Overexploitation
From Wikipedia
Overexploitation is a term used in fisheries, where it means that a fish stock has been fished down to the point of diminishing returns. It is also a term used in ecology, as one of the five main activities threatening global biodiversity.[citation needed] In conservation ecology the term means the taking of biological resources, or organisms, in larger numbers than their populations can withstand.[1]
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Use in fisheries
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".[2] 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.[3]
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.[4] 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.[5]
Use in 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.[6] 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[7], eco-development[8][9]and deep ecology.[10][11]
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.[12]
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[13]. Overexploitation often occurs rapidly as markets open, utilising previously untapped resources, or locally used species. The illegal trade in wildlife is valued at $10 billion per year, not including edible fish[14]
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[15][16]
Examples of 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
Other examples
Overall, 50 bird species that have become extinct since 1500 (approximately 40% of the total) have been subject to overexploitation,[17] 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.
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[18]. 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
- ↑ Oxford. (1996). Oxford Dictionary of Biology. Oxford University Press.
- ↑ NOAA: FishWatch glossary Retrieved 2 February 2010.
- ↑ Bolden, E.G., Robinson, W.L. (1999), Wildlife ecology and management 4th ed. Prentice-Hall, Inc. Upper Saddle River, NJ. ISBN 0138404224
- ↑ Grafton RQ, Kompas T and Hilborn RW (2007) "Economics of Overexploitation Revisited" Science, 318 (5856): 1601.
- ↑ Rosenberg AA (2003) "Managing to the margins: the overexploitation of fisheries" Frontiers in Ecology and the Environment, 1(2): 102-106.
- ↑ Townsend, C.R. Begon, M. and Harper, J.L. (2003). Essentials of Ecology, 2nd edition. Page 474. Blackwell Publishing. Oxford.
- ↑ Larkin, P.A. 1977. An epitaph for the concept of maximum sustained yield. Transactions of the American Fisheries Society 106: 1-11
- ↑ Lubchenco, J. 1991. The Sustainable Biosphere Initative: An ecological research agenda. Ecology 72: 371-412.
- ↑ 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.
- ↑ 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.
- ↑ Sessions, G. (ed.) 1995. Deep Ecology for the 21st Century: Readings on the Philosophy and Practice of the New Environmentalism. Shambala Books, Boston.
- ↑ Redford 1992, Fitzgibon etal 1995, Cuarón 2001
- ↑ Frankham, R., Ballou, J.D. and Briscoe, D.A. (2002). Introduction to Conservation Genetics. Cambridge University Press.
- ↑ Hemley 1994
- ↑ Wilcove, D.S., D.Rothstein, J. Dubow, A. Phillips, and E. Losos. 1998. Quantifying threats to imperialised species. BioScience 48: 607-615.
- ↑ Primack, R.B. (2002). Essentials of Conservation Biology, 3rd edition. Sinauer Associates Inc.
- ↑ The LUCN Red List of Threatened Species (2009).
- ↑ Krebs, C.J. (2001). Ecology (5th ed.). Benjamin Cummings.
References
- FAO (2005) Overcoming factors of unsustainability and overexploitation in fisheries Fisheries report 782, Rome. ISBN 9789251054499
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