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Re: More tyrant Q & A's



At 12:40 PM 12/9/98 -0500, Norton, Patrick wrote:
>Stan Friesen  wrote:
>
>>On ecological grounds, forms that co-occur in one time-place can really   
>only be distinct species if they differ sufficiently to be able to   
>minimize competition.<
>
>Many highly competitive, morphologically similar species have territories   
>that overlap to some extent for one reason or another.  _Canis lupus_ and   
>_Canis latrans_ , for example, co-exist across broad ranges of Canada and   
>the US, probably due to a gradual process of displacement.

Well, partially.  But they can co-exist indefinitely, as the changes to
Yellowstone ecology since the re-introduction of the wolf have shown.  For
one thing, they differ in size by more than the minimum amount necessary to
establish resource partitioning.

>  Several   
>species of _Sciuris_ (also highly competitive and morphologically   
>similar) co-occur in the forests of the northeast.

Again, virtually always with either size differences or with differences in
microhabitat.  For instance, in Kansas (where I am most familiar with these
things) one common squirrel inhabits forest edges, brushy fields and the
like, and the other common squirrel inhabits mature closed forests.  In
squirrels differences in the part of the tree they forage in might also be
sufficient resource partitioning.  But I have a hard time imagining
tyrannosaurs partitioning on the basis of tree zones :-)

>  I think that, over   
>time, in a stable environment, minimization of competition would   
>naturally among the inhabitants.

The basic conclusion of ecological research is that truly intense
competition is always *extremely* short-lived, and almost never observed
directly, and that exclusivity is the general rule in all but the most
disturbed habitats.  (In heavily disturbed habitats, where populations
levels are *far* below the carrying capacity, exclusion fails to occur,
since there are excess resources to go around, but even moderately
disturbed habitats still show competitive exclusion and/or competitive
divergence).

>  But I'm not sure it's safe to assume   
>that morphologically similar Tyranosaurid species found in the same   
>paleoenvironment were engaged in only minimal competition.

If they are truly in the same time and place, the chances of a
fossilization event preserving a strictly momentary interval of competition
is vanishingly small.  Note the qualifier is rather nasty - even being at
levels separated by 6 inches vertically is enough temporal separation to
allow for a shift in geographic ranges in between.


Thus only cases like Sue and Stan, or the Ghost Ranch fossils, actually
provided enough specificity to establish true sympatry.  Merely being in
the same formation is not quite enough.

--------------
May the peace of God be with you.         sarima@ix.netcom.com