Broadening the extinction debate: Population deletions and additions in California and Western Australia

Current discussions of biodiversity frequently center on the question of species extinction, and much of conservation biology focuses on this topic. We argue that species extinction often represents the endpoint of a process of population extinctions, and that the deletion of populations over much of a species' range is likely to be of as much or more concern than the final extinction of that species. Population extinctions often result from habitat destruction and modification, which can be widespread. The result is that species can be deleted from most of their former range but continue to persist in small refuge areas. Moreover, species additions in the form of invasive species are frequently more numerous than extinctions in any given area. Such invasions often result in dramatic changes in ecosystem structure or function and can be instrumental in hastening the extinction of native populations. We examine these premises using two examples from California and Western Australia. These two contrasting areas show broadly similar trends in species extinctions, range contractions, and invasions, and they illustrate the fact that, by concentrating on species extinctions, many of the important human effects on biodiversity can be overlooked.