The Sixth Extinction: how large, where, and when?

Over the last few thousand years, humans have eliminated over 10% of the world’s bird species and locally, over 90% of them. Double-digit extinction percentages are part of our history, not merely a prediction about our future. Low numbers make species vulnerable to disaster, but so do small geographical ranges. Human impacts destroy habitats locally. Species with small geographical ranges typically have relatively low densities. Nature has put her eggs into a few baskets, the hot spots. Many species are rare and local, and so at particular risk form humanity’s impact. Such species are not spread evenly; extinctions therefore will be geographically clumped, like broken eggs in small dropped baskets. The fraction of species that will go extinct will depend critically on whether we lose or protect aggregations of range-restricted species. The good news is that vulnerable species are concentrated, so saving them requires relatively little area. The bad news is that many of these areas have rapidly growing human populations and are in less-developed countries that have sparse resources to protect them. How large the sixth extinction will be is still a matter of human choice, not of predestination.
Myers informally identified 18 host spots (Myers, 1998, 1990). More recently, there have been many efforts worldwide to identify these key areas formally. Areas rich in species are typically not those rich in range-restricted species. Equivalently, areas that have similar numbers of species can differ greatly in their numbers of range-restricted species. The Hawaiian Islands, eastern North America, and Great Britain have broadly similar numbers of forest-living bird species (about 150); the percentages of species restricted to those areas are 100%, 17%, and less than 1% respectively. Nor are areas rich in range-restricted species in one group always rich in another: eastern North America is a hot spot for salamanders bout not for birds. We still have much to learn about the geography of hot spots. And we cannot protect hot spots if we do not know where they are .
On the other hand, extensive modern experience shows that populations numbering in the thousands have risks of extinction observable within human lifetimes. Isolated habitat fragments (certainly fragments of tropical rain forest) will have suffered most of their extinctions by 100 years after isolation. We know that over 10% of the world’s roughly 10,000 bird species are threatened with extinction, with habitat loss and fragmentation as the main causes. We therefore predict that about 500 of these bird species will go extinct in the next 50 years, producing an extinction rate of 1,000 extinctions per million species per year. The rates for other groups of species will likely be higher in that they have much greater rates of current endangerment. To prevent species extinctions in fragmented habitats we must act immediately, for after a narrow window of only a century, it will be too late.
We can now give answers to the questions posed in the title. How soon the extinctions will occur? Very soon we can expect to see widespread extinctions in fragmented habitats within 50 years, with the extinction rate about 1,000 to 10,000 greater than the background rates. Where will extinctions strike hardest? In the hot spots of biodiversity in the tropics. How many species will be lost if current trends continue? Somewhere between one-third and two-thirds of all species, easily making this event as large as the planet’s previous five mass extinctions.

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