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body size and competition cont.



        First, I would like to thank everyone for their responses to my remarks 
on
body size and competition.  Your critical, knowledgeable, thoughtful
comments are of a caliber I have come to expect from the people on this
mailing list.
        With regard to the problems of whose body size to measure, there are
indeed a number of problems.  There is a considerable range of adult body
size in many species, and some have strong sexual dimorphisms.  But I
strongly suspect that such problems will affect only the fine details and
not the big picture.  Incidentally, I don't think I would call dinosaurs
r-selected animals, any more than tortoises are r-selected.  They seem to
generally have a mixture of r- and K-selected traits, like crocodilians and
ostriches, producing large numbers of eggs but being large and long-lived.
        As to whether we should divide up the Cretaceous land masses in some way
to make comparisons, I suppose it would be interesting but again I suspect
that it will not change the big picture.  It will not increase the size of
Triassic dinosaurs.
        With regard to adaptations and the survival abilities of mammals and
dinosaurs, I should have been more clear.  When I said I saw nothing to
indicate that an insectivorous mammal was more or less likely to survive
than an insectivorous dinosaur, I meant as a competitor.  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.  There is often
a tendency (although I have not seen it in posts here) to consider
endothermy as inherently superior to ectothermy over a range of
environments.  Ectotherms do poorly in polar regions, for reasons that have
nothing to do with competition.  Other than that, ectothermic vertebrates
do quite well and are more diverse than mammals in most terrestrial
systems.  Again, the reasons probably have nothing to do with competition
between the two.
        Never apologize for citing a reference!  Cite at will!  A professor of
mine was fond of saying "Give me 3 references."  I will not be so brutal,
but by all means feel free to give references at any time.
        With regard to the size of Tertiary mammals, I will save most of my
remarks for "stay tuned."  But notice that dinosaurs did decrease in size
in the Cretaceous.  Since the end of the Jurassic there has been a global
advancement of open herb-dominated habitats at the expense of forests.
This has major implications for the evolution of large herbivores and their
predators.  My remark about grassland-type habitats was in reference to
prairies, savannahs, desert-grasslands, some deserts, and even some
"forests."  All fire-frequented plant communities are sometimes lumped
together in the plant ecology literature as "grasslands."  Admittedly there
were no grasses in the Cretaceous. 
        I was using the phrase predator escape in the sense it's used in the
evolutionary ecology literature, to mean being unavailable to a predator.
This often occurs via increasing size.  The term predator refugium is also
used to mean the same thing.
        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.  Whether a predator
species has a measurable effect on the prey population over time is more
problematic, but at least we have a way of approaching it.  With
competition, though, things are far from clear-cut.  An animal dies and
there is generally no obvious way to tie this to the population level of a
syntopic species.  It becomes even more nebulous if we try to look at how
population changes in one species affect those in another.  The density of
my species goes up and down, the density of your species goes up and down.
All God's chilluns is goin' up and down.  It requires a long time series to
even determine whether there is a trend, much less compare the series and
sort out who is affecting who.  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.
        The reference to the "nocturnal niche" brings up exactly the problems 
with
the whole niche concept I was alluding to.  If I'm a diurnal snake that
eats only one species of toad, and you're a nocturnal snake that eats the
same species of toad, how does it ease any competition between us that we
never see each other?  We could take any characteristic and place a species
along a "niche axis," but it is quite unclear how such separation
ameliorates competition between species.
        Even in plant communities (and encrusting animals, which are 
ecologically
similar in many ways), where we can clearly see competition occurring
between individuals or clones, niche theory doesn't seem to work.  Studies
in tropical forests, which are supposed to be the classic examples of
intense competition and biotic as opposed to abiotic factors at work, show
that, far from segregating into niches, closely related species tend to
have similar "niches."  They are gap exploiters, or slow-growing shade
tolerators, or highly dispersive disturbance opportunists.  Everybody is
competing with everybody and the competition between any two taxa gets lost
in the complexity and stochasicity of the dynamics.  The fact that two
species have similar lifeways seems to have little effect on their evolution.
        Where I grew up, there are 6 species of water snakes, 4 of the genus
Nerodia and 2 of the genus Regina.  The 4 Nerodia are fish and frog eaters,
the 2 Regina crayfish specialists.  All occur together in the same
waterways.  Yet niche theory dictates that no 2 species can occupy the same
niche.
        It may be tempting to accuse me of a bias here (it is to me).  Perhaps 
it
appears that I am leaning away from competition as a hypothesis because it
is difficult to test.  But I truly feel that the complexities of nature are
such that competitive effects are much broader and blunter than has been
suggested.  We cannot explain the disappearance of species, or any other
taxa, by appealing to competition with another taxa.  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.
        I should mention that I do not feel that intraspecific competition is
unimportant in evolution.  Intraspecific competition is what natural
selection is all about.  But interspecific competition in much more
problematic.

Best regards,

Dave