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[[Image:Atlantic-Cod-Stocks.jpg|thumb|300px|right|[[Atlantic cod]] stocks were severely overexploited in the 1970s and 1980s, leading to their abrupt collapse in 1992]]
[[Image:Atlantic-Cod-Stocks.jpg|thumb|300px|right|[[Atlantic cod]] stocks were severely overexploited in the 1970s and 1980s, leading to their abrupt collapse in 1992]]


'''Overexploitation''' is a term used by a number of disciplines. In [[ecology]], it describes one of the five main [[http://en.wikipedia.org/wiki/Overexploitation#Other_threats_to_biodiversity]] threatening global [[biodiversity]].<ref>Wilcove, D.S., Rothstein, D., Dubow, J. Phillips, A. and Losos, E. (1998). Quantifying Threats to Imperilled Species in the United States. BioScience. 48, 607-615.</ref>. Ecologists use the term to describe populations that are harvested at a rate that is unsustainable, given their natural rates of mortality and capacities for reproduction. This can result in harvesting to the point of diminishing returns and 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.<ref>Oxford. (1996). Oxford Dictionary of Biology. Oxford University Press.</ref> The term is also used and defined somewhat differently in [[fisheries]], [[hydrology]] and [[resource management]].
'''Overexploitation''' is a term used by a number of disciplines. In [[ecology]], it describes one of the five main [[activities|#Other_threats_to_biodiversity]] threatening global [[biodiversity]].<ref>Wilcove, D.S., Rothstein, D., Dubow, J. Phillips, A. and Losos, E. (1998). Quantifying Threats to Imperilled Species in the United States. BioScience. 48, 607-615.</ref>. Ecologists use the term to describe populations that are harvested at a rate that is unsustainable, given their natural rates of mortality and capacities for reproduction. This can result in harvesting to the point of diminishing returns and 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.<ref>Oxford. (1996). Oxford Dictionary of Biology. Oxford University Press.</ref> The term is also used and defined somewhat differently in [[fisheries]], [[hydrology]] and [[resource management]].


==Tragedy of the commons==
==Tragedy of the commons==

Revision as of 16:30, 3 February 2010

File:Atlantic-Cod-Stocks.jpg
Atlantic cod stocks were severely overexploited in the 1970s and 1980s, leading to their abrupt collapse in 1992

Overexploitation is a term used by a number of disciplines. In ecology, it describes one of the five main #Other_threats_to_biodiversity threatening global biodiversity.[1]. Ecologists use the term to describe populations that are harvested at a rate that is unsustainable, given their natural rates of mortality and capacities for reproduction. This can result in harvesting to the point of diminishing returns and 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.[2] The term is also used and defined somewhat differently in fisheries, hydrology and resource management.

Tragedy of the commons

Overexploitation can occur naturally, as in the overgrazing caused by native fauna shown in the upper right.

The tragedy of the commons refers to a dilemma described in an article by that name written by Garrett Hardin and first published in the journal Science in 1968.[3] The article describes a situation where multiple individuals, acting independently and rationally consulting their own self-interest, will ultimately overexploit a shared limited resource, even though it is clear that it is not in anyone's long-term interest for this to happen.[4]

Central to Hardin's essay is an example, first sketched in an 1833 pamphlet by William Forster Lloyd, of a hypothetical and simplified situation based on medieval land tenure in Europe, of herders sharing a common parcel of land, on which they are each entitled to let their cows graze. In Hardin's example, it is in each herder's interest to put each succeeding cow he acquires onto the land, even if the carrying capacity of the common is exceeded and it is temporarily or permanently damaged for all as a result. The herder receives all of the benefits from an additional cow, while the damage to the common is shared by the entire group. If all herders make this individually rational economic decision, the common will be overexploited or even destroyed to the detriment of all. However, since all herders reach the same rational conclusion, overgrazing with immediate losses occurs, and the pasture may be degraded to the point where it has very little valuee.

In the course of his essay, Hardin develops 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".[5] A major theme running throughout the essay is the growth of human populations, with the Earth's finite resources being the3 general common.

The tragedy of the commons is a useful parable for understanding how we overexploit to the brink of environmental catastrophe. This dangerous situation is created not by malicious outside forces but by the apparently rational and innocent behaviours of many individuals acting independently.

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. (Hardin, 1968)[3]

The tragedy of the commons has intellectual roots that trace back to Aristotle, who noted that "what is common to the greatest number has the least care bestowed upon it",[6] as well as to Hobbes and his leviathan.[7]

Ecology

In conservation biology the terms ‘overharvesting’ and ‘overexploitation’ are interchangeable. Essentially they mean populations are harvested at a rate that is unsustainable, given their natural rates of mortality and capacities for reproduction.[8] Overexploitation resulted in the gradual emergence of the concept of sustainable development and sustainability, which has built on a range of other concepts, e.g. sustainable yield[9], eco-development[10][11] and deep ecology.[12][13]

An Ecological definition of ‘overexploitation’ differs in that Ecology deals with living organisms and the relationships and interactions between those organisms. In the strictest of terms an ecologists would not deal with overexploitation of water resources, unless in the context of its affect on species other than humans. When talking about sustainability we factor in a human element because it is usually an economic activity that is the cause and needs addressing. This is when overexploitation deviates from an ecological perspective. An ecologist, for example, would not be overly interested in the depletion of oil reserves, but would be interested in the effects of burning that oil has on ecosystems. It is also important to note that in Ecology, overexploitation is not limited to human activities. Introduced predators and herbivores, for example, can overexploit the native flora and fauna.


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 have needed to survive. Human populations, historically, were small, and methods of collection limited to small quantities. With an exponential increase in human population, expanding markets and increasing demand, combined with improved access and techniques for capture, are causing the exploitation of many species beyond sustainable levels.[14] In practical terms, if continued, it reduces valuable resources to such low levels that their exploitation is no longer sustainable and can lead to the extinction of a species, in addition to having dramatic, unforeseen effects, on the eco-system[15]. Overexploitation often occurs rapidly as markets open, utilising previously untapped resources, or locally used species.

Overexploitation threatens one-third of endangered vertebrates, as well as other groups. Excluding edible fish, the illegal trade in wildlife is valued at $10 billion per year. Industries responsible for this include the trade in bushmeat, the trade in Chinese medicine, and the fur trade.[16] The Convention for International Trade in Endangered Species of Wild Fauna and Flora, or CITES 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.

Where there is substantial human migration, civil unrest, or war, controls may no longer exist. With civil unrest, for example in the Congo and Rwanda, firearms have become common place and the breakdown of food distribution networks in such countries leaves the resources of the natural environment vulnerable to whoever can exploit them[17] Animals are even killed as target practice sometimes, or simply to spite the government. Populations of large primates, such as gorillas and chimpanzees, ungulates and other mammals, maybe reduced by 80% or more by hunting and certain species maybe eliminated all together[18] This decline has been called the “bushmeat crisis”.


Overexploitation is not a new phenomenon. It has been observed for millennia. For example, ceremonial clocks worn by the Hawaiian kings were made from the mamo bird (Drepanis sp.); a single clock used the feathers of 70,000 birds of this now-extinct species. The dodo (Raphus cucullatus), a flightless bird from Mauritius, is another well know example of overexploitation. As with many island species, it had predatory naivety, allowing humans to approach it and kill it with ease[19].

In fact there is a whole history of overexploitation. The "Blitzkrieg" or "Overkill" hypothesis (New World Pleistocene extinctions) explains why the megafaunal extinctions occurred within a relatively short period of time. This can be traced with human migration. The most convincing evidence of his theory is that 80% of the North American large mammal species disappeared within 1000 years of the arrival of humans on the Western Hemisphere continents.[20] Again in New Zealand, the Moa were 10 species of giant birds, which are believed to have been hunted to extinction by the Māori by 1500AD. A second wave of extinctions occurs later with European settlement.


Today, overexploitation and misuse of natural resources is an ever present threat for species richness. This is more prevalent when looking as islands and the species that inhabit them, as islands can be viewed as the world in miniature. Island endemic populations are more prone to [extinction]] from overexploitation, as they often exist at low densities with reduced reproductive rates[21]. A good example of this are island snails, such as the Hawaiian Achatinella and the French Polynesian Partula. Achatinelline snails have 15 species listed as extinct and 24 critically endangered[22]whilst 60 species of partulidae are considered extinct with 14 listed as critically endangered.[23]

The WCMC have attributed over-collecting and very low lifetime fecundity for the extreme vulnerability exhibited among these species.[24]

Equally, the humble hedgehog (Erinaceus europaeus), when introduced to the Scottish island of Uist, the population greatly expanded and taken to consuming and overexploiting shorebird eggs, with drastic consequences to their breeding success. Twelve species of avifauna are affected, with some species numbers being reduced by 39%[25].

Fisheries

The northern bluefin tuna is currently seriously overexploited. Scientists say 7,500 tons annually is the sustainable limit, yet the fishing industry continue to harvest 60,000 tons.

In wild fisheries, overexploitation 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".[26] However, overexploitation can be sustainable.

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. If the harvest rate is increased, then the stock biomass will further decrease. 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.[27]

It is possible to fish the stock down further, to say 15% of the pre-harvest biomass, and then adjust the harvest rate so the biomass remains at that level. In this case, the fishery is sustainable, but is now overexploited, because the stock has been run down to the point where the sustainable yield is less than it could be.

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.[28] These depleted fisheries can often recover if fishing pressure is reduced until the stock biomass returns to the optimal biomass. At this point, harvesting can be resumed near the maximum sustainable yield.[29]

Forest resources

Water resources

Other resources

Overexploited species

It is estimated that a quarter of the endangered vertebrates in the United States of America and half of the endangered mammals is attributed to overexploitation[30][31]

Overall, 50 bird species that have become extinct since 1500 (approximately 40% of the total) have been subject to overexploitation,[32] including:

  • Great Auk- The penguin of the north, hunted for its feathers, meat, fat and oil.
  • Carolina Parakeet - The only parot species native to the eastern United States, was hunted for crop protection.

Other species affected by overexploitation include:

  • The international trade in fur: chinchilla (Chinchilla spp.), vicuña (Vicugna vicugna), giant otter (Pteronura brasiliensis) and numerous cat species.
  • Insect collectors: Butterflies
  • Horticulturists: New Zealand mistletoe (Trilepidia adamsii) orchids, cacti and many other plant species.
  • Shell collectors: Marine Mollucks
  • Aquarium hobbyists: Tropical fish
  • Chinese medicine: Bears, tigers
  • Novelty pets: Snakes, parrots and primates

Reversal

During the 18th and 19th centuries, the sea otter was heavily exploited, prized universally for its exceptionally warm and tremendously valuable pelt, which could fetch a price of between $2000 – $2500 US dollars[33]. Fur traders hunted the sea otters so intensively for their skins, that they were exterminated over huge areas. Their disappearance led to dramatic changes in the environment. Sea urchins, with no otters to control their numbers, increased explosively. Urchins eat kelp and they began to destroy the underwater forests. As the kelp disappeared, so did the other animals that relied upon it. Soon all that was left was a bare seabed carpeted with urchins.

One small group of 32 individuals survived in a remote cove and hunting the sea otters was eventually banned. Under heavy protection, those 32 multiplied to 2,377 repopulating the depleted areas, which eventually made a full recovery. In addition, 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.

Other threats to biodiversity

See also

Notes

  1. ^ Wilcove, D.S., Rothstein, D., Dubow, J. Phillips, A. and Losos, E. (1998). Quantifying Threats to Imperilled Species in the United States. BioScience. 48, 607-615.
  2. ^ Oxford. (1996). Oxford Dictionary of Biology. Oxford University Press.
  3. ^ a b Garrett Hardin, "The Tragedy of the Commons", Science, Vol. 162, No. 3859 (December 13, 1968), pp. 1243-1248. Also available here and here.
  4. ^ Alderman, Naomi (Tuesday 7 April 2009 15.32 BST). "Encarta's failure is no tragedy:Wikipedia has succeeded where Microsoft's Encarta failed, and seems to be a reversal of the 'tragedy of the commons'". News:Technology:Wikipedia. guardian.co.uk. Retrieved 2009-04-07. {{cite news}}: Check date values in: |date= (help)
  5. ^ Samuel Bowles: Microeconomics: Behavior, Institutions, and Evolution, Princeton University Press, pp. 27–29 (2004) ISBN 0691091633
  6. ^ Ostrom, E (1992) "The rudiments of a theory of the origins, survival, and performance of common-property institutions" In D. W. Bromley (Ed.) Making the Commons Work: Theory, Practice and Policy. San Francisco, ICS Press.
  7. ^ Feeny, D. et al., (1990) "The tragedy of the commons – 22 years later", Human Ecology, 18: 1-19.
  8. ^ Townsend, C.R. Begon, M. and Harper, J.L. (2003). Essentials of Ecology, 2nd edition. Page 474. Blackwell Publishing. Oxford.
  9. ^ Larkin, P.A. 1977. An epitaph for the concept of maximum sustained yield. Transactions of the American Fisheries Society 106: 1-11
  10. ^ Lubchenco, J. 1991. The Sustainable Biosphere Initative: An ecological research agenda. Ecology 72: 371-412.
  11. ^ Lee, K.N. 2001. Sustainability, concept and practice of. In S.A. Levin (ed.), Encyclopedia of Biodiversity, 5: 553-568. Academic Press, San Diego, CA.
  12. ^ Naess, A. 1986. Intrinsic value: Will the defenders of nature please rise? In M.E. Soulé (ed.), Conservation Biology: The Science of Scarcity and Diversity, pp.153-181. Sinauer Associates, Sunderland, MA.
  13. ^ Sessions, G. (ed.) 1995. Deep Ecology for the 21st Century: Readings on the Philosophy and Practice of the New Environmentalism. Shambala Books, Boston.
  14. ^ Redford 1992, Fitzgibon etal 1995, Cuarón 2001
  15. ^ Frankham, R., Ballou, J.D. and Briscoe, D.A. (2002). Introduction to Conservation Genetics. Cambridge University Press.
  16. ^ Hemley 1994
  17. ^ Jones, R.F. 1990. Farewell to Africa. Audubon 92: 1547-1551.
  18. ^ Wilkie, D.S. and J.F. 1999. Bushmeat hunting in the Congo Basin: An assessment of impacts and options for migration. Biodiversity and Conservation 8: 927-955.
  19. ^ Jonathan Fryer (2002-09-14). "Bringing the dodo back to life". BBC News. Retrieved 2006-09-07)
  20. ^ Paul S. Martin
  21. ^ Dowding, J.E. and Murphy, E.C. (2001). The Impact of Predation be Introduced Mammals on Endemic Shorebirds in New Zealand: A Conservation Perspective. Biological Conservation. 99: 47-64.
  22. ^ IUCN Red List. (2003b). http://www.iucnredlist.org/search/search.php?freetext=Achatinella &modifier=phrase&criteria=wholedb&taxa_species=1&redlistCategory%5B%5D=allex&country%5B%5D=all&aquatic%5B%5D=all&regions%5B%5D=all&habitats%5B%5D=all&threats%5B%5D=all&Submit.x=97&Submit.y=2.
  23. ^ IUCN Red List. (2003c). http://www.iucnredlist.org/search/search.php?freetext=Partula& modifier=phrase&criteria=wholedb&taxa_species=1&redlistCategory%5B%5D=allex&country%5B%5D=all&aquatic%5B%5D=all&regions%5B%5D=all&habitats%5B%5D=all&threats%5B%5D=all&Submit.x=95&Submit.y=4. Accessed 9th December 2003.
  24. ^ WCMC. (1992). McComb, J., Groombridge, B., Byford, E., Allan, C., Howland, J., Magin, C., Smith, H., Greenwood, V. and Simpson, L. (1992). World Conservation Monitoring Centre. Chapman and Hall.
  25. ^ Jackson, D.B., Fuller, R.J., Campbell, S.T. (2003). Long-term Population Changes Among Breeding Shorebirds in the Outer Hebrides, Scotland, In Relation to Introduced Hedgehogs (Erinaceus europaeus). Biological Conservation.
  26. ^ NOAA: FishWatch glossary Retrieved 2 February 2010.
  27. ^ Bolden, E.G., Robinson, W.L. (1999), Wildlife ecology and management 4th ed. Prentice-Hall, Inc. Upper Saddle River, NJ. ISBN 0138404224
  28. ^ Grafton RQ, Kompas T and Hilborn RW (2007) "Economics of Overexploitation Revisited" Science, 318 (5856): 1601.
  29. ^ Rosenberg AA (2003) "Managing to the margins: the overexploitation of fisheries" Frontiers in Ecology and the Environment, 1(2): 102-106.
  30. ^ Wilcove, D.S., D.Rothstein, J. Dubow, A. Phillips, and E. Losos. 1998. Quantifying threats to imperialised species. BioScience 48: 607-615.
  31. ^ Primack, R.B. (2002). Essentials of Conservation Biology, 3rd edition. Sinauer Associates Inc.
  32. ^ The LUCN Red List of Threatened Species (2009).
  33. ^ Krebs, C.J. (2001). Ecology (5th ed.). Benjamin Cummings.

References


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