Conservation biologists agree that small populations are at risk. Many people would like to take this farther and be able to say exactly how small a population can be before it is too small. But we can't--and shouldn't--base conservation policies on exact numbers, says John Wehausen of the University of California's White Mountain Research Station in Bishop.

"Everyone seems to be searching for simplistic predictive models that they can blindly apply to conservation problems," says Wehausen. "It is my opinion that many ecologists suffer from physics envy." But ecology is not as precise as physics and we can't really predict what will happen to a given population based solely on its size, he says.

Wehausen assessed a 1990 model of the relationship between population size and the likelihood of extinction in bighorn sheep. Developed by Joel Berger of the University of Nevada in Reno, the model predicts that populations smaller than 50 will go extinct within 50 years. But Wehausen analyzed 68 California bighorn populations that were once this small and found that well over half had persisted for 50 years.

Rather than setting numerical cut-offs for conservation action, Wehausen recommends monitoring small populations individually and stepping in if they begin to decline precipitously. "The question is not whether to attempt to help small populations with augmentation, but at what point this should occur," says Wehausen. "Policies based on Berger's analysis would require augmentation of all populations below 50 sheep. The resources that would be necessary to carry out such a policy in California would be enormous."

Another problem with conservation by numbers is that agencies might decide that bighorn populations smaller than 50 are effectively extinct and write them off.

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Contact: John Wehausen
johnw@wmrs.edu
760-873-4563
Society for Conservation Biology
30-Mar-1999