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. 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 Abundance ecology Conservation biology Local extinction Overfishing Overexploitation Small population size Threatened species 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 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 Environmental terminology Cat ... more details
Refimprove date December 2008 Conservation status Image Status iucn3.1 threatened.svg thumb left The 3 Categories of the threatened species IUCN Red List . Threatened species are any species including animal s, plant s, fungus fungi , etc. which are vulnerable to endangerment in the near future. The World Conservation Union IUCN is the foremost authority on threatened species, and treats threatened species not as a single category, but as a group of three categories, depending on the degree to which they are threatened Vulnerable species Endangered species Critically endangered species Species that are threatened are sometimes characterised by the population dynamics measure of critical depensation , a mathematical measure of biomass related to population growth rate . This quantitative metric is one method of evaluating the degree of endangerment. Less than threatened categories are Near Threatened , Least Concern , and the no longer assigned category of Conservation Dependent . Species which have not been evaluated NE , or do not have sufficient data Data Deficient also are not considered threatened by the IUCN. Although threatened and vulnerable may be used interchangeably when discussing IUCN categories, the term threatened is generally used to refer to the three categories critically endangered, endangered and vulnerable , while vulnerable is used to refer to the least at risk of those three categories. They may be used interchangeably in most contexts however, as all vulnerable species are threatened species vulnerable is a category of threatened species and, as the more at risk categories of threatened species namely endangered and critically endangered must, by definition, also qualify as vulnerable species, all threatened species may also be considered vulnerable. Threatened species are also referred to as a red listed species, as they are listed in the IUCN Red List IUCN Red List of Threatened Species . Subspecies , populations and stocks may also be class ... more details
pp vandalism small yes File Surexploitation morue surp cheEn.jpg thumb 300px right Atlantic cod stocks were severely overfished in the 1970s and 1980s, leading to their abrupt collapse in 1992 Overfishing occurs when fishing activities reduce fish stock s below an acceptable level. This can occur in any body of water from a pond to the oceans. Ultimately overfishing can lead to resource depletion in cases of subsidised fishing, low biological growth rates and critical low biomass ecology biomass levels e.g. by depensation critical depensation growth properties . For example, overfishing of sharks has led to the upset of entire marine ecosystems. ref http news.nationalgeographic.com news 2007 03 070329 sharks shellfish.html Shark Declines Threaten Shellfish Stocks, Study Says , National Geographic News. 29 March 2007. ref The ability of a fishery to recover after overfishing depends on whether the ecosystem 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 other than those that had been present before. 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. Types There are three recognized types of overfishing growth overfishing, recruit overfishing and ecosystem overfishing. Growth overfishing is when fish are harvested at an average size that is smaller than the size that would produce the Maximum sustainable yield maximum yield per recruit. This makes the total yield less than it would be if the fish were allowed to grow to a reasonable size. It can be countered by reducing fishing mortality to lower levels and increasing the average size of the fish harvested to a length that will allow maximum yield per recruit. ref name Pauly 1983 Pauly 1983 ref Recruit overfishing is when the mature adult spawning Biomass ecology biomass population is ... more details
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