Re: The extinction of small dinosaurs (long again)

[Michael Bruce Habib <mbh3q@virginia.edu>]

> > 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.

Ah, now this is a VERY familiar subject for me, at least.
(I'm doing similar work right now, though not in birds at 
the moment).  I'm NOT arguing that biological traits are 
not important.  Quite the contrary, I would argue that some 
groups were far more vulnerable to what happened at the K/T 
than others.  Notice that they still found habitat loss to 
be the most important factor.  They found it did not act 
randomly, or alone, however.  Small species were hit harder 
by habitat loss.  There are a number of reasons why this 
might be, one being dispersal ability (wing morphology is 
important in bat extinction risk, probably for this 
reason).  Disturbance effects are not random in what they 
kill, and certain kinds of disturbances select against (or 
for, depending on how you think) certain traits. 

The fact that biological traits are important does not mean 
that it's an invasive scenario.


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

Still, they don't have continental-sized or global ranges, 
so a global predation-mediated extinction event seems 
unlikely, unless somehow all of the bird-pterosaur 
interactions worked out the same way.


> 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.

Ok, let me explain what I mean in a different way.  I'm not 
talking about specialization.  What I'm saying is this: You 
seem to be suggesting that a given species might radiate 
into an _occupied_ niche, because it has a competitive 
advantage.  However, competition is actually an individual 
trait, though we speak of it at the species level at times. 
For a species to radiate into an occupied niche, you would 
have to select _for_ individuals that are similar to the 
species that is the current 'niche-holder'.  This should be 
rare, because it would put increased pressure on those 
individuals.  

That scenario also requires that individuals appear which 
have a competitive advantage immediately.  It'd be like 
birds being able to fly better than pterosaurs right off 
the bat.  

> I mean, competitive exclusion refers to almost
> identical niche utilization--I just don't think this was the case with bird
> v pterosaurs.

Not always.  If the two groups are competing heavily enough 
to have extinction consequences, then that should be enough 
for competitve exclusion to be a factor.

> > 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.

Ah, agreement is good.

I would argue that, given the fern spikes, iridium, and 
crater, that a bolide strike was the deciding factor at the 
K/T.  HOWEVER, a bolide strike at another time might not 
have led to a dinosaur extinction.  Had a bolide hit at 
another point in time, it could have been far more severe 
as well.  The reason is that the biology of the organisms 
present, and their interactions, are very important.  Now, 
I think that a major bolide strike is likely to pretty much 
wipe most of the larger terrestrial vertebrates, so it 
likely would have killed, say, the Late Jurassic fauna if 
it had hit then, but we can't say that for sure.  In the 
same way, perhaps whatever affected pterosaurs going into 
the Early Cretaceous would not have been as severe without 
birds in the picture.  However, I really don't see the 
evidence that they were basically predated to death at a 
family level.

Mike