[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: body size yet again



> 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.

I think that the factors you cite sound like plausible pieces of the
puzzle.  I have additional points I would like to contribute to the mix. 
One is that (ignoring the flighted birds and marine animals) long-distance
seasonal terrestrial migrations would tend to be practiced mostly by those
larger animals for whom such migrations are most necessary and most
feasible.  Mice and other small creatures may hibernate to get through
periods of low plant productivity, but I'm not aware of them doing habitual
cross-country trips.  (Lemmings are the only counterexample that comes to
mind, though I don't know enough about them to say).  Hence, I would expect
a typical small, terrestrial animal to establish a home base and stick to
it.  As I see it, there would be an ongoing dispersal of high metabolism,
high population growth animals, ever seeking new food sources, which would,
as individual family groups, establish and maintain territories.  So there
would be a spreading of the animal type, perhaps followed by environmental
stresses and, from time to time, geographic isolation.  As I see it, some
obstacles (such as rivers)  which are easily crossed by larger animals
would prove impassable for very small animals.  As the animals establish
new habitats, the populations of microfauna might adapt to a variety of
environmental conditions and evolve over time to produce distinct species.

Moreover, as individuals, smaller animals require much less from the
environment: less food, less water, a smaller home range.  So not only are
such animals more likely to stay put if conditions are suitable, but they
are also more likely to survive as a population when the going gets tough. 
They may be less conspicuous on the landscape than the megafauna, but their
numbers are legion, so _some_ individuals are likely to survive just about
anything, and live on to propagate and speciate (breeding like rabbits, as
the expression goes).

And a large breeding population makes for greater genetic diversity.  This
is helpful in fighting off plagues, starvation, predation, climatic
changes, you name it.  More variety within the population means not only a
greater resistance to environmental stresses, but also a greater ability to
ultimately produce distinct varieties of animals in response to new
environments and circumstances.

Perhaps one's level on the food chain figures into the equation as well. 
Larger animals are fed upon by a smaller number of species, and a smaller
number of individuals of any given species, than are smaller animals.  In
other words, everybody nips at the little guys, whether the little guys in
question eat plants, meat, or both.  Smaller terrestrial animals (unlike
elephant seals or people) therefore don't usually establish large, noisy,
gregarious colonies out in the open, but instead tend to keep to
themselves, settling for a less flamboyant lifestyle.  Burrows are often
the preferred home, and camouflage is commonly employed.  There certainly
is some amount of interaction between various clans of small animal types,
but nothing comparable to the plein air gregariousness seen among large
herbivores today or suggested by _Maiasaura_ assemblages.   The consequent
intraspecies isolation apparent in some smaller prey species may represent
another factor in the higher speciation rate you observe.  

How much of this would apply to the Cretaceous, or to non-avian dinosaurs
in particular, I don't know.  When I think of a small animal today, I tend
to think of a mouse or a small gecko.  With the smallest full-grown n-a
dinosaurs being in the range of a turkey, the above considerations may not
be relevant.  I welcome any corrections to preceding.

One could argue that some of the smallest creatures on earth are the most
successful, and the most difficult to exterminate (if you know what I mean,
and I think you do).

-- Ralph Miller III     gbabcock@best.com

"Small, cheap, and out of control"