New Caledonia: a very old Darwinian island?

Abstract

New Caledonia has generally been considered a continental island, the biota of which largely dates back to Gondwanan times owing to its geological origin and the presence of phylogenetic relicts. This view is contradicted by geological evidence indicating long Palaeocene and Eocene submersions and by recent biogeographic and phylogenetic studies, with molecular or geophysical dating placing the biota no older than the Oligocene. Phylogenetic relicts do not provide conclusive information in this respect, as their presence cannot be explained by simple hypotheses but requires assumption of many ad hoc extinction events. The implication of this new scenario is that all the New Caledonian biota colonized the island since 37 Ma Local richness can be explained by local radiation and adaptation after colonization but also by many dispersal events, often repeated within the same groups of organisms. Local microendemism is another remarkable feature of the biota. It seems to be related to recent speciation mediated by climate, orography, soil type and perhaps unbalanced biotic interactions created by colonization disharmonies. New Caledonia must be considered as a very old Darwinian island, a concept that offers many more fascinating opportunities of study.

Figure 1

(a) Distribution of ultramafic rocks (shaded areas) in New Caledonia. (b) Orography in New Caledonia showing several chains of mountains, peaking at more than 1600 m in the north and south.

Figure 2

A time scale for the major geological events including the emersion of the New Caledonia mainland, 37 Ma ago (horizontal dotted line), and estimated ages (horizontal bars) for New Caledonian clades according to the studies cited (confidence intervals are shown if provided by the authors). Smith et al. (2007) provided a confidence interval for the age of scincid lizards of the whole of Tasmantis, indicated here with a vertical dotted line (the age of the New Caledonian clade itself was not provided but is necessarily more recent). Note that all these ages inferred independently from geology have been found to post-date the emersion of the island.