On the Challenge of Comparing Contemporary and Deep-Time Biological-Extinction Rates

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To assess whether Earth is currently experiencing a human-induced “sixth” mass extinction, scientists over the past 20 years have compared modern rates of extinction with the widely accepted average global background rate of 1 extinction (E) per million species-years (MSY). The application of the comparative method has led to the widely iterated estimate that contemporary global extinction rates are 100–1000 times higher than the background rate. Recent analyses indicate that the average background rate is closer to 0.1 E/MSY, suggesting that the difference between contemporary and background extinction is actually about 10 times greater than previously thought. Here, we review the historical development and mathematical underpinning of these estimates and show that, regardless of the baseline measure, there have been fewer documented extinctions in the recent 100–500 years than the comparative measure implies. Although anthropogenic activities have reduced the abundance and distribution of countless species and have caused more species extinctions than would be expected in the absence of humans, we conclude that the most appropriate interpretation of the existing data is that the global rate of contemporary extinction is closer to 100 times greater than the (revised) background rate of extinction rather than 1000 times greater.

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