When is an “Extinct” Species Really Extinct? Gauging the Search Efforts for Hawaiian Forest Birds and the Ivory-Billed Woodpecker

Rare species, particularly those in inaccessible habitat, can go years without being observed. If we are to allocate conservation resources appropriately to conserving such species, it is important to be able to distinguish "rare" from "extinct. " Criteria for designating extinction, however, tend to be arbitrary or vaguely defined. This designation should not be made unless the search effort has been sufficient to yield a high degree of confidence that the species is in fact absent. We develop models to assess the probability of extinction and the search effort necessary to detect an individual in a small population. We apply these models to searches for nine potentially extinct Hawaiian forest birds and for the Ivory-billed Woodpecker ( Campephilus principalis) in intensively searched areas in Arkansas. The Hawaiian forest bird survey was extensive, providing excellent information on population sizes and habitat associations of species encountered during the survey. Nonetheless, we conclude that the survey effort was not sufficient to conclude extinction ( p > 0.90) for populations of 10 or fewer individuals for those species that were not encountered during surveys. In contrast, our analysis for Ivory-billed Woodpeckers suggests that, unless there were actually two or fewer birds present, the search effort was sufficient to conclude ( p > 0.95) that Ivory-billed woodpeckers were not present in the intensively searched area. If one assumes distributions other than uniform, there is a greater chance that Ivory-billed Woodpeckers may persist in the intensively searched areas. Conclusions regarding occupancy of suitable habitat throughout the rest of the former range will require similarly intensive survey efforts. The degree of confidence in the absence of the Ivory-billed Woodpecker depended in part on our assumptions about the distribution of birds in the search area. For species with limited detection distance and small populations, a massive search effort may be required to conclude with confidence that a species is unlikely to be extant. RESUME. Les observations d'especes rares peuvent etre separees par plusieurs annees, particulierement lorsque leur habitat est inaccessible. Si des ressources sont consacrees a la conservation de ces especes, il est important de pouvoir etablir des criteres propres aux especes « rares » et « disparues ». Par contre, les criteres utilises pour determiner l'extinction tendent a etre arbitraires et plutot vagues. Cette designation ne devrait jamais etre appliquee, a moins que les efforts de recherche aient ete suffisants pour affirmer avec confiance que l'espece est effectivement absente. Nous avons developpe des modeles afin d'estimer la probabilite d'extinction ainsi que l'effort necessaire pour la detection d'un individu provenant d'une petite population. Nous avons applique ces modeles a neuf especes d'oiseaux forestiers potentiellement disparus de Hawai ainsi qu'au Pic a bec ivoire ( Campephilus principalis) dans des sites de recherche intensive de l'Arkansas. L'inventaire des oiseaux forestiers d'Hawai etait extensif, ce qui nous a procure de l'excellente information sur l'effectif des populations ainsi que l'association aux habitats des especes rencontrees durant l'inventaire. Neanmoins, nous estimons que l'effort d'inventaire etait insuffisant pour conclure a l'extinction (p > 0.90) chez les populations comprenant 10 individus ou moins parmi les especes non detectees durant l'inventaire. A l'oppose, notre analyse pour le Pic a bec ivoire suggere qu'a moins que la population ne compte que deux individus ou moins, l'effort de recherche etait suffisant ( p > 0.95) pour conclure a l'absence de l'espece dans le site de recherche intensive. Lorsqu'on suppose que la distribution de l'espece n'est pas uniforme, la probabilite de persistance du Pic a bec ivoire est plus elevee dans les sites de recherche intensive. Les conclusions sur l'occupation de l'habitat favorable dans le reste de l'aire de repartition exigeront des efforts d'inventaire d'intensite comparable. Le degre de confiance quant a l'absence du Pic a bec ivoire dependait en partie de nos suppositions par rapport a la