Disjunct Distribution Patterns

Certain present day anomalies in the distribution patterns of New Zealand species are thought by some to have been caused by these climatic fluctuations.

Wardle¹⁰⁸ noted that the northern North Island and northern and southern South Island regions have much higher numbers of endemic species (90-110) than the southern North Island and central South Island (25-30). The endemics of the northern North Island are mostly woody and lowland and those of the northern and southern South Island mostly herbaceous and alpine.

Wardle also observed that there are a number of species disjunct between the regions of high endemism, for example tanekaha (Phyllocladus trichomanoides) and Astelia trinervia between the northern regions of page 248the two islands, and Celmisia traversii and species of Nothofagus between the northern and southern South Island.

It is suggested that these anomalies have the same explanation. During the last glaciation the central South Island with its high mountains would have been more severely glaciated than the neighbouring regions. Its flora would have been more reduced with the local extinction of some species that survived to the north and south. The low endemism of the southern North Island is more difficult to explain, but a somewhat lower treeline at the present time than would be expected and some fossil evidence indicates that climatic conditions were relatively severe there during the last glaciation.

Recently McGlone²⁰⁸ has proposed a quite different explanation. He believes that the causes of these anomalous patterns are tectonic not climatic and must be sought much further back in time in the late Tertiary before the glacial/interglacial fluctuations had commenced. At this time much of the southern North Island had become submerged beneath the sea and this would explain the low endemism and some of the disjunctions. In the South Island the uplift of particularly high mountains in the central region would have greatly altered habitats there and 'as a result the once continuous, upland flora was split into northern and southern regions, where the rate of uplift was low and average age of surfaces older, and a rapidly uplifting central zone'.