The Extinction Puzzle

With six billion humans and counting – and our individual consumption growing – we face enormous challenges in using our natural resources sustainably. And there is one clear measure of how we perform at that task: extinction rates. For this reason, some colleagues and I have calculated the rate at which bird species became extinct in the recent past and the rate at which they will likely become extinct in the future.

Bird extinctions are our best window onto humanity’s massive and irreversible environmental impact. For every one of the world’s 10,000 bird species, there may be 10 or perhaps even 100 other unknown species of animals, plants, or fungi. Birds’ popularity means that they provide an unrivaled source of information about which species live where and how well those species are doing.

Estimates of the number of extinctions have tended to vary wildly, owing to differences in the assumed total number of species, which range from the one million species that have scientific names to an implausibly high estimate of 100 million species. To avoid such uncertainties, my colleagues and I introduced the extinction rate – the number of extinctions per year per species or, to make the numbers more reasonable, extinctions per million species-years — E/MSY. With the exception of the past five mass extinction events, estimates from the fossil record suggest that an approximate background rate is one extinction per million species-years, (1 E/MSY). For the 10,000 bird species, there should only be one extinction in each century.

Before European exploration, the Polynesian expansion across the Pacific probably exterminated species at the rate of one every year or two, which is 50 to 100 times more extinctions than should occur naturally. For example, parrots, rails, and doves once occured across the Pacific on sufficiently large islands. Those islands that have been well explored archaeologically yield bones of species the Polynesians exterminated, while the islands now lack them.