[Taxacom] Shorter trans-oceanic dispersal of Nothofagus

Michael Heads michael.heads at yahoo.com
Thu Jul 29 05:23:29 CDT 2010

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

Wellington, New Zealand.

My papers on biogeography are at: http://tiny.cc/RiUE0

--- On Thu, 29/7/10, Geoffrey Read <gread at actrix.gen.nz> wrote:

From: Geoffrey Read <gread at actrix.gen.nz>
Subject: Re: [Taxacom] Shorter trans-oceanic dispersal of Nothofagus
To: Taxacom at mailman.nhm.ku.edu
Received: Thursday, 29 July, 2010, 7:44 PM

>>> On 28/07/2010 at 5:51 p.m., Michael Heads  wrote:

>  [...] Some relevant facts are: Nothofagus grows down to sea-level in SW
> Zealand, and this area has one of the highest rainfalls in the world.
> Flooding, landslides, storms that break off large branches etc. happen
> all the time. And yet on Stewart Island (the third main island of New
> Zealand), just 25 km away across Foveaux Strait, Nothofagus is absent
> and probably never grew there (McGlone & Wilson, New Zealand J Botany
> 34: 369. 1996). The whole island comprises excellent Nothofagus habitat.
> If trans-oceanic dispersal is the key process, why has it not worked
> here?

Just to note that Foveaux Strait was dry for 10,000 yrs or so during the
glacial maximum, prior to the Holocene period considered by Wilson &
McGlone, so Nothofagus menziesii, if it could have survived the climate
then, could in theory have marched across the hills year by year onto
Stewart Island. No need for water voyages.  But it apparently didn't. Why
not? Climate likely wasn't suitable and it hadn't yet become present close
by on the mainland?



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