The Alpine Fault biogeographic hypothesis revisited

The New Zealand Alpine Fault is a major tectonic feature located on the mutual boundary of the Australian and Pacific Plates which is hypothesized to have undergone some 470 km of right-lateral displacement. The Nelson and Westland provinces have moved north-east relative to the rest of the South Island. Many plant and animal taxa show a conspicuous distribution gap in the central South Island, and traditionally this has been explained by glacial extirpation of the central populations. However, the gap is usually filled by a closely related taxon. Furthermore, the taxa involved occupy a wide ecological range, and include intertidal algae, shorefishes, lowland trees, and alpine herbs and insects that thrive around glaciers. The Alpine Fault biogeographic hypothesis [Heads, M., 1998a. Biogeographic disjunction along the Alpine Fault, New Zealand. Biol. J. Linn. Soc. 63, 161-176; Heads, M., 1998b.Coprosma decurva (Rubiaceae), a new species from New Zealand. NZ J. Bot. 36, 65-69.], based on track and phylogenetic analyses, proposes that the disjunction has instead been caused by movement on the fault pulling apart plant and animal populations. Wallis and Trewick (2001) [Wallis, G.P., Trewick, S.A., 2001. Finding fault with vicariance: a critique of Heads (1998). Syst. Biol. 50, 602-609.] provided a critique of this idea and pointed out what they felt were nine problems with it. These problems are answered here and shown to result from Wallis and Trewick's misunderstanding of aspects of geology and biology involved in the hypothesis. In particular, they have confused the age of inception of the fault with the age of movement along it, and, by neglecting the related central taxa in the "gap", have assumed the biogeographic hypothesis to be an uninformative two-area statement, whereas in fact it is an informative three- or four-area statement.