For populations of endangered species, how small is too small?

A recent paper by Richard Frankham, ALERT member Corey Bradshaw, and Barry Brook is the latest volley in a vigorous ongoing debate about endangered species.  When managing such species, what guidelines should we follow to ensure the chances of their irretrievable extinction are acceptably small?

Imperiled species: Clouded Leopard in Borneo (photo by Ch'ien Lee).

Imperiled species: Clouded Leopard in Borneo (photo by Ch'ien Lee).

The intellectual passion on both sides of this debate is impressive.  Bradshaw summarizes some key elements of the controversy in a lively blog on his excellent ConservationBytes website.  For an opposing viewpoint, see this earlier paper by Ian Jamieson and Fred Allendorf.

At the heart of this debate is the famous '50/500' rule, in which an effective population size of 50 individuals is suggested to be sufficient to limit short-term extinction proneness from inbreeding and random demographic events, while 500 individuals are needed to sustain genetic variation over the long term.

It goes beyond the scope of this brief summary to delve further into the debate, but like many such scientific dust-ups there's a good reason that feelings are running high: the stakes for conserving endangered species are potentially very large.