Answers to Questions about Extinction Criteria
- How did we handle subspecies?
- Why did we use AD 1500 as a cut-off date?
- Why wait 50 years before declaring a species' extinction fully resolved?
- What about species that are functionally extinct, regionally extinct, or extinct in the wild?
We chose "species" as the basic unit to count for practical reasons. It is the most widely accepted and recognized biological unit, particularly in terms of having the greatest acceptance and practical value to conservation policy. Thus, we chose not to count "subspecies" or populations. If we include these units, we risk producing lists that are too confusing and complicated to be understood or used by relevant consuming groups.
Why did we use AD 1500 as a cut-off date?
For practical reasons, we defined recent extinctions as those occurring since AD 1500. Extinctions after AD 1500 represent those most pertinent to current biodiversity issues. This date also marks a time when recorded evidence for the loss of species in association with human movement and activities became more common. However, there may be cases of earlier extinctions that are relevant to the evaluation of current patterns and processes of extinction, and these events will also be evaluated (e.g. extinctions associated with Polynesian expansion 1000 to 4000 years ago; Pimm et al., 1994, 1995; Steadman, 1995; Diamond, 1997)
Why wait 50 years before declaring a species' extinction fully resolved?
The 50 year rule was introduced by CITES (Convention on International Trade in Endangered Species) and the IUCN, but dropped from the 1996 IUCN Red List. We have re-adopted it here as an insurance policy (albeit arbitrary) against prematurely declaring a species extinct. We decided on this protocol because it provides a lengthy time period during which the species, if still extant, might be seen or collected, purely by chance. This can be particularly important when it is difficult to judge whether sufficient sampling has been conducted to confirm the species' absence. We are concerned about wrongly declaring an extinction in situations where:
- the species is particularly difficult to collect regardless of sampling -- for example, because it is cryptic or naturally rare.
- the habitat is particularly difficult to survey -- for example, marine environments.
- the species is temporally absent from its typical habitat, but not extinct -- for example, when environmental change or ecological shift result in the movement of the species to a different, unexpected (and unsurveyed) habitat.
Use of the 50 year rule as the only criterion for judging extinctions can lead to errors. Therefore, we propose application of the 50 year rule in conjunction with other criteria.
What about species that are functionally extinct, regionally extinct, or extinct in the wild?
Some species have been described as "functionally extinct," where only senescent and non-reproductive adults represent the taxon. CREO will adopt a policy of drawing attention to those species that are on an irreversible path to extinction, but listing them separately from those species that are truly extinct.
Species-level extinctions refer to the loss of all representatives of that species, such that the shared, unique genetic heritage of that species has completely disappeared. Species that have been extirpated regionally, or have been eliminated in the wild but still exist in captivity, may have lost important genetic material and/or may be in grave danger of extinction, but they cannot be considered extinct. However, the regional extirpation of these species can be measured using similar criteria as those proposed here.