Essay Elemental Conservation Units: Communicating Extinction Risk without Dictating Targets for Protection CHRIS C. WOOD ∗ AND MART R. GROSS† ∗ Conservation Biology Section, Fisheries and Oceans Canada, Pacific Biological Station, Nanaimo, BC V9T 6N7, Canada, email woodc@dfo-mpo.gc.ca †Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario M5S 3B2, Canada Abstract: Conservation biologists mostly agree on the need to identify and protect biodiversity below the species level but have not yet resolved the best approach. We addressed 2 issues relevant to this debate. First, we distinguished between the abstract goal of preserving the maximum amount of unique biodiversity and the pragmatic goal of minimizing the loss of ecological goods and services given that further loss of biodiversity seems inevitable. Second, we distinguished between the scientific task of assessing extinction risk and the normative task of choosing targets for protection. We propose that scientific advice on extinction risk be given at the smallest meaningful scale: the elemental conservation unit (ECU). An ECU is a demographically isolated population whose probability of extinction over the time scale of interest (say 100 years) is not substantially affected by natural immigration from other populations. Within this time frame, the loss of an ECU would be irreversible without human intervention. Society’s decision to protect an ECU ought to reflect human values that have social, economic, and political dimensions. Scientists can best inform this decision by providing advice about the probability that an ECU will be lost and the ecological and evolutionary consequences of that loss in a form that can be integrated into landscape planning. The ECU approach provides maximum flexibility to decision makers and ensures that the scientific task of assessing extinction risk informs, but remains distinct from, the normative social challenge of setting conservation targets. Keywords: conservation policy, designatable unit, distinct population segment, ecological exchangeability, elemental conservation unit, endangered species, evolutionarily significant unit, replaceability Unidades de Conservaci´ on Elementales: Comunicando el Riesgo de Extinci´ on sin Imponer Objetivos para Pro- tecci´ on Resumen: Los bi´ ologos de la conservaci´ on en general est´ an de acuerdo con la necesidad de identificar y proteger la biodiversidad por debajo del nivel de especies pero aun no han resuelto cu´ al es el mejor m´etodo. Abordamos dos temas relevantes de este debate. Primero, distinguimos entre la meta abstracta de preservar la m´ axima cantidad de biodiversidad ´ unica y la meta pragm´ atica de minimizarla p´erdida de bienes y servicios ecol´ ogicos considerando que la p´erdida de biodiversidad parece inevitable. Segundo, distinguimos entre la tarea cient´ ıfica de evaluar el riesgo de extinci´ on y la tarea normativa de seleccionar objetivos para protecci´ on. Proponemos que se proporcione asesor´ ıa cient´ ıfica sobre el riesgo de extinci´ on en la escala significativa m´ as peque˜ na: la unidad de conservaci´ on elemental (UCE). Una UCE es una poblaci´ on demogr´ aficamente aislada cuya probabilidad de extinci´ on en una escala de tiempo de inter´es (por decir, 100 a˜ nos) no es afectada sustancialmente por la inmigraci´ on natural desde otras poblaciones. Dentro de este marco de tiempo, la p´erdida de una UCE pudiera ser irreversible sin la intervenci´ on humana. La decisi´ on de la sociedad de proteger una UCE debiera reflejar valores humanos que tienen dimensiones sociales, econ´ omicas y pol´ ıticas. Los cient´ ıficos pueden informar sobre esta decisi´ on proporcionando consejo sobre la probabilidad de que una UCE se pierda y de las consecuencias ecol´ ogicas y evolutivas de esa p´erdida de manera que puedan ser Paper submitted October 31, 2006; revised manuscript accepted August 14, 2007. 36 Conservation Biology, Volume 22, No. 1, 36–47 Journal compilation C 2008 Society for Conservation Biology No claim to original Canadian government works DOI: 10.1111/j.1523-1739.2007.00856.x