Comparative analysis of an adaptive radiation: Origins of the variation in extinction risk among Hawaiian honeycreepers

The endemic Hawaiian honeycreepers (Drepanididae) are known for their spectacular phenotypic diversity and their history of decline and extinction. Identifying the factors that explain why some species are more threatened than others is an important step in increasing the efficiency of conservation efforts. To investigate the origins of the variation in threat status in honeycreepers, we searched for significant associations between 3 categories of threat measure at various spatial and temporal scales, and 26 potential predictors of extinction relevant to characteristics of the life history, ecology, and geographic range of honeycreepers. Variation in the threat status of honeycreepers is linked with both intrinsic and extrinsic factors. Some of these factors are behind the global processes of honeycreeper extinction, while other predictors are specifically linked to a few indices of threat only. Extinction risk in Drepanididae is most significantly associated with geographic ranges that are composed of few patches and restricted to few islands, followed by restricted diet breadth, and small clutch size. The loss of significance of body size when extinct species are excluded suggests that honeycreepers may have experienced an extinction filter. General predictors of honeycreeper extinction correlate with indices of threat regardless of the temporal and spatial scales of these indices, while predictors exclusive to few indices tend to be scale-dependent and more likely to have two correlation directions. These results highlight the importance of incorporating threat measures at various temporal and spatial scales, at least in studies that search for predictors of extinction at fine taxonomic scales.