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Re: The extinction of small dinosaurs (long again)



> Date: Sat, 14 Dec 2002 14:10:43 -0500
> From: Michael Habib <mbh3q@cms.mail.virginia.edu>
> To: jbois@umd5.umd.edu
> Cc: Dinosaur Mailing List <dinosaur@usc.edu>
> Subject: Re: The extinction of small dinosaurs  (long again)

Bois said:
> > 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<
>
> This is actually not the case.  Invasions are important in the modern
> world in leading to extinction, but the overwhelming majority of
> extinctions in recent times has been due to human-mediated habitat
> destruction.  Closer to a bolide effect in some ways, interestingly
> enough.

>From the latest "Bible" on this subject: "It is generally thought that the
current extinction crisis is largely the result of human disturbance to
natural environments...Thus, it was plausible that extinction risk might be
randomly distributed among bird species...However...we found that taxa
differ in the extent to which they are extinction-prone and these
differences are apparently influenced by the biology of the species
themselves."
Two examples they give are: large body size (in birds) is correlated with
risk from predation/human persecution; while small body size is correlated
with habitat loss.  Their analysis indicates that less than 50% of
threatened bird species are threatened by habitat-loss alone.
Source: Bennett, P.M.., I.P.F. Owens.  2002 Evolutionary Ecology of Birds:
life histories, mating systems, and extinction. pages 174-179.

> Also keep in mind that we are moving massive numbers of
> organisms across all corners of the globe.  It would take some major
> interchange events to do the same without humans (and this has
> happened, of course).

Except that birds are great dispersers (not saying there aren't limits to
this).

> > 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.
>
> Fair enough, good call. I would say the global scale, in this case,
> rules out predation.

If we're talking about pterosaurs vs. birds, I don't think we can rule out
predation for the same reason (dispersal).

>>If a new behavior evolves that results
> >in a competitive advantage, that species' range will increase at the
> >expense of others'
>
> Probably does not happen very often, actually.  Selection should
> favor REDUCTION in interspecific competition.

I'm not understanding. If a species has a competitive advantage it _has_
reduced the competition.  If you mean that a species may utilize a new
resource and thus reduce competition, I would agree--but specialization is
only one way to gain a living; many, if not most organisms share the need
for _similar_ prey, resources, nest sites, etc. with other species.

>  The mechanism you cite
> is well-known and common regarding intraspecific competition, but is
> probably rare between species.

Certainly not among invasive species; granted that it might be less common
among the home crowd.  However, I would argue that over geological time
novel traits arise that the local species can't handle.

> I find it hard to believe that small birds, just getting into the
> air, somehow pushed aside small pterosaurs.  Again, competitive
> exclusion principles would imply this is unlikely.

Birds lived with pterosaurs.  They likely shared some niche requirements,
but not others.  A bird species may have lived a completely separate life
until nesting season when it needed the rock ledge occupied by a pterosaur.
This particular bird, and several of its mates, kept swooping on the hapless
pterosaur until it flew off.  I mean, competitive exclusion refers to almost
identical niche utilization--I just don't think this was the case with bird
v pterosaurs.

> Selection for
> flight would be awfully weak if birds were radiating into an occupied
> niche.  This is probably another case of opportunisitic replacement.
 > In fact, this seems to be a consistent trend: we keep looking at
> processes that were once thought to be a competitive replacement, and
> are finding them to be opportunistic.

What is a known example of this?

> Likely what happened in the
> Late Triassic, probably the case with pterosaurs, and seems to be the
> case at the K/T.

Wide distribution of birds makes opportunistic hypothesis less likely in my
view.

> Of course, this does not mean that competitiion has nothing to do
> with it.  If some species are less vulnerable to a changing climate,
> then one could make the argument that they had a competitive
> advantage.  After all, on some scale, everything competes for space.

Yes.  Ecospace.