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