Rates of extinction are very difficult to estimate, because we don't even know within an order of magnitude how many species there are. Nonetheless, we can be quite sure that a large proportion of taxa are threatened with extinction. A little less than 10 years ago, The Nature Conservancy [9] assessed the status of 20,892 species in groups that are taxonomically well-characterized in the United States (freshwater mussels, crayfishes, vertebrates, vascular plants, tiger beetles, butterflies/butterflies, and dragonflies/damselflies; Table 3).
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These statistics don't tell us how many extinctions are happening. And depending on whether you're an optimist or a pessimist you can take this as good news or as bad news. Two out of three species in the United States are apparently secure. That's the good news. One out of three species in the United States is vulnerable or imperiled. That's the bad news.
But how do these numbers compare with what might have characterized the biota of the U.S. prior to the arrival of humans? Do they, in fact, tell us that extinction rates are elevated, or are they simply characteristic of what we would have found at any time in the geological past? Are a large fraction of species on the edge of extinction during most of geological history? Two different approaches have been employed to try to compare current rates of extinction with those inferred from the fossil record.