Regional patterns of plant species richness in southern New Zealand

We examine the proposition that spatial heterogeneity, as modelled by land area, productivity-influencing climatic variables, and elevation, is a useful predictor of regional patterns of species richness in 56 regions of southern New Zealand stretching from mid Canterbury to Campbell Island. We also examine whether regional patterns of taxonomic richness support the concept of a Late Cenozoic radiation of upland floras and that this was sourced from just a few genera in a few widespread to cosmopolitan families. We found several contrasting geographical patterns in taxonomic richness. Several mainly mountain regions just east of the main divide in western Canterbury, Otago, and Southland are unusually species-rich, but not genus- or family-rich. Conversely, several coastal regions of eastern South Island are unusually genus- and family-rich, but not species-rich. Four geographically distinct groups of regions share relatively species-, genus-, and family-depauperate floras: maritime-influenced regions of Fiordland, the Chatham and subantarctic islands, inland and lowland basins of Otago and Southland, and eastern Otago's block mountains. Various combinations of elevation range, climatic variability, and land area explain 54% to 61% of the variability in richness of species, genera, families, and life-form groups. Mean daily temperature of the coldest month appears the strongest climatic variable at depressing floristic richness, although moderate degrees of continentality of climate increase it. We conclude that both propositions are supported by the present study. Databases of regional-scale patterns of plant species richness should be useful for selecting mainland islands for ecosystem restoration, estimating ecosystem or guild diversity, and testing the ecological distinctiveness of ecological regions and districts.