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Re: body size and competition cont.



I respond to this with interest for two reasons.  Firstly, I have
just taught a unit on ecology in A.P. Biology and have immediacy to spark
my interest, and secondly, I believe strongly that predation plays a
stronger role in shaping communities than is generally recognized.  As
such I am generally sympathetic to your apparent wish to de-emphasize
competition.  So, with the seemingly contradictory desire both to learn
and to beat my drum, I'll proceed. 

On Sun, 3 May 1998, Colette H. Adams wrote:

> Animals have all
> kinds of adaptations to their environments, but this has nothing to do with
> interspecific competition.  The fact that there are polar bears in Alaska
> and no crocodiles is not because they outcompeted them.  Competition means
> that the density of one population affects that of another. 

But that is a very stark example.  Take Darwin's finches.  Two species
living sympatrically show character displacement in beak depth.  That is, 
they avoid competition by evolving in opposite directions, one toward
smaller seeds, the other to large.  But on islands where each of the two
species lives alone their beak size is medium, i.e., suitable for both
large and small seeds.  This is a case where the density of one species
affects the other. And I believe that such partitioning of niches is a
fairly common phenomenon (known at least in Anole lizards). 

>       There is an important difference between competition and predation, in
> terms of testable effects.  When a predator makes a kill, it has had an
> easily measurable effect on the prey population.  

But a more important effect may be predator avoidance behavior.
The distribution and density of prey communities may be profoundly
affected by their desire to stay off the dinner menu.  Some species may
avoid patches to escape predation (Lima, S.L. 1998 Nonlethal effects in
the ecology of predator -prey interactions. Bioscience 48 No.1 pp.25-34.)
For me, this is where the distinction between predation and
competition gets blurry. Say species A is in competition with B for a
certain patch and this keeps the numbers of both quite low.  Now a
predator enters the picture.  Sp. A has adequate predator defences but sp.
B must leave to avoid predation  and so gives the patch up to up to A.  A
thrives, B is banished to less productive patches and perhaps becomes
extinct.  Is this competition or predation?

> We can do a controlled experiment,
> introducing two species together and looking at their effects on each
> other, but there is always the question of how applicable this is to the
> natural world, with all of its complexities and subtleties.

I sympathize.  Have you thought of going into something a little more
predictable, say, accounting!

> Most species occur in
> a variety of habitats, and each habitat has different players.  The
> composition of animal communities varies so much from place to place that
> it is difficult to see how a consistent pattern of competition could emerge.

And yet arguing against your stochastic vision, C4 plants do better in
tropical biomes; bats do better, generally,
at night; small birds (under 1M) are vastly more diverse than large birds;
I mean it is possible to make such broad generalizations about many
classes of animals and to attribute this to strategies or
morphologies (correctly or not).
Your frustration with being unable to observe such moments of triumph in
"ecological time" is understandable.
Wiens, in the paper I cited in previous post notes that most
changes in species composition happen in times of high stress.
The species in your forest may appear to all be fairly equal competitors
until something nasty happens.  Perhaps some important triumphs occur at
this time.
Finally, from me, there is a tremendous growth right now in literature
devoted to the effect of predation on communities, particularly avian
communities. It is my view that predation greatly effects the competitive
success of creatures both today and in the Mesozoic.