Humans have a history of driving once abundant species to extinction. The passenger pigeon is thought to have once been the most abundant bird on the planet, with several billion in North America when Europeans arrived. But by the end of the nineteenth century the species had been exterminated through hunting and habitat loss. Extinctions approaching this magnitude may be under way with some sea-turtle populations. It is estimated, for example, that there were tens of millions of green turtles in the Caribbean when Columbus arrived in the fifteenth century, but human harvesting has since reduced this number by around 95%. Some populations have already been driven to extinction: the last green turtles nested in Bermuda in the 1930s.
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