Asked by: Auriol Matthews, Twickenham
We often hear of species being on the brink of extinction, with reported remaining numbers from several thousand to a few dozen. But their fate depends on whether they can be protected from three key threats. The most potent threat is environmental, such as changes in food and water supplies. The other two are disease and the emergence of a new predator. Populations below 10,000 can be quickly wiped out by such challenges.
Below a few hundred, species become vulnerable to genetic inbreeding, while species with fewer than around 50 members can be wiped out by otherwise normal blips in birth and death rates. Even then, their fate is not sealed: conservation efforts have rescued species such as the whooping crane of North America, which in the 1940s numbered fewer than 23 individuals.
Image caption: This whooping crane chick is being fed by its disguised surrogate ‘mother’.
Get more fascinating Q&As from BBC Focus magazine by following @sciencefocusQA