Goodbye Gondwana? New Zealand biogeography, geology, and the problem of circularity.

Vicariance is potentially a key biogeographic process (Rosen, 1978) that physically disrupts populations— typically through geological upheaval—and leads to genetic divergence and speciation (Avise, 2000). One of the most dramatic and best-known cases of vicariance is the perceived role of plate tectonics in mediating the widespread “Gondwanan” distribution of taxa currently limited to southern landmasses such as Africa, South America, Australia, New Zealand, and New Caledonia. Following the general acceptance of plate tectonic theory, vicariance biogeographers (e.g. Rosen, 1978; Nelson and Ladiges, 2001; Humphries, 2000; Ebach et al., 2003) have explained the wide southern distributions of ratite birds, freshwater fishes, and southern beeches, for example, as essentially passive phenomena shaped by geology. Vicariant biogeographic inferences should ideally be based on a combination of biological and geological information. When such distinct fields of scientific research intersect, however, there is potential for associated “multidisciplinary” inferences to be clouded by circular reasoning. Bishop (1995) noted such a problem when considering geological versus biological evidence for river capture, a geomorphological process thought to effect vicariant isolation of freshwater-limited taxa (Mayden, 1988; Waters et al., 2001). To demonstrate a causal relationship between these geological and biological phenomena (e.g. cladogenesis of freshwater fish lineages mediated by vicariance), Bishop (1995) argued that geomorphological and organismal evidence should be assessed independently. If not, he explained, there is potential to invoke a geologically-mediated process which may not have occurred. Although a few studies of freshwater vicariance have indeed examined geological and biotic data independently (e.g. Mayden, 1988; Waters et al., 2001; Burridge et al., 2006), it is more common to merely invoke a geological explanation a posteriori on the basis of biological data (e.g. McDowall, 1990; Musyl and Keenan, 1992; Waters et al., 1994; Gollmann et al., 1997; Hurwood and Hughes, 1998; Engelbrecht et al., 2000; Kreiser et al., 2001). In many of the above cases that lack independent geological analysis, there are plausible biogeographic alternatives to drainage rearrangement, including migration across flooded divides, aridification, or lowland/marine dispersal (Bishop, 1995). More broadly, a similar tendency towards circularity may apply to biogeographic inference at the intercontinental scale. The isolated landmass of New Zealand (NZ), 2000 km east of Australia, presents a fascinating system for biogeographers, as a variety of biological and geological processes could potentially explain the origin of its unique biota (e.g. Cooper and Millener, 1993; Pole, 1994). We suggest that NZ biogeography represents another case in which geological and biological explanations apparently lack independence. Specifically, we argue that the “Gondwanan” ancestry typically ascribed to components of NZ’s biota (Cooper and Millener, 1993; Bellamy et al., 1990) lacks geological support. Instead, the geological “evidence” has largely been driven by the general assumption that NZ has a Gondwanan biota (Fleming, 1962, 1979), which results in circular logic. We therefore argue that the NZ-Gondwana story has achieved a level of prominence, driven by this circularity, where the whole is considerably greater than the sum of the parts.