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Re: Harmonious depredations




On Wed, 7 Jun 2000, Ronald Orenstein wrote:

> >The point that you have not admitted is that ostrich nesting is dependent
> >on predator presence.  Until you do so, you are saying that they have
> >nothing to fear from predators.  My argument depends on this
> >dependence.
> 
> Nonsense.  Ostrich nesting is dependent on maximizing the production of 
> young ostriches.  As for predation, of course nesting behaviour can be 
> influenced in an evolutionary sense by predator pressure (and while we're 
> at it let's distinguish between egg predation and predation on brooding 
> adults).  That doesn't make predation the only selective factor involved.

I'm not saying it does.  I am saying predation limits the areas that
oviparous species nest.  In the case of ostriches, this has been, and
remains, places with or near grass.  Nesting density does indeed appear
determined by both predator density and forage.  But if they didn't need
to conceal their nest from predators, they could nest in many more places,
places with better quality forage.  So, they are limited in this sense by
predation.

> Your argument is not that reproductive behaviour is influenced by 
> predation.  Everyone admits that.
> Your argument is that egg predation 

...and hatchling predation...

> exterminated the dinosaurs, and as far as the ostrich is concerned you
> are trying to avoid the evidence that ostriches tolerate continental
> predation levels...

The ostrich's mating system has been shaped (according to Bertram) by
predation.  Ostriches are a living example of the tyranny of predation on
large oviparous species.  Far from avoiding this evidence, I embrace
it.  It demonstrates my argument perfectly.  However, if I am to maintain
that ovivory has a primary role in structuring/limiting large oviparous
communities, I must explain the continued existence of the ostrich.  I do
so with the authority of the field's leading researcher who says ostrich
nests are extremely difficult to find because they nest in grass at low
predator density.  Dinosaurs didn't have grass to nest in.  Do you want me
to _ignore_ this evidence?

>  As such, it is you who are arguing that ostriches have nothing to 
> fear from predators because their current nesting behaviour is so cryptic 
> that predators cannot locate their nests...

No.  No.  No!!!  Most of their nests are located inspite of grass.  But a
critical number survives.  My point is that grass provides that margin, a
margin dinosaurs did not have!

>... and that therefore their survival 
> does not affect your position that dinosaurs, which (you claim) did not or 
> could not exhibit such behaviour, must have been exterminated by such 
> predators.

I am simply stating as a fact that the medium of grass was unavailable to
dinosaurs.  I am sure crypticity was employed.  But, without grass, it
could not have been as effective.  Supporting evidence for this position
is that ONLY ONE large (ratite-size) bird nests in an area where grass is
not a significant part of its range.

> Bertram's study, however valid, was not (I believe) meant to apply to every 
> ostrich population wherever it occurs (and historically that was from the 
> Arabian deserts to the Cape).  I have already stated that predation is very 
> high - but the point is, it does not exterminate ostriches asnd did not 
> stop them from evolving (probably in Asia) in the first place.

And I claim that grass was critical in all of this.  Ostriches do not
appear before grass.

> > > Though rheas frequently hide their nests, "the male sometimes uproots all
> > > the vegetation within a radius of 2-3 meters around the nest, 
> > apparently to
> > > isolate it, in case there is a fire".
> >
> >Relevancy?
> 
> The fact that rheas will on some occasions not only not try to hide their 
> nests in vegetation but will actually remove what might have been 
> protecting vegetation to create a bare patch up to six meters across 
> centering on the nest hardly suggests that they rely on crypticity to avoid 
> nest predators!

The crypticity stems from being lost in space.  A predator scans the
horizon in the Serengeti, and sees nary an ostrich.  They are there, but,
from a distance, they appear not to be.  I suggest the rhea is
similar.  As with the ostrich, it is not the aerial aspect but the
horizontal which is important.  From a distance this behavior would not
make the nest more visible.

> >"Open site" lacks definition.  Open site with grass? On sand dunes?  I'm
> >not being critical of HBW, only stating what we say to all our
> >students: you must go deeper than encyclopedias.
> 
> I am restricted by the material to hand in my home library.  However, I 
> spent two years doing field work in Australia and can certainly testify 
> that emus are often found in areas where there is little or no tall 
> grass.

Anecdotal.

>  Also the HBW is far more than an "Encyclopedia", and I have not 
> seen you provide any citations whatever on emu nest site preference.

Again, I'm not criticizing the excellent HBW.  There is little published
on emus--I'll dig up what I have tonight.  But you must admit, its
description lacks fullness.