Wednesday, 4 April 2012

Panbiogeography of New Caledonia

Map of New Caledonia and the Loyalty Islands. Courtesy of Eric Gaba.
Two weeks ago, I reviewed Nattier et al's paper concerning the dispersal of the eneopterine crickets to New Caledonia. This week I discuss the opposing view of Michael Heads, who vociferously promotes the idea that the biota of New Caledonia has its origins in a vicariance framework; i.e. plate tectonic processes have had more of an influence on organism distribution than chance dispersal processes. In particular, Heads is a practicioner of the method known as panbiogeography—a method that emphasises the importance of recurring patterns in the distribution of organisms.

After giving an overview of New Caledonian geology, Heads discusses the various distributional patterns displayed by a variety of taxa in New Caledonian mainland, including the Loyalty Islands. He identifies 10 primary patterns, which can be broadly summarised as restricted to the Loyalty Islands; shared between the Loyalty Islands and Grande Terre; and distributions corresponding to the geology of Grande Terre. Of particular note is his observation that the strange shrub Amborella is restricted to central Grande Terre, on what Heads calls the basement terranes.

I enjoy reading Head's papers. His perspective is an interesting one, his promotion of mapping distributions and having an understanding of geological processes is important and his papers are full of fascinating examples. However, I do see a something of a contradiction in some of his views. He's a proponent of the metapopulation theory, whereby organisms jump between islands that are emerging and disappearing as part of island building processes, resulting in organisms having a longer evolutionary history than the islands that they currently inhabit. I don't have problems with that. However, once land gets accreted, his explanations rely on organisms remaining on those terranes, and not moving far from them at all. The combination of these two views sits somewhat uneasily with me.

Biogeography is a fascinating subject. What I also find amazing is that debates regarding biogeographic processes and methods become incredibly passionate. Panbiogeography is one of those sub-disciplines that is fiercely defended by its proponents and viciously denigrated by its critics. I don't count myself in either camp, preferring to take the useful bits out of any research and always bearing in mind that our perception of the past will always be incomplete, and that discussions regarding the past should be conducting in the light of that fact.

Heads M. 2008. Panbiogeography of New Caledonia, south-west Pacific: basal angiosperms on basement terranes, ultramafic endemics inherited from volcanic island arcs and old taxa endemic to young islands. Journal of Biogeography 35: 2153–2175

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