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Understanding how global plant biodiversity arose and is maintained requires an understanding of plant species radiation. This is a phenomenon in which genetically similar, but morphologically and ecologically diverse species arise and go extinct over short periods of geological time. These radiations provide unparalleled opportunities for studying how evolution works. Studies of macro- and micro-fossil record (1-3), genetics of plant development (4, 5), molecular ecology and systematics (6-15) converge in helping to explain how explosive radiations have occurred in certain places and times in the past. New Zealand is one of those places. The radiation is happening now, and there is at present great potential to study to it.

Recent findings in New Zealand and overseas indicate that plant evolution is far more rapid and dynamic than generally appreciated. It has a history intimately linked with local geological and global climate change (1,16-22). This realization has stimulated scientific thinking. Answers are being sought to questions including:

When and why does species radiation occur? What is its relationship to geological and climatic change? What genetic processes lead to morphological, physiological and ecological diversification? How important is the underlying geology (and associated soils) as an ecological driver for diversification? What roles do refugia play in influencing species distributions and diversification? What role do different plant breeding systems have on evolutionary dynamics? Why have some plant groups radiated and diversified more than others; do they respond differently to evolutionary selection pressures? What is the evolutionary and ecological significance of hybridization during radiation? How does the relative coding capacity of genomes affect the potential of lineages to evolve? How plastic are plant genomes? How can genetic and genome information be integrated into biodiversity conservation strategies? Can endemic plants react to the new evolutionary pressures generated by weeds, pests, and climatic change?

 

The importance of New Zealand for understanding plant biodiversity

Volcanic islands have long been useful for scientists in the development of evolutionary theory. Geologically young and tractable for study, archipelagoes such as the Hawaiian islands have provided important models for understanding plant biodiversity (2, 6-9, 12, 14, 15, 23-26). However, a shortcoming of volcanic islands as model systems for studying evolution is that they are short-lived; they have virtually no fossil records; and many classes of plants - such as conifers, are absent.

The New Zealand archipelago represents a more complex model system. It is home to approximately 2200 native plants - a flora twice the size of Hawaii. New Zealand has an 'oceanic island character' which parallels island systems such as Hawaii, Juan Fernandez, Canary and the Galapagos Islands. However, New Zealand is not an island. It is a continental remnant and thus more relevant to the world as a whole. In contrast to volcanic islands, New Zealand has an excellent plant fossil record - extending back to Gondwana. It is habitat-rich with latitudinal and climatic zones ranging from tropical to alpine and sub-antarctic. Unlike studying the phenomenon of radiation on volcanic islands (which typically means tracking the pattern of dispersion between islands), studying the phenomenon in New Zealand also means evaluating the impact of novel habitats (e.g. alpine, dry land, high rainfall, scree) on plant evolution. This includes studying plant groups that have radiated extensively (e.g. Ranunculus, Epilobium, Hebe, Celmisia, Craspedia, Dracophyllum, Asplenium, Blechnum) as well as studying groups that have survived for long periods of geological time without radiating (e.g. Agathis, Knightia, Pseudowintera, Dacrydium ).

Originating from an ancient continental land fragment which became geographically isolated in the South Western Pacific more than 65 million years ago, New Zealand was first settled by Polynesians perhaps less than 1000 years ago (27,28). A consequence of this is that some habitats, particularly alpine, are little affected by human activities - thus the study of these habitats promises considerable insight into ecological and evolutionary processes important in alpine radiation. Further, many lowland habitats in New Zealand have been significantly impacted by human settlement, and the long term effects of these ecological disturbances can more easily be studied in New Zealand than in larger, more complex and less isolated continental landscapes. Thus the human-disturbed biotic systems of New Zealand are in some ways as interesting for study as the more pristine habitats.

Recently, the unique geology and floristic diversity of New Zealand has caught the attention of film makers with its magic captured in film epics such as “Walking with Dinosaurs” (produced by the BBC) and Tolkien's “Lord of the Rings” (produced by New Line Cinema).

Behind the scenes: BBC 'Walking with Dinosaurs'. Filmed in an ancient landscape? Nothofagus forests on the Volcanic Plateau; with 'Mt Doom' from 'Lord of the Rings' in the background Extinct mega fauna in New Zealand

Behind the scenes: BBC 'Walking with Dinosaurs'. Filmed in an ancient landscape?

Nothofagus forests on the Volcanic Plateau; with 'Mt Doom' from 'Lord of the Rings' in the background.

Extinct mega fauna in New Zealand.

However, the potential of New Zealand for understanding plant evolution has been appreciated for much longer (29), as indicated by comments by the noted biologist Gareth Nelson from the American Natural History Museum.

“With regard to general problems of biogeography, the biota of New Zealand has been, perhaps, the most important of any in the world. It has figured prominently in all discussions of austral biogeography, and all notable authorities have felt obliged to explain its history: explain New Zealand and the world falls into place around it.” -- Nelson, G. (1975)(29)

 

 

Understanding the New Zealand flora

In recent years, molecular descriptions of the New Zealand flora have changed our understanding of its plant biodiversity and origins. These studies have shed new light on the traditional view of the New Zealand biota. New Zealand has long been thought of as a "Moa's Ark" home to relic species undergoing slow changes over long periods of time. However, we now realise that this view is far too simplistic, and a more dynamic, almost tumultuous view of New Zealand's biodiversity is emerging (30).

Some plant lineages in New Zealand are very old, possibly ancient Gondwanan relics. Agathis (or Kauri as it is commonly known) is our best example of this (31). In contrast, other species are clearly more recent immigrants to New Zealand - and despite our current poor understanding of the mechanisms involved, these plants clearly arrived in New Zealand by transoceanic dispersal (30). Fossil records suggest trans-Tasman Sea dispersals from Australia to New Zealand throughout the Tertiary period. Over this time many plant groups first appear in the fossil record in Australia and then subsequently in New Zealand. Presumably seeds were dispersed passively or actively eastward with prevailing circumpolar winds and sea currents (32,33).

Molecular data, like the fossil evidence, record many events of dispersal during the Tertiary, with the late Tertiary being a particularly important period for origins of the modern flora of New Zealand. DNA studies suggest that genera such as the alpine Ranunculus (buttercups) established with the onset of Pliocene mountain building (by 5 mya) in New Zealand and soon after began to radiate extensively into many distinctive habitats (12). During radiation and range expansion within New Zealand, species were dispersed from New Zealand to other Southern Hemisphere lands. This pattern of late Tertiary arrival, radiation and dispersion from New Zealand has been repeatedly inferred in the study of numerous endemic plant groups (30, 34, 35).

Until the findings of recent molecular investigations, the geological age of species-rich groups in New Zealand had been uncertain. Doubt always existed that the lack of ancient presence in the fossil record for certain plant groups might be misleading. However, evidence from fossil studies (1, 3, 17, 36-38) and studies of genetic diversity (30); are consistent in suggesting that species radiations are geologically young in New Zealand. Many seem to have occurred sometime during the late Tertiary to late Quaternary period - within the last 5 million years - a time of dramatic geological and climatic change in New Zealand (17,39,40).

Study of the New Zealand flora provides a model system for understanding plant evolutionary and ecological processes as they operate in a more global context. Taking advantage of this opportunity requires interdisciplinary research. It involves biological studies in palynology, morphological, ecological, physiological and genetic variation, and requires the collaboration of biologists, mathematicians, physicists, computer scientists, climatologists and geologists.

 

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