Moon, you cut to the heart of the study of biodiversity. It seems that one
could answer with a qualified "yes, sometimes" to most of your questions. An
area like the Tennessee Valley has a high diversity because the
geomorphology seems to have been stable for at least 70 million years or
more, with no glaciations to disrupt existing communities. The same analysis
can be made of many tropical areas, especially in southeast Asia, which have
been continuously tropical for over 200 million years. This kind of
stability seems to facilitate speciation through adaptive radiation, i.e.
filling (and creating?) ecological niches.
My question about rainbow darters in Maryland several weeks ago is a smaller
scale example of these dynamics. As Dave Neely pointed out, many Atlantic
drainage streams are relatively depauperate of species like darters. These
streams run basically west to east, and during the last several glaciations
became sub-Arctic for periods of time. There was no way for fishes to easily
retreat south ahead of the glaciers, so either they adapted or more likely
were extirpated. These streams would be slow to be recolonized, because fish
need direct water connections for easiest recolonization. If these streams
ran north-south, recolonization would probably be faster. A lot of plants on
the eastern seaboard had an easier time, because they were able to retreat
south ahead of the ice and survive in Georgia or Florida and then move north
again.
--Bruce Stallsmith
Huntsville, AL
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