Going or gone: defining ‘Possibly Extinct’ species to give a truer picture of recent extinctions

The IUCN Red List is widely regarded as the most authoritative classification of species by their extinction risk (Lamoreux et al. 2003, Hambler 2004, Rodrigues et al. 2006), including those species known to have become extinct in recent times. Birds are the best-documented class of organisms on the Red List, and the fourth complete assessment of the status of the world’s birds was recently published (BirdLife International 2004, IUCN 2004), and updated (at www.birdlife.org) for the 2005 IUCN Red List. As well as 1,208 threatened bird species in the categories of Critically Endangered, Endangered and Vulnerable (in order of decreasing risk of extinction), it lists 131 species as having become Extinct since 1500 (for which ‘there is no reasonable doubt that the last individual has died’: IUCN 2001), and an additional four species as Extinct in the Wild (‘known only to survive in captivity’: IUCN 2001). However, extinction—the disappearance of the last individual of a species—is very difficult to detect (Diamond 1987). For a species to be listed as Extinct requires that exhaustive surveys have been undertaken in all known or likely habitat throughout its historic range, at appropriate times (diurnal, seasonal, annual) and over a timeframe appropriate to its life cycle and life form (IUCN 2001). Listing as Extinct has significant conservation implications, because conservation funding is, justifiably, not targeted at species believed extinct. Following a precautionary approach, conservationists are therefore reluctant to designate species as Extinct if there is any reasonable possibility that they may still be extant, in order to avoid the ‘Romeo Error’ (Collar 1998), where we might give up on a species before it is too late...