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Re: The extinction of small dinosaurs (long again)
> > 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
> 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
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
> 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