Andrew V.,

The correlation of floras and faunas between eastern North America and
southeastern Asia (especially south China) is well-known. Travelers from
the US to Szechwan, for instance, have been delighted to see forest trees
similar to their own -- same genera, different species. The key is that
similar climates support similar plants and animals. If you transplant a
gingko from south China to France or Kentucky, it will grow.

For instance, there are several areas with Mediterranean climates (parts of
California, Chile, the Mediterranean basin, part of Cape Province in South
Africa, the area around Perth in Western Australia), and they all have
plants that look and act like chaparral, whether or not they are closely
related. In the case of California and Chile, many of the plants are in the
same genera, not present in between, and biogeographers scratch their heads
and talk bemusedly about the possibility of migrating birds transporting
seeds.

So some of the basic biogeographic problems are 'How did they get there?'
and 'Why aren't they everywhere that they could live?' The answers to these
questions are necessarily a matter of the details of Earth history -- what
happened and when. In the case of Northern temperate-zone floras (E North
America, S China, Europe), the plants were able to move from continent to
continent whenever the climate was relatively warm and the continents
connected by land bridges (or the seeds helped along by migrating birds).
The reason that Europe seems like the odd man out is that much of the
temperate zone was recently covered by a thick ice sheet, and most of the
remainder was too cold for growing warm-temperate trees. (The Mediterranean
area survived with relatively little change.) We have fossils that show
that many kinds of 'Amero-Chinese' trees became extinct in Europe during
the Ice Ages, e.g., Liriodendron (tulip tree), Magnolia, Ginkgo, Taxodium
(baldcypress), and I think Metasequoia. Meanwhile, Metasequoia and Gingko
became extinct in North America (although Sequoia, once very widespread,
survived in a few pockets), and others became extinct in China. So the
three north-temperate areas used to have even more similar floras a million
or more years ago.

The situation is similar with marine creatures such as horseshoe crabs
(limulids), which also have 'climatic' (temperature and salinity) barriers
as well as geographic barriers like the Panamanian isthmus. To
oversimplify, the faunas were once able to spread more easily because there
was an open band of ocean girdling the earth -- no blockages at Suez or
Panama -- and so, for instance, Caribbean and eastern Pacific faunas are
very similar, but different in almost every detail.

So there are broad patterns that mark the distribution of plants and
animals on earth, but a lot of what we want to know requires knowing
considerable detail. To the students out there, I'd say this is a powerful
argument for memorizing a lot of facts rather than just knowing 'where to
look them up' -- if you don't have the facts in your head, you may not make
the interesting correlations and see the bigger patterns.

Andrew R.

Andrew K. Rindsberg
Geological Survey of Alabama