Forest Vines to Snow Tussocks: The Story of New Zealand Plants
Immigrants from The Northern Hemisphere
Immigrants from The Northern Hemisphere
However, if it is true that only a minor part of the New Zealand alpine flora is derived from plants that were already present here, where did the majority of the alpine species come from? Raven148 suggests that many of them are derived from ancestors which reached New Zealand by long distance dispersal after the mountains had formed here. He believes that the ultimate source of the genera to which these ancestral colonisers belonged was temperate Asia and that they were only able to reach Australasia in recent geological times after mountains, contemporary with those of New Zealand, had formed in south east Asia, Indonesia, New Guinea and eastern Australia. Species of these genera 'mountain hopped' across the tropics to eastern Australia and then across to New Zealand by the agency of the strong, frequent, westerly winds of the 'Roaring-Forties'. Raven explains the fact that some of the genera concerned have many more species in New Zealand than in page 198eastern Australia by the higher, ecologically more diverse mountains in New Zealand providing a setting for extensive radiation of the immigrants after their arrival from Australia. In view of the direction and strength of the prevailing winds he regards migration from New Zealand to Australia as improbable.
Some of the genera that Raven believes reached New Zealand in this way are represented here by only one or a few species, for example, Anemone, Geranium, Linum, Stellaria and Viola, but others have dozens of species and include Epilobium, Ranunculus, Myosotis, the grass genus Poa, the sedge Carex and the rush genus Juncus. Other large genera are not known in north temperate regions, but at least some of their species were originally placed in northern genera from which they may have evolved within Australasia, for example, Celmisia (Aster), Anisotome and Aciphylla (Ligusticum), and Hebe (Veronica).
In a response to Raven's theory Wardle149 claims that Raven underestimates the possibility of plants dispersing from New Zealand to Australia. He points out that vigorous easterly winds can flow from New Zealand to Australia when a tropical depression meets an anticyclone over New Zealand and also notes that there are several bird species migrating annually between New Zealand and Australia, which might serve as seed transporters. There is also the possibility of direct transport between New Zealand and northern Asia and north-west America by a number of migratory birds. A few mountain species of grasses, which are the same as or closely related to north temperate species may have arrived in this way.
Wardle further states that it is necessary to know something about patterns of evolution within a genus before one can hypothesise about directions of migration. If for instance there were in a genus a mixture of primitive and advanced species in New Zealand, but only a few advanced species in Australia, then it is likely that the latter would have been derived from New Zealand.
The patterns in Epilobium indicate that Raven's interpretation could be correct with the exception of two species in Australia which may have been derived from New Zealand. However, since Wardle's article, abundant fossil pollen of Epilobium has been found in New Zealand of Oligocene age, a period long before the formation of the present mountains. So if Epilobium persisted in New Zealand until colder climates developed, then some at least of the present species may derive page 199from that early stock, though others may have come from more recent colonisations.
In Ranunculus the affinities of the lowland and montane species are with Australia, but the alpines are most like species in South America suggesting a different origin.
Wardle suggests that an ultimate South American derivation is also possible for the snow grass genus Chionochloa, which exhibits cytological and other similarities with Cortaderia (which includes Pampas grass) of South America and New Zealand. Similarly most of the New Zealand species of Plantago belong to an otherwise South American section of the genus and so may have been derived from there rather than from temperate Asia via Australia.