There's a rule that states a (sub)species can't officially be declared Extinct until at least 50 years after the last confirmed sighting- though I think there are some exceptions.
~Thylo
Yes, for a long time that was conventional "wisdom". Now of course we've basically dispensed with that notion in favour of more accurate criteria, such as that used by the IUCN:
"EXTINCT (EX)
A taxon is Extinct when there is no reasonable doubt that the last individual has died. A taxon is presumed Extinct when exhaustive surveys in known and/or expected habitat, at appropriate times (diurnal, seasonal, annual), throughout its historic range have failed to record an individual. Surveys should be over a time frame appropriate to the taxon’s life cycle and life form."
Source:
http://jr.iucnredlist.org/documents/redlist_cats_crit_en.pdf
Now the problem is making sure that species actually fulfil those criteria before being listed as extinct. But such cases as that of the Aldabra banded snail (
Rhachistia aldabrae) show that species are still being prematurely listed as extinct:
Battarbee, Richard W. (2014).
The rediscovery of the Aldabra banded snail, Rhachistia aldabrae. Biology Letters 10: 20140771.
Even government's are guilty of the same thing:
"Not so fast, say the Center for Snake Conservation and the Center for Biological Diversity. These organizations claim that the USFWS made their official announcement before enough was done to confirm the [South Florida rainbow]snake's absence. Ideally, the feds should have carried out multiple, intensive, surveys before concluding that the animal no longer existed, but they didn't do that."
Source:
Living Alongside Wildlife: Is the South Florida Rainbow Snake Really Extinct?
Exactly 50 years after the death of the last known individual. And now National Threatened Species Day each year
