In the year 1865, Père Armand David, a French Catholic missionary to China and a naturalist, caught sight of a strange-looking herd of deer with mulish faces and long tufted tails. Once, such deer must have been common in forests all over China. But mankind cleared the forests for farms. So, Père David found every remaining member of the species living within the walls of the Imperial Chinese Hunting Park near Peking. He described the species for science and somehow got living specimens back to European zoos. During a war, soldiers ate the last Père David deer living in China. Zoos had the survivors. But for all practical purposes, the species was extinct.

By the early 19th century, geologists had unearthed so many extinct fossil species that 82% of all species known to science were extinct (Lyell, 1833, p. 54). By the mid-20th century, so many more fossils had been discovered that Romer (1949) offered the following shocking estimate: probably, more than 99% of known tetrapods from the mid-Mesozoic became extinct without leaving any descendants in our age. Simpson (1953, p. 281) called the Earth ‘a charnel house for species’.

Extinction is not extraordinary. It is as certain as gravity. What forms does it take? What mechanisms power it? What wards it off? What are its rates?

Basic causes

Although a welter of specific problems can extinguish a species, I find it useful to group them into two categories, accidents and population interactions.