Catherine Clabby in American Scientist:
A group of Australian researchers say they have nailed the best figure achievable with the available data: 5,000 adults. That’s right, that many, for mammals, amphibians, insects, plants and the rest.
Their goal wasn’t a target for temporary survival. Instead they set the bar much higher, aiming for a census that would allow a species to pursue a standard evolutionary lifespan, which can vary from one to 10 million years.
That sort of longevity requires abundance sufficient for a species to thrive despite significant obstacles, including random variation in sex ratios or birth and death rates, natural catastrophes and habitat decline. It also requires enough genetic variation to allow adequate amounts of beneficial mutations to emerge and spread within a populace.