Content area
Full Text
Palaeontologists characterize mass extinctions as times when the Earth loses more than three-quarters of its species in a geologically short interval, as has happened only five times in the past 540 million years or so. Biologistsnowsuggest that a sixth mass extinction may be under way, given the known species losses over the past fewcenturies and millennia. Here we review how differences between fossil and modern data and the addition of recently available palaeontological information influence our understanding of the current extinction crisis. Our results confirm that current extinction rates are higher than would be expected fromthe fossil record, highlighting the need for effective conservation measures.
Of the four billion species estimated to have evolved on the Earth over the last 3.5 billion years, some 99% are gone1. That shows how very common extinction is, but normally it is balanced by speciation. The balance wavers such that at several times in life's history extinction rates appear somewhat elevated, but only five times qualify for 'mass extinction' status: near the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous Periods2,3. These are the 'Big Five' mass extinctions (two are technically 'mass depletions')4. Different causes are thought to have precipitated the extinctions (Table 1), and the extent of each extinction above the background level varies depending on analytical technique4,5, but they all stand out in having extinction rates spiking higher than in any other geological interval of the last,540 million years3 and exhibiting a loss of over 75% of estimated species2.
Increasingly, scientists are recognizingmodern extinctions of species6,7 and populations8,9. Documented numbers are likely to be serious underestimates, because most species have not yet been formally described10,11. Such observations suggest that humans are now causing the sixth mass extinction10,12-17, through co-opting resources, fragmenting habitats, introducing non-native species, spreading pathogens, killing species directly, and changing global climate10,12-20. If so, recovery of biodiversity will not occur on any timeframe meaningful to people: evolution of new species typically takes at least hundreds of thousands of years21,22, and recovery from mass extinction episodes probably occurs on timescales encompassing millions of years5,23.
Although there are many definitions of mass extinction and gradations of extinction intensity4,5, here we take a conservative approach to assessing the seriousness of the ongoing extinction crisis, by setting...