Re: Mystery of Mass Extinctions Is No Longer Murky

[Michael Habib <mhabib5@jhmi.edu>]

> 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