[Taxacom] Shorter trans-oceanic dispersal of Nothofagus

Kenneth Kinman kennethkinman at webtv.net
Thu Jul 29 22:23:03 CDT 2010


Michael, Geoff, and others:
       The failure to disperse further might APPEAR to be simply due to
chance or "for no particular reason", but such a view may simply be due
to a lack of information or even a lack of scientific imagination to
figure out the reason (or both).            
       More so than climate, I suspect that limits on Nothofagus
distribution have been (perhaps still are) most likely due to particular
insect distributions.  It could be a combination of the presence of
insects that are bad for Nothofagus and/or even the absence of insects
that are good for Nothofagus.  Regarding the former, nothing seems to
wipe out a population of trees faster than an insect pest getting the
upper hand, especially if its numbers dramatically increase for some
reason.  Insects can rapidly increase their numbers so much faster than
trees can.  It may have been even more so in the past, before various
angiosperm taxa evolved ways to fight such threats.  Insects evolved
before angiosperms, and therefore insects have probably more often taken
the offensive (attacking) and angiosperms perhaps more often reacting to
it defensively.  When insects and angiosperms become interdependent, it
can be great for both, but in other cases, insects can easily get the
upperhand.  What may appear to be suitable habitat for a tree may
actually be unsuitable if particular insect pests are present.       
           ---------Ken Kinman                   
=====================================
Michael Heads wrote:
Dear Geoff and colleagues, 
      Considering the rather wide ecological range that Nothofagus
occupies it seems unlikely that climatic factors would keep
Nothofagus out of Stewart I., montane Borneo or the central-Northern
Andes. The vast majority of taxa do not occupy all the ecologically
suitable habitat that exists, e.g. hummingbirds endemic east of the
Andes in very similar habitat to sister species west of the
Andes. This general phenomenon suggests that after an early period of
widespread migrations (e.g. with the maximum marine transgressions in
the Cretaceous along with rifting and much new coastline) many taxa
have stopped migrating, differentiated, and not (yet) entered into a
new phase of mobilism. The worldwide Fagales (in which Nothofagus =
Nothofagaceae is basal), overlaps more or less completely with its
sister group, the worldwide Cucurbitales. There must have been a phase
of massive range expansion of both orders after they diverged (unless
you accept sympatric differentiation), but before the modern families
differentiated (as groups such as Nothofagus are largely allopatric with
their relatives).  
   The key phrase is 'stopped migrating' . What caused this
cessation, or at least considerable slowing, of dispersal? In the
dispersal model it is simply chance - after the founder 'disperses
across a barrier' (a contradictory concept) dispersal stops or slows,
but for no particular reason. In a vicariance model the cessation of
dispersal is due to events in Earth history, climate change
etc.     
Michael Heads 





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