Copeing with mammals

[John Bois <jbois@umd5.umd.edu>]

Article in Oct 1 2004 Science pp. 101...Van Valkenburgh et al. _Cope's
Rule, Hypercarnivory, and Extinction in NA Canids_ notes that consecutive
canid species get bigger and therefore predispose themselves to extinction.
This process resulted in the extinction of hesperocyonines and
borophagines.
But missing from the article is a discussion of why they move _out_ of the
smaller niche.  I'm not sure why they ignore this question.  Instead, they
settle for Cope's--somewhat Lamarckian--Rule.  Lamarck posited that
each of today's organisms started in their own little evolutionary
pond...and they have been on their own separate escalator-like
evolutionary track ever since.  Cope's Rule as applied to the dogs is
similar: mammals (particularly) start out small, experience an
ineluctable, clade-wide increase in size, and then, as if falling off the
end of a production line, become extinct.
I buy the idea that big creatures are more susceptible to extinction.  I
buy the ratchet: once a species is very big it is difficult to re-enter
the very small niche.  I also buy the idea that there are advantages to
large size.  But all these considerations don't make Cope's Rule a rule.

The article argues for increase in size, extinction, and _then_ a
replacement by other species.  Their data (graph B and C on page
102) suggest a different mechanism.  Hesperocyanines live comfortably in
the small niche for ten or more million years.  They are then _replaced_
(either due to competition of predation) by borophagines.  Indeed, if
tooth-length can be extrapolated to niche usage, at no time do the two
clades share a niche.

These data argue for a competition _for_ the small niche, not a romantic
force to get out of it.

Species don't say: "Adieu small niche...I gotta be big now".  They are
squeezed out of it by other, more effective clades.

And it occurred to me that if a similar thing happened toward the end of
the Cretaceous...i.e., that small sub-clades of dinosaurs (birds) and
newly evolving mammal types squeezed other dinosaur and pterosaur out of
these niches and in effect pushed them into big-only niches...that this is
enough reason in itself to provide an extinction mechanism.  In other
words, if a clade surrenders its small niche, it becomes extinct.  Except,
of course, dinosaurs did not surrender the small niche and are in fact
still preeminent in it!