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Re: Mystery of Mass Extinctions Is No Longer Murky



If we want to rank ourselves amongst mass extinction causes, then we'll really have to lift our game. Extinction events caused by direct or indirect human activity are nothing compared to those in the geologic past. The Permian/Triassic extinctions in particular really raised the bar.

Our "ranking" as a mass extinction cause depends a great deal on how you deal with the differing temporal scales. The total diversity loss due to humans, to date, is modest compared to any of the mass extinctions of the past (especially the Permian/Triassic, as mentioned). However, the *rate* of extinction may be comparable. Thus, if we assume that the current rate holds for tens of thousands of years, the total percentage loss could be comparable to at least some of the mass extinctions.


It depends a great deal on how fast the mass extinctions of the geologic past really were, which is hard to get a precise measure on. Some were clearly very fast, geologically speaking, but to make comparisons to the modern rate, we need even better precision than is currently available. The comparison is also made difficult by the issues of taxon sampling. And, of course, there is the issue of extrapolation: do we make the comparison assuming that all living species that are threatened bite the dust, or some fraction? Can we take the rate over the last 100 years and extrapolate to get the expected numbers in 1 million years, or should we expect the rate to taper after those species most susceptible to human mediated effects perish?

The upshot is that one can get numbers that are comparable between "expected" human-mediated extinctions, and the estimated mass extinctions of the past, but only with a given set of assumptions. Other sets (which are not objectively better or worse) give very different numbers.

Cheers,

--Mike


Michael Habib, M.S. PhD. Candidate Center for Functional Anatomy and Evolution Johns Hopkins School of Medicine 1830 E. Monument Street Baltimore, MD 21205 (443) 280 0181 habib@jhmi.edu