Overexploitation
Overexploitation, also called overharvesting or ecological overshoot, refers to harvesting a renewable resource to the point of diminishing returns. Continued overexploitation can lead to the destruction of the resource, as it will be unable to replenish itself. The term applies to various natural resources such as water aquifers, grazing pastures and forests, wild medicinal plants, fish stocks, and other wildlife.
In ecology, overexploitation describes one of the five main activities threatening global biodiversity. 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. In conservation 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. The term is also used and defined somewhat differently in fisheries, hydrology, and natural resource management.
Overexploitation can lead to resource destruction, including extinctions, however, it can also be sustainable, as [|discussed 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" in forest management, "overdrafting" in aquifer management, and "endangered species" in species monitoring. Overexploitation is not an activity limited to humans. Introduced predators and herbivores, for example, can overexploit native flora and fauna.
History
Concerns about overexploitation have their roots in ancient practices that are deeply embedded in human history, although they are relatively recent in the annals of modern environmental awareness. Contrary to the notion that overexploitation is an exclusively contemporary issue, it has been documented for millennia and is not limited to human activities alone. Historical evidence reveals that various 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 of Hawaiian kings, which were adorned with the feathers of the now-extinct mamo 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.
The story of the dodo bird from Mauritius 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.
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. The overkill hypothesis, which addresses the Quaternary extinction events, explains the relatively rapid extinction of megafauna after human arrival. This hypothesis posits that these extinctions were intricately associated with human migration and population growth. One strong piece of evidence for this theory is that about 80% of North America's large mammal species became extinct within about a thousand years of humans arriving in the Western 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. A 2018 study says that large mammals started to become extinct in Africa 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.
The fastest-ever recorded extinction of megafauna occurred in New Zealand. By 1500 AD, a mere 200 years after the first human settlements, ten species of the giant moa birds were driven to extinction by the Māori. This swift extinction underscores the significant impact humans can have on native wildlife, particularly 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 extinction. 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. 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.
In more recent times, overexploitation has led to the gradual emergence of the concepts of sustainability and sustainable development, which have built upon other concepts, such as sustainable yield, eco-development, and deep ecology.
Overview
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.Tragedy of the commons
In 1968, the journal Science published an essay by Garrett Hardin. It was based on a parable that William 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. Lloyd described a simplified hypothetical situation based on medieval land tenure in Europe. Herders share common land on which they are each entitled to graze 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 the carrying 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 concluded:
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.
Hardin developed the theme, drawing in many examples of latter-day commons, such as national parks, the atmosphere, oceans, rivers, and fish stocks. The example of fish stocks had led some to call this the "tragedy of the fishers." A major theme running through the essay is the growth of human populations, along with the idea that Earth's finite resources are commonly shared.
The tragedy of the commons has intellectual roots tracing back to Aristotle, who noted that "what is common to the greatest number has the least care bestowed upon it," as well as to Hobbes and his Leviathan.
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."
Sectors
Fisheries
In wild fisheries, overfishing occurs when a fish stock has been fished down "below the size that, on average, would support the long-term maximum sustainable yield of the fishery".When a fishery starts harvesting fish from a previously unexploited stock, the biomass 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 the maximum 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.
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. Even though fishing has ceased, the cod stocks have failed to recover. The absence of cod as the apex predator in many areas has led to trophic cascades.
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. 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.
The tragedy of the commons can be avoided within the context of fisheries if fishing effort and practices are regulated appropriately by fisheries management. One effective approach may involve assigning individual transferable quotas 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.