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body size yet again
Once again, critical, knowledgeable, thoughtful responses. I'm getting
With regard to body size trends, it did not occur to me to compare
outliers. If we look at outliers I would probably have to eat my words
that regardless of what we measure, we will see the same big picture. But
if we look at means, medians, modes, or any other measure of central
tendency, I believe we will see the trends I have described. I don't
really know what to say about outliers, other than the fact that very few
scientific studies use them to make comparisons. The tails of most
distributions are relatively flat, so the values of outliers can vary a
great deal. Generally scientists use measures of central tendency and/or
variation in making comparisons. If I were to compare the body size of the
largest lizard with that of the largest snake, I would find that the former
is larger. This doesn't seem to tell us much about lizards and snakes as
groups. Lizards are on average much smaller than snakes, and this
difference relates to important differences in their ecology.
In terrestrial plant communities, the relative amounts of biomass do not
necessarily reflect the physical structure. There are many "forests" which
under natural conditions are open-canopied and have abundant herbaceous
growth, yet trees constitute the overwhelming bulk of the biomass.
Examples include the longleaf pine "forests" of the southeastern U.S. and
many of the eucalypt communities of Australia. Note that I consider ferns
with only underground rhizomes to be herbaceous plants. But in the context
of herbivore evolution, the physical structure is as important as the
relative abundances of herbs vs. woodies.
Herds of Triceratops horridus did not feed on tree leaves. It requires a
great deal of vegetation near ground level to sustain numbers of such
creatures. The very presence of large herds of ceratopsians and hadrosaurs
in forests would maintain a lot of canopy gaps.
My hypothesis that closed-canopied forests decreased in area after the
Jurassic is based on the literature I have read, but I have to admit I am
not up on paleobotany terribly well. I would like nothing better than to
see the DinoFest papers cited, but this is not a very likely possibility
given my current interlibrary loan situation.
The Darwin's finch example may well be a good case of niche partitioning.
With small islands, caves, and some other relatively simple ecosystems, the
competition between two species my be straightforward and intense. As for
Anolis, I have to say I am not convinced by what I have seen, nor am I
convinced by studies of partitioning of vegetation zones by passerine
birds. The fact that closely related species have different habitat
preferences is not evidence of interspecific competition at work. For
every such example, I could probably cite half a dozen in which closely
related species are the MOST similar in a given area, in term of habitat
preference, diet, activity patterns, and morphology.
The examples of C4 plants and bats do indeed contradict a stochastic
vision, but only as it applies to ADAPTATION. Taxonomic and other trait
groups most certainly do show consistent adaptations. Within a taxon there
is always a certain degree of genotypic consistency which leads to a
consistent pattern of adaptation. There are most certainly consistent
selective forces at work over a range of habitats. Cold is cold whether
it's in Alaska or Siberia. Terrestrial animals lose water. Energy
efficiency is selected for in every environment.
But the remark about small birds being diverse has sparked my interest. In
ALL taxa that I'm aware of, including dinosaurs, species richness increases
exponentially with decreasing size. Why? The answer is not at all clear
to me. It may be that small species tend to have shorter generation times,
leading to more rapid rates of evolution and speciation. Or perhaps large
species have a higher extinction rate. Or is it a combination of factors?
I would be interested in hearing opinions.