Re: The extinction of small dinosaurs (long again)

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

On Thu, 12 Dec 2002, Michael Bruce Habib wrote:

> Invasions are involved in regional extinctions all the 
> time, but there is no evidence that they can be a primary 
> killer in a global extinction event that kills all 
> populations of thousands of species in a short period of 
> time.

A sobering fact is that there is no direct evidence for _any_ extinctions
except those recorded by humans--and they are almost all due to
invasions, i.e., species being unable to cope with the
strategies, phenotypes of other species.  A strange science it
is that rejects _known_ extinction causes (perhaps because they are less 
spectacular and are going on, as you say, all the time) in favor of
never-observed phenomena.  I'm not saying catastrophies don't cause
catastrophic extinctions; I am saying that specific direct causes
involving competing and preying species cannot be ruled out without good
reason.

> Also keep in mind that such invasions (of which 
> there are many presently) are essentially migration or 
> dispersal events.  Speciation and divergence patterns are 
> far too slow to constitute a true biological invasion...  

I don't see the distinction: if a new behavior evolves that results in a
competitive advantage, that species' range will increase at the expense of
others'.

> Your example above requires that there be no adaptive 
> response from terrestrial dinosaurs.  It also requires that 
> the same competitive advantage exist everywhere that the 
> competing species meet, and that it is significant enough 
> among different groups to increase extinction risk across 
> entire families. To date, there has been no strong data to 
> show that invasions work in this manner.

Pinning down specific causal agents is a wonderful field of study.  But,
I believe there are no rules, only hypotheses as yet.  Some species go out
quickly, some slowly.  Only the most radical of proponents of punctuated
equilibrium would suggest that _all_ speciation involves catastrophy.  If
it is true that speciation may be slow or fast--depending on the
individual circumstances, my feeling is that as species develop new, more
complex, strategies and behaviors, the previously successful strategies of
earlier species become a liability to them.  The great changes in
utilization of ecospace through geological time seem to reflect these
sorts of forces rather than the random action of huge intervening
rocks--no matter how frequent, how massive.  But, hey, that's just me.

> You do not need a major disturbance to cause extinction, as 
> HP Bois points out; invasions and other processes cause 
> changes in biotas consistently.  However, to kill major 
> groups on a global scale in a short time period _does_ 
> require some major shift in conditions.

And yet, we see a disappearance of smaller pterosaurs without a major
shift in _physical_ conditions.  There may well have been a major shift in
biological conditions: competition/predation of birds.  And if birds could
do this to small pterosaurs, why not small grounded dinosaurs?