[Taxacom] New Caledonia as a classic lesson in dispersal

[John Grehan]

The New Caledonia paper raises quite a few issues of general relevance for
biogeography. One of these concerns claims that 'short distance' dispersal
involve a more complex and speculative combination of low probability
compared with long distance dispersal. Ironically (there is quite a bit of
irony in this paper) Heads points out that “‘short-distance’ dispersal is
neither complex nor speculative, and is observed in almost all organisms
using their normal means of dispersal (unlike the unique long distance
dispersals used to explain allopatric speciation). He noted that Grandcolas
(2017) and Nattier et al. (2017) both rejected normal, short-distance
dispersal (and, presumably, the idea of metapopulations) around New
Caledonia as ‘ad hoc scenarios’, but at the same time they explained the
biogeography of New Caledonia by long-distance dispersal.

Another critically important issue for biogeography is the concept of
metapopulation persistence, Critical because it represents a process by
which taxa persist despite landscapes emerging and submerging. It is the
very quality of mobility by which organisms are able to persist in within a
dynamic geology and topography (essentially Croizat’s concept of life as
the uppermost geological layer). Heads points out that Nattier et al.’s
(2017, p. 2) comparison of metapopulation theory with the ‘island-hopping
scenarios’ for  Wallace (1881)  et al ignored the fact that the concept of
island-hopping was unidirectional migration allowing very rare dispersal
events across ocean basins (by non-standard means and with speciation)
which is a very different process from metapopulation survival in situ,
without speciation, and caused by regular dispersal by normal ecological
means.

Ironically (again) Nattier et al. (2017) accepted that the process might
explain some New Caledonia groups with respect to six molecular clock
studies proposing that New Caledonia clades and New Caledonia + Pacific
islands clades that were older than 37 Ma (the supposed age of subaerial
New Caledonia, the oldest island in the region). Nattier et al. took things
both ways, either appealing to older lineages that long ago occurred
somewhere else where they went extinct, but survived by hopping to New
Caledonia - the details of which “remain to be established”. But then
Nattier et al. (2017) also suggested that the six older groups ‘evolved by
hopping/local dispersal mechanisms between terrestrial lands in the region”
and have extinct members either on vanished islands or nearby continents.
Heads points out that this view is actually consistent with metapopulation
survival in the New Caledonia region, and with the idea that former islands
(some still represented by seamounts) are critical in understanding
contemporary biogeography. So this is a classic case where panbiogeographic
models are condemned on the one hand and then used on the other. This is
the kind of confusion that is often found in biogeographers who look to
explain speciation by chance dispersal rather than understanding the role
of ecological dispersal in the origin, establishment, and persistence of
populations rather than as a mechanism for speciation.

John Grehan

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