Overexploitation of species can result in knock-on orcascade effects. This can particularly apply if, through overexploitation, a habitat loses itsapex predator. The loss of the top predator can lead to adramatic increase in the population of itsprey species. In turn, the unchecked prey can then overexploit their food resources until population numbers dwindle, possibly to the point of extinction.
A classic example of cascade effects occurred withsea otters. Starting before the 17th century and not phased out until 1911, sea otters were hunted aggressively for their exceptionally warm and valuable pelts, which could fetch up to $2500 US. This caused cascade effects through thekelp forest ecosystems along the Pacific Coast of North America.[60]
One of the sea otters' primary food sources is thesea urchin. When hunters caused the sea otter populations to decline, this led to anecological release of the sea urchin populations. The sea urchins then overexploited their main food source,kelp, creating urchin barrens, areas of seabed denuded of kelp but carpeted with urchins. Consequently, the sea urchin population also becamelocally extinct due to the lack of food. Also, since kelp forest ecosystems are homes to many other species, the loss of the kelp caused other cascade effects of secondary extinctions.[61]
In 1911, when only one small group of 32 sea otters survived in a remote cove, an international treaty was signed to prevent further exploitation of the sea otters. Under heavy protection, the otters multiplied and repopulated the depleted areas, which slowly recovered. More recently, with declining numbers of fish stocks, again due to overexploitation,killer whales have experienced a food shortage and have been observed feeding on sea otters, again reducing their numbers.[62]
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