[Taxacom] Dispersal... again

[Michael Heads]

Hi Karl,
?
Concerning?Clague's (1996) estimates of the former heights of the Hawaiian Islands, you wrote:
?
'that number is the maximum height attained, which doesn't take into account what is removed by
erosion'.  Agreed.
?
'Oahu and Kauai have a long way to go before submergence,
but they've already had most of their tops removed so that only the
core of the volcanic pipes remain at the highest points'.  Agreed.
?
'So if anything, Clague's 1996 underestimate is probably an *overestimate*
of the relative height between the original shoreline and the peak
at submergence'.?
?
Clague estimated the maximum heights of the 129 islands and seamounts based on their original areas, indicated?by the offshore breaks-in-slope. Larger area implies the island was higher. For example, he calculated that one of the Emperor seamounts reached 3190 m above sea level.?Low-lying Laysan was 1020 m. This makes sense so far. But using this method?he estimated that the maximum height of Mauna Kea was 3050 m, whereas its actual, current height is 4205 m.?He estimated Hualalai had a maximum height of?1040 m, but it is currently 2521 m. He also underestimated the heights of Kilauaea, Mauna Loa,?and E Maui. If his method could underestimate the heights of these islands/volcanoes by so much (up to >100%), it could also underestimate the heights of the former islands. This means the seamounts?could have lasted longer as islands than he thought. This means his main conclusion - that there was a period from 34-30 Ma when there were no islands - may
 not be correct.? 
?
Michael


Wellington, New Zealand.


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


________________________________
From: Karl Magnacca <kmagnacca at wesleyan.edu>
To: Michael Heads <michael.heads at yahoo.com>
Cc: "taxacom at mailman.nhm.ku.edu" <taxacom at mailman.nhm.ku.edu>
Sent: Friday, 28 October 2011 1:41 AM
Subject: Re: [Taxacom] Dispersal... again


On Wed, October 26, 2011 2:11 pm, Michael Heads wrote:
> Hi Karl,
>
> You don't need continuous land (e.g. a lost continent) for
> vicariance. For example, two metapopulations on an active island
> arc (surviving in situ on individually ephemeral islands) can be
> separated by rifting.

They can be, but it depends on the organism.? For many things that
have speciated on islands, even crossing a 50-mile channel between
islands is such a rare event that they don't exist as
metapopulations between oceanic islands.

> The dispersal model for Hawaii relies entirely on the suggestion
> that there was a period (33-29 Ma) in which no islands (Clague et
> al, 2009:1) or no high islands (Clague et al., 2009: 10) were
> emergent, but the calculation this was based on is dubious.

No, this is an interesting part of the story but irrelevant.? Even
granting the most wildly optimistic past scenario of islands
scattered across the Pacific, you're still left with having things
cross 500 miles of ocean from the nearest seamounts that could
possibly have ever been high islands with any significant diversity.
By my definition, that still counts as long-distance dispersal.

Also, the 2009 paper was based on coral growth, not the approximate
calculated height.? On top of that, even that number is the maximum
height attained, which doesn't take into account what is removed by
erosion.? Oahu and Kauai have a long way to go before submergence,
but they've already had most of their tops removed so that only the
core of the volcanic pipes remain at the highest points.? So if
anything, Clague's 1996 underestimate is probably an *overestimate*
of the relative height between the original shoreline and the peak
at submergence.

Karl