07 July 2010

The origin of the Chatham Islands’ flora

The Chatham Island archipelago lies over 800 km to the east of mainland New Zealand. There are two major islands, Chatham and Pitt, and several smaller islands. Humans have lived on the islands for a few hundred years. There is a good level of diversity of species in the archipelago. For example, there are about 410 plant species, subspecies and varieties present although the vast majority are the same as mainland New Zealand species. There are 36 endemic plant species (species only found on the Chathams), including 17 trees and shrubs and 11 herb species. A recent study led by Peter Heenan (Landcare Research), with Anthony Mitchell (University of Otago), Peter de Lange (Department of Conservation), Jeanette Keeling (University of Auckland) and Adrian Paterson (Lincoln University) has used molecular methods to look at the evolutionary history of these endemic plant groups. Their work has now been published in the NZ Journal of Botany. In this study the group analysed 35 endemic species and their separate closest relatives using information from several gene regions to estimate evolutionary relationships and time since last common ancestor for each group.

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Not surprisingly, the vast majority of species had their closest relatives on the New Zealand mainland. It had been argued that most species colonising the Chathams would have originally come from northern New Zealand but this was not supported. Rather, closely related species were often those that were widespread around the mainland, perhaps indicating their ability to colonise successfully. Most of the speciation of Chatham species has occurred as allopatric speciation where the colonising population has since become isolated from mainland populations. Only six species pairs appear to have arisen within the Chathams themselves (from the genera Aciphylla, Coprosma, Hebe, and Olearia) and this speciation may be driven by niche differentiation into wet and dry habitats. These results are all consistent with the isolation of the Chatham Islands and the predominant winds and currents moving from the west (New Zealand) to east (Chathams) making it easy to colonise the Chathams but difficult to get back to the mainland.

The age of the endemic Chatham flora was investigated using molecular clocks. First, 14 species showed virtually no DNA divergence when compared to their mainland closest relative. This suggests that these Chatham species are recent arrivals. Second, 19 species showed low levels of divergence (less that 1%) which indicated a relatively recent colonisation of the Chathams over the last 3 million years. Finally, four species had much higher levels of divergence that implied 4-7 million years since the last common ancestor.

These results fit well with what we know about the geology of the Chatham Islands. The Chathams have a long history. They were part of Gondwanaland and then Zealandia (as seen by terrestrial dinosaur bones found on the islands). However, most of the last 60 million years have been spent underwater with short periods of volcanic island building. In recent times, the Mangere volcano produced land about 6 million years ago which had largely sunk again by 4 mya. The Rangitihi volcanics raised land about 2-3 mya and land has persisted since then. The plant evidence certainly supports this idea of most plant species arriving in a newly emergent land over the last two million years. Coupled with the fact that about 350 plant taxa are identical to mainland species, these findings support the idea of a young flora and the hint at the tremendous power of long distance dispersal and colonisation to establish diverse biotas on isolated oceanic islands.


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