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Re: a little background



On Sun, Mar 10, 2002 at 04:47:18AM -0500, Edels sent:
> Some things to consider:
[snip]
> 5)    The larger herbivores appear to have traveled in large herds
> (Jack Horner has often claimed  that the _Maiasaura_ nesting grounds
> had over 15,000 individuals buried by a massive mudslide - and the
> animals came back the next year[?] to make new nests).  This tactic
> DOES protect the majority of the herd.

It does; it's also an arguement that there was major predator pressure
or migratory behaviour, if we can argue by analogy to modern systems.

> 6)    Not all of the apparent prey animals were available to be eaten
> by the predators, and not all of them were DESIRED by the predator.
> (For instance, who here really wants to eat Horseshoe Crab?  I know
> where you can find millions of them).  Animals also have things that
> they like to eat and other things that they don't.

Well, yes, but I don't think any predatory dinosaur would turn down a
chance to eat any other dinosaur.

> 7)    If we focus on the Late Maastrictian, we have mostly _T. rex_ and
> _Triceratops_ dominating the Western American continent.  While there are
> other dinosaurs around (most notably hadrosaurs, and there were also some
> smaller sauropods), these 2 dinos were the majority of the megafauna for
> nearly the last 500,000 years of the Cretaceous.
> 
> What do I mean by all this?
> 
> The likely situation is that prey vastly outnumbered prey, much like most
> environments today.  The predators, possessing superior weapons and speed,
> are STILL usually outnumbered by the prey.  The prey animals continue to
> multiply and survive (at least most of them).  [There is currently only one
> known super-predator - MAN, who seems to be able to remove prey entirely
> from an environment].

Sure, no argument.

What I'm looking at is not 'why are there big herbivores' -- if you're
already big, you want to get bigger -- but 'why did big herbivores so
dominate ecologically if the smaller herbivores were in an ecosystem
where running like hell was such a good defense?'

Dinosaurian ecologies already look like they have a suspiciously high
proportion of large herbivores; they also look like getting to adult
size as a large herbivore was really bad odds.  Since we don't know when
in the life cycle the primary mortality occurred, there's a lot we can't
infer about predation, but the indication seems to be that smaller
herbivores -- and that includes things about to about 500 kg!! -- were
in marginal, speciality niches, despite being at lower risk of
predation, anywhere except really low-energy (arid or arctic/antarctic)
environments.

If the top predators are too fast to escape, this makes a kind of
ecological sense; the best defense for a herbivore at that point is a
combination of size and herding. (and there is a plausible tendency
among ceratopsians to get bigger over the later cretacous.)

If the top predators are *not* too fast to escape, one expects to see a
lot of smaller, faster herbivores; most antelope are small and fast, and
most lions eat the *larger* antelope, since those are the ones worth the
effort of hunting if you're lion sized.  Being small and fast is a good
strategy.

> The mega-predators would eat the other mega-fauna, as available.  They would
> also eat any of the smaller prey when they could.  (People eat rabbit,
> pigeons, small trout - not just cattle, sheep, etc.)

Large predators pretty much *have* to eat large prey, if I understand
the energetic arguments; only when small prey species are super-abundant
can large predators survive by preying on animals much smaller than
themselves.

So is it just preservational bias, or is something odd going on in the
dinosaurian ecology, skewing it to large body sizes?

-- 
graydon@dsl.ca   |  Hige sceal þe heardra, heorte þe cenre,
                 |  mod sceal þe mare þe ure maegen lytlað.
                 |   -- Beorhtwold, "The Battle of Maldon"