Wednesday, 21 March 2012

Dispersal of crickets to New Caledonia

Eneopterine cricket
An eneopterine cricket in the genus Cardiodactylus. Photo courtesy of Guido Bohne

New Caledonia is one of the largest islands in the Pacific, being exceeded only by New Guinea, New Britain, and the main islands of New Zealand. Like New Zealand, the composition of the biota, which includes several relict groups such as the Amborella shrub and the Kagu (Rhynochetus jubatus), has lead many to believe that the island is a fragment of Gondwana, with a long biological history. Also like New Zealand, this story has been questioned in recent years by geological evidence that the island underwent submergence at various points in its history, and that the biota is a result of long-distance dispersal. Cue a classic dispersal-vicariance stoush that is a trademark of biogeographic discussion.

In the dispersal corner is Romain Nattier and coauthors, who present evidence of a "recent" arrival in New Caledonia of the eneopterine crickets. This subfamily of crickets have a wide range throughout several islands of the Pacific, South-East Asia and South America. Their analysis of four genes show that the New Caledonian eneopterine crickets are most closely related to species on other Melanesian islands, and that they've likely been in New Caledonia for "only" 5–16 million years. They then use this evidence to indicate argue against an ancient presence of New Caledonia.

I've got no quarrel with their conclusions regarding this group of crickets. The evidence for a recent arrival of these guys seems pretty clear. However, I do wonder why the authors chose this group of insects to test their hypothesis. The distribution of the non-New Caledonian species suggests that this group is fairly vagile. Their presence in archipelagoes such as Vanuatu, a young island group by most people's standards, hints at their dispersal ability. Much more convincing with regard to their New Caledonia drowning hypothesis would be one of the groups that is less likely to move around as much. Of course, with this criteria, the taxon of choice would be likely to be one of the relict species—which by definition are species poor and with few close relatives—leading to a biased outcome.

I was surprised that none of the Australian representatives were closely related to the New Caledonian crickets. An Australia–New Caledonia link is a fairly common pattern in New Caledonian biogeography, but it hasn't held up in this instance. I was also surprised by the sister-taxon relationship between a genus in Fiji and Samoa, and a couple of genera in Central and South America. Those sorts of relationships seem fairly strange, but not too much stock can be placed on it without knowing more details about the group, and the sampling regime used in the study.

As always, this is not the last word on the subject of the origin of New Caledonia and its biota. It isn't even the definitive on the evolution of this group of crickets. This is one piece of the puzzle that is New Caledonia's biota, and one that will illuminate further research on the natural history of the island, as well as being a valuable addition to the literature of eneopterine crickets.

Nattier R, Robillard T, Desutter-Grandcolas L, Couloux A, Grandcolas P. 2011. Older than New Caledonia emergence? A molecular phylogenetic study of the eneopterine crickets (Orthoptera: Grylloidea). Journal of Biogeography 38: 2195–2209

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