Re: Armadillos at the K/T!

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

> Dinosaurs could be a lot smaller than
> elephants and still be a lot bigger than they are now.  I mean there are
> lots of niches between elephant and ostrich.

The fact that there are no dinosaur decendents larger than 
an ostrich may simply mean that the selective advantage 
associated with that increase in size does not exist at 
this time.  The mere existence of a niche does not mean 
there is a selective advantage to occupying it.
 
> But ratites are excellent analogues for non-avian dinosaurs.  If animals
> can eat their offspring, despite being completely overpowered, then
> similarly-sized animals may have had similar effects in earlier times. In
> other words, extant species demonstrate that predators may be vastly
> smaller than prey adults.

It is true that some animals eat young ratites and do not 
feed on the adults. It is also true that some animals eat 
young mammals, reptiles, and others, without preying on the 
adults of these species.  I see nothing outstanding 
about that fact that ratite young and eggs are eaten.  A 
lot of species are fed upon.  So, my question is this: is 
there any data to show that infant mortality is 
significantly higher for ratites than for other bird 
species, or mammals, or crocodilians, or any other 
vertebrate group?

The second set of data of use is any that shows a 
correlation between extinction risk and predation (infant 
mortality in particular.)  The extinction risk studies with 
which I am familiar (done with threatened extant species) 
have found geographic range size to be a common 
correlate, as well as body size to some extent, but I have 
not seen predation levels cited as being correlated with 
extinction risk (perhaps they were not tested).  I doubt 
that predation mortality correlates at all with extinction 
risk, just from a quick glance at modern species:

Example: Rabbits/Hares, or Passerine birds.  Some are doing 
very well, many are critically endangered.  The species 
that are doing well are in no less danger from predation as 
adults or young as those that are critically endangered, 
as far as population studies have covered (ie. common 
species get eaten a lot too). In fact, the game species we 
keep shooting all the time, such as the Eastern 
Cottontails, are doing very well indeed.  Thus, while 
predation is important for the dynamics at the population 
level for these animals, its importance vanishes at the 
species level.

I am excluding island invasion models, because dinosaur 
taxa, as a group, had a world-wide distribution (and thus 
they break essentially all of the rules of these models, 
when taken as a group.)

--Michael Habib
mbh3q@virginia.edu
University of Virginia