Re: Armadillos at the K/T!

[Graydon <graydon@dsl.ca>]

On Sun, Sep 30, 2001 at 09:21:00PM -0400, John Bois scripsit:
> On Sun, 30 Sep 2001, David Marjanovic wrote:
> > > This is especially difficult to argue when we realize that by far
> > > the most severe cause of mortality in many if not most species is
> > > _predation on the young_.
> > 
> > Of mortality, maybe (in crocodiles it's AFAIK flooding of the eggs).
> > Of extinction?
> 
> First comes mortality, then extinction.  Right?  If extant organisms
> suffer much mortality due to a certain factor, that factor should be
> suspect if extinction occurs.

If it's a factor at all.  It's very hard to sugguest a way to test rates
of nest predation for species known through fossils!

If nest predation became a suddenly decisive reproductive constraint for
*all* large oviparious terrestrial species -- which includes the ones
whose nesting habits we know nothing about! -- you're forced to
postulate a guild of small organisms that's completely ubiquitous, and
which is equally effective against all the strategies evolved over the
previous 100 Myears to avoid nest predation becoming too severe,
strategies which will range from complete R strategy abandonment to
active nest guarding by extended family groups.

That's very hard to postulate; an *increased* success, yes, that's not
hard to postulate in the case of a nocturnal burrowing animal, but
ubiquitous success has several arguments against it.

One is that some of the nesting strategies will be ones for which the
new tactic does not apply; nesting on hard ground, frex, or soft sand
which won't support burrowing, or the nesting animal being night-active
and an active nest guardian.

Another is that the life cycles of the small or relatively small
predators will be much shorter than the life cycles of the large
dinosaurs, and that if a predator does get into a 'green field' food
supply situation, that predator's population will surge and crash,
leaving the large dinosaurs with years in which nest predation will be
substantially *less* than formerly.  (Since the competing nest predators
will have starved, relatively, in the years in which the predator with
the new strategy was wildly successful.)  That pattern argues that if
they're to produce an extinction event, they have to do it very quickly,
before the average clutch size rises and other evolutionary
countermeasures available to large R-strategist animals are taken.  That
would be a very surprising degree of success against a *single* species;
for that to be ubiquitous is wildly improable.

Third is that there will already be nest predators, and as has been
noted, almost all egg eaters do so opportunistically, as part of a
generalist feeding strategy.  If they're put under pressure by a highly
successful egg predator, they're going to need a new source of food; the
young of the newly successful and rapidly proliferating egg predator, or
the predator itself, are obvious candidates.

The only way this works *globally* is by invoking the McLaughlin
Hypothesis -- that the demise of the dinosaurs is consistent with a
predatory dinosaur species having become intelligent to at least the
level of _Homo erectus_, which wiped itself out through ecological
instability brought on by devouring everything about it.

-- 
                           graydon@dsl.ca
               To maintain the end is to uphold the means.