[OGD] Re: orchids Digest V3 #5

  • From: Oliver Sparrow <director@xxxxxxxxxxx>
  • To: orchids@xxxxxxxxxxxxx
  • Date: Wed, 06 Jan 2010 10:29:02 +0000


>So in recent times, both India and Africa have had a great effect on

There are actually two, rival interpretations of why Asian and African plant
populations have so much in common. We are snow-bound today, so here is an
overview. On re-reading, rather a *long* overview. 

The block that is India / Madagascar (and a great sunken bit, of which the
Seychelles the only bit above sea level) spilt off the super-continent
together. Madagascar didn't get very far, the Seychelles got half way and
India crashed into the hapless little Tethys sea, the bed of which is now
Tibet. This quite incidentally creating the monsoon wet-dry sequence at the
same time, and by weathering the newly-created Himalayas sequestering carbon
from the atmosphere and so triggered the long sequence of ice ages that began
about 30 MYBP. But I digress. Rocks in the Seychelles and in Sri Lanka exactly
match the granite formations that are found in Zimbabwe.

The story then runs: African genera are freighted across, and some - such as
Ansellia - show up in Asia as Grammatophyllum, Cymbidium and the rest of that
group, upward pointing roots and all. Acampe rigida made the Asia trip
unchanged, as did Calanthe masuca; and so on. That is quite a tall order, for
the following reasons. 

The first rupture happened about 110 MYBP, and the collision with Asian, 60
MYBP, so the Indian freight car was isolated for around 60 million years.
However, flowering plants were in their infancy before the CT "death of the
dinosaurs" event 64 MYBP, with - allegedly - only things like magnolias and
water lilies around. That makes the 'orchid freight' story a bit ... strange.
(But I am also concerned to explain how things as taxonomically different as
lilies and magnolias were around when - allegedly - all the stuff in the
middle was not. )

The other story is to do with ice ages and changing forest patterns. Broadly,
interglacial climate - as today - has air ascend over the equatorial and the
temperate regions, and descend bitterly dry over the tropics and the poles.
Thus, a belt of desert that follows the Northern tropics strongly and the
Southern one rather less so, perhaps due to the extent of the Southern oceans.
And hence the Sahara, the dry Middle East and US-Mexico border region. 

During ice ages, of which there have been plenty, this patterns shifts North,
and the dry belt around the Tropics becomes wet. The Sahara was covered in
savannah until the current bubble of warmth, the Holocene. I look out of my
window at Windsor castle, perched on patch of glacial moraine left there about
8000 years ago, when all of this was ice. Scotland was then under a kilometre
of ice. The alps were being ground down to make the fertile plains of Germany.
Glaciers came across the US to the Mason-Dixon line. 

The resulting green band connected Africa with Asia. The ocean shores were
much lower - perhaps 60 metres lower. The sea is, of course, colder during ice
ages than it is today, and so it contracts in volume. Equally, a lot of water
sits on the land as glaciers, which melt in warm times, as ours are continuing
to do today.  Consequently, 12,000 years ago, all of the Indonesian islands
were a single land mass and Australia was separated from them by a relatively
narrow channel. The Red sea was dry, the Mediterranean a vast cold salt plain.
Forest covered the entire Indonesian plain, a boundary with the Northern Asian
savannah which is today marked by the Wallace line, where species change
abruptly. It is therefore easy to see how plants could exchange between Africa
and India. Madagascar was, (I think) still isolated by sea, but people came
and went and (orchid) seeds would blow across the straight. 

Which of these models is correct - or anyway, dominant - is not clear. The
Americas split away at roughly the same time. Nigerian and Suriname oil is
very much a part of the same original province, for example. However, the
relatedness of plants and animals is not at all clear, as compared to Asia.
Even the fossil animals are different. My own guess is, therefore, that the
land bridge model is the correct one. Madagascar is like but separate from
Africa because of its geographical proximity and modest isolation, and not
through ancient continental rupture. 

Oliver Sparrow
+44 (0)1628 823187

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