In population dynamics , depensation is the effect on a population or Fish stock stock whereby, due to certain causes, a decrease in the breeding population mature individuals leads to reduced survival and production of eggs or offspring. The cause may be either predation levels rising per offspring given the same level of overall predator pressure , or the allee effect , which is the reduced likelihood of finding a mate. citation needed date September 2011 Although depensation is often considered in relation to the population being harvested especially fish , the actual level of harvesting, by definition, is not part of depensation. Citation needed date February 2007 Critical depensation When the level of depensation is high enough that the population is no longer able to sustain itself, it is said to be a critical depensation . This occurs when the population size has a tendency to decline when the population drops below a certain level known as the Critical depensation level . Ultimately this may lead to resource depletion , the population or fishery s collapse, or even local extinction . The phenomenon of critical depensation may be modelled or defined by a negative second order derivative of population growth rate with respect of population Biomass ecology biomass , which describes a situation where a decline in population biomass is not compensated by a corresponding increase in marginal growth per unit of biomass. See also div col 2 Abundance ecology Conservation biology Local extinction Overfishing Overexploitation Small population size Threatened species div col end External links http cat.inist.fr ?aModele afficheN&cpsidt 16127115 Optimal harvesting in the presence of critical depensation http www.oceansatlas.org world fisheries and aquaculture html glos terms 796.htm On line source of definition Category Population Category Extinction Category Ecological processes biology stub modelling ecosystems expanded other ... more details
A rare species is a group of organisms that are very uncommon or scarce. This designation may be applied to either a plant or animal taxon, and may be distinct from the term endangered species endangered or threatened species but not extinct . Designation of a rare species may be made by an official body such as a national government, state or province. However, the term is used more commonly without reference to a specific criteria. The IUCN does not normally make such designations but may use the term in its scientific discussions. ref http www.iucnredlist.org info categories criteria2001 Categories & Criteria version 3.1 IUCN, 2001. ref The concept of rarity is established from having a very small number of organisms worldwide, usually a number less than 10,000 however, the concept is also influenced by having a very narrow Endemic ecology endemic range and or Habitat fragmentation fragmented habitat . ref R.MacNally and G.W.Brown, Reptile s and Habitat Fragmentation in the Box ironbush Forests of Central Victoria, Australia Predicting Compositional Change and Faunal Nested ness , Oecologia 128 116 125 2001 ref A species may be endangered or vulnerable, but not considered rare if, for example, it has a large, dispersed population but its numbers are declining rapidly or predicted to do so. Rare species are generally considered threatened simply because the inability of small population size s to recover from stochastic events, and the potential for a rapid decline in population. See Also Biodiversity Action Plan Critical depensation Endangered Species Recovery Plan Common species Abundance ecology References Reflist Further reading Gorbunov, Y.N., Dzybov, D.S., Kuzmin, Z.E. and Smirnov, I.A. 2008. http www.bgci.org files Worldwide Publications reintroduction manual engl.pdf Methodological recommendations for botanic gardens on the reintroduction of rare and threatened plants Botanic Gardens Conservation International BGCI Category Conservation Category Environmen ... more details
pp vandalism small yes Image Chilean purse seine.jpg thumb 300px center 400 tons of Trachurus murphyi jack mackerel caught by a Chilean purse seiner center Image Overfished US stocks 2010.png thumb 300 px right center Overfished US stocks 2010 center Overfishing is the act whereby fish stock s are depleted to unacceptable levels, regardless of water body size. Resource depletion , low biological growth rates, and critical low biomass ecology biomass levels e.g. by depensation critical depensation growth properties result from overfishing. For example, overfishing of sharks has led to the upset of entire marine ecosystems. ref cite web url http news.nationalgeographic.com news 2007 03 070329 sharks shellfish.html title Shark Declines Threaten Shellfish Stocks, Study Says work National Geographic News first Helen last Scales date 29 March 2007 accessdate 2012 05 01 ref The ability of a fishery to recover from overfishing depends on whether the ecosystem s conditions are suitable for the recovery. Dramatic changes in species composition can result in an ecosystem shift, where other equilibrium energy flows involve species compositions different from those that had been present before the depletion of the original fish stock. For example, once trout have been overfished, carp might take over in a way that makes it impossible for the trout to re establish a breeding population. Instances Image Fishing down the food web.jpg thumb 350px Fishing down the food web Examples of overfishing exist in areas such as the Fishing in the North Sea North Sea of Europe , the Grand Banks of Newfoundland Grand Banks of North America and the East China Sea of Asia. ref cite web url http www.gov.cn english 2006 08 16 content 363493.htm title Pollution, overfishing destroy East China Sea fishery editor Lu Hui work Xinhua on GOV.cn date 16 August 2006 accessdate 2012 05 01 ref In these locations, overfishing has not only proved disastrous to fish stocks but also to the fishing communities re ... more details
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