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Re: The extinction of small dinosaurs (long again)




On Mon, 16 Dec 2002, Michael Bruce Habib wrote:

> 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. 

So what is it that selected against enantiornithines and small dinos at
the K/T, or against small pterosaurs during the Cretaceous?  And what
disturbance effects were there _during_ the K/T such that ithe balance of
power so dramatically shifted from pterosaurs to birds?

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

Right.
 
> > 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.

My understanding is that pterosaurs did not disappear in an event--rather
a gradual loss of diversity--first small then large species becoming
extinct.  Is this not true?

> You seem to be suggesting that a given species might radiate 
> into an _occupied_ niche, because it has a competitive 
> advantage.

Yes.  I just don't agree with the view that species residing in a
niche maintain dominion over it (because they can always adapt to a
threat from the surrounding competing species) unless turfed out by
some environmental change.  Again, I feel the environmental change is
the threat posed by the surrounding species.  I'm not sure you would
argue with this in "normal" times--I suppose I'm saying the ptero/bird 
shift happened in normal times.  But I suppose this may not be true.

> 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.  

But there is _always_ competition for resources.  Perhaps the idea of
fundamental vs. realized niche is helpful here.  I would argue that most
species are under pressure from other species for many of their
resources.  They are able to exist because they outcompete others for
essential resources.  Now an individual is slightly more competitive for a
specific crucial resource that you were already in competiton for.  That
individual has more offspring because of it and--bang, zoom--pretty soon
you have a new species whose entire population is more
competitive.  Depending upon the resource, this might put the former
resident out of business.

> > 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.

Agreed.  But this is a _process_.  Species may compete for non-critical
resources, and gradually encroach on each other's territory.  Arms races
may ensue and one species may be better positioned (by virtue of its
pre-existing body plan) to usurp the other.

> 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.

Agreed.  But I'm trying to argue that because pterosaurs became extinct
over the Cretaceous, that the biology of the organisms was the major
factor.  I think this is the prevalent view (right???).  And then, if
birds did do in the pterosaurs, they must also be considered in other
extinctions--especially for those extinctions that are not yet tied to the
K/T (as in no bottleneck in birds at the K/T), i.e., enantiornithines,
and perhaps, small dinosaurs).  

> 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.

Right, there is no evidence.