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Quantification of Extinction Risk: IUCN's System for Classifying Threatened Species

Mace, Georgina M., Collar, Nigel J., Gaston, Kevin J., Hilton-Taylor, Craig, Akcakaya, H. Resit, Leader-Williams, Nigel, Milner-Gulland, Eleanor J., Stuart, Simon N. (2008) Quantification of Extinction Risk: IUCN's System for Classifying Threatened Species. Conservation Biology, 22 (6). pp. 1424-1442. ISSN 0888-8892. (doi:10.1111/j.1523-1739.2008.01044.x) (Access to this publication is currently restricted. You may be able to access a copy if URLs are provided) (KAR id:15506)

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The International Union for Conservation of Nature (IUCN) Red List of Threatened Species was increasingly used during the 1980s to assess the conservation status of species for policy and planning purposes. This use stimulated the development of a new set of quantitative criteria for listing species in the categories of threat: critically endangered, endangered, and vulnerable. These criteria, which were intended to be applicable to all species except microorganisms, were part of a broader system for classifying threatened species and were fully implemented by IUCN in 2000. The system and the criteria have been widely used by conservation practitioners and scientists and now underpin one indicator being used to assess the Convention on Biological Diversity 2010 biodiversity target. We describe the process and the technical background to the IUCN Red List system. The criteria refer to fundamental biological processes underlying population decline and extinction. But given major differences between species, the threatening processes affecting them, and the paucity of knowledge relating to most species, the IUCN system had to be both broad and flexible to be applicable to the majority of described species. The system was designed to measure the symptoms of extinction risk, and uses 5 independent criteria relating to aspects of population loss and decline of range size. A species is assigned to a threat category if it meets the quantitative threshold for at least one criterion. The criteria and the accompanying rules and guidelines used by IUCN are intended to increase the consistency, transparency, and validity of its categorization system, but it necessitates some compromises that affect the applicability of the system and the species lists that result. In particular, choices were made over the assessment of uncertainty, poorly known species, depleted species, population decline, restricted ranges, and rarity; all of these affect the way red lists should be viewed and used. Processes related to priority setting and the development of national red lists need to take account of some assumptions in the formulation of the criteria.La Lista Roja de Especies Amenazadas de la UICN (Union Internacional para la Conservacion de la Naturaleza) fue muy utilizada durante la decada de l980 para evaluar el estatus de conservacion de especies para fines politicos y de planificacion. Este uso estimulo el desarrollo de un conjunto nuevo de criterios cuantitativos para enlistar especies en las categorias de amenaza: en peligro critico, en peligro y vulnerable. Estos criterios, que se pretendia fueran aplicables a todas las especies excepto microorganismos, eran parte de un sistema general para clasificar especies amenazadas y fueron implementadas completamente por la UICN en 2000. El sistema y los criterios han sido ampliamente utilizados por practicantes y cientificos de la conservacion y actualmente apuntalan un indicador utilizado para evaluar el objetivo al 2010 de la Convencion de Diversidad Biologica. Describimos el proceso y el respaldo tecnico del sistema de la Lista Roja de la IUCN. Los criterios se refieren a los procesos biologicos fundamentales que subyacen en la declinacion y extincion de una poblacion. Pero, debido a diferencias mayores entre especies, los procesos de amenaza que los afectan y la escasez de conocimiento sobre la mayoria de las especies, el sistema de la UICN tenia que ser amplio y flexible para ser aplicable a la mayoria de las especies descritas. El sistema fue disenado para medir los sintomas del riesgo de extincion, y utiiza cinco criterios independientes que relacionan aspectos de la perdida poblacional y la declinacion del rango de distribucion. Una especie es asignada a una categoria de amenaza si cumple el umbral cuantitativo por lo menos para un criterio. Los criterios, las reglas acompanantes y las directrices utilizadas por la UICN tienen la intencion de incrementar la consistencia, transparencia y validez de su sistema de clasificacion, pero requiere algunos compromisos que afectan la aplicabilidad del sistema y las listas de especies que resultan. En particular, se hicieron selecciones por encima de la evaluacion de incertidumbre, especies poco conocidas, especies disminuidas, declinacion poblacional, rangos restringidos y rareza; todas estas afectan la forma en que las listas rojas deberian ser vistas y usadas. Los procesos relacionados con la definicion de prioridades y el desarrollo de las listas rojas nacionales necesitan considerar algunos de los supuestos en la formulacion de los criterios.

Item Type: Article
DOI/Identification number: 10.1111/j.1523-1739.2008.01044.x
Additional information: Review Article
Uncontrolled keywords: conservation priority setting; extinction risk; IUCN Red List; threatened species
Subjects: G Geography. Anthropology. Recreation > GE Environmental Sciences
Divisions: Divisions > Division of Human and Social Sciences > School of Anthropology and Conservation > DICE (Durrell Institute of Conservation and Ecology)
Funders: Natural Environment Research Council (
Pew Charitable Trusts (
Depositing User: Maureen Cook
Date Deposited: 14 Apr 2009 11:43 UTC
Last Modified: 12 Jul 2022 10:39 UTC
Resource URI: (The current URI for this page, for reference purposes)

University of Kent Author Information

Leader-Williams, Nigel.

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