Vegetation history of Tawhiti Rahi helps to inform management of degraded offshore islands
Wednesday 25 Sep 2013
Conservation palaeoecology is increasingly being recognised as a tool that can help to inform both restoration and conservation of island reserves by providing prehuman vegetation baselines.
Islands play a key role globally in the conservation of endemic species. Many island reserves have been highly modified since human colonisation, and their restoration and management usually occur without knowledge of their prehuman state. However, conservation palaeoecology is increasingly being recognised as a tool that can help to inform both restoration and conservation of island reserves by providing prehuman vegetation baselines.
Many of New Zealand’s mammal-free offshore islands are focal points for the conservation of biological diversity and, like many islands in the Polynesian region, were deforested following initial human settlement. Current efforts to restore, replant and manage offshore island reserves are guided either by historic vegetation descriptions or the current presence of native species on forested islands. However Janet and her colleagues have just published a paper that shows this may be misguided – significantly so for the Poor Knights.
The Poor Knights Islands (25-30k east of Whangarei) home to endemic fauna and flora (the Islands have been separated from the North Island for at least 2 million years). They have no introduced mammalian predators, including rats, and are the only known breeding site for Buller’s Shearwater. Ngatiwai are kaitiaki (guardians) of the islands, which they declared tapu (sacred covenant) in the early 1800s after being invaded by northern iwi, which prohibited further habitation and gardening on the islands. Gardens and other signs of former habitation have long grown over in a succession of vegetation, which has culminated in the current dominance of pohutukawa and other native flowering trees – vegetation that is generally assumed to be climax forest and which is similar to that found on many other small, forested, offshore islands in northern New Zealand.
Tawhiti Rahi is the largest of the Islands (163 ha, maximum elevation 191 m) and has several shallow valley systems with ephemeral streams. Janet and colleagues from Ngatiwai and the Department of Conservation collected a soil core from the head of one of these streams. While this core was only 85 cm long, it produced a 2000-year record of vegetation change that began >1200 years before human settlement and spanned 550 years of human occupation and the last 180 years since Ngatiwai permanently lived on the island. The analyses of pollen and ancient plant DNA in the soil core revealed that, over this time, a near-complete species turnover occurred and two once-dominant forest species (a conifer and a palm tree) became extinct on the island. There is no modern equivalent of the prehuman forests on any northern New Zealand island yet the analyses provide clear evidence for conifer- and palm-rich forests in the prehuman era. Obviously this challenges the assumption that what currently exists is climax forest – at least not the climax forest that would have occurred in the total absence of people.
Vegetation reconstructions such as these are becoming increasingly important for conservation managers responsible for decisions about the long-term future of degraded offshore islands. For example the pollen based reconstructions can help to determine the likely rate and direction of successional changes that have occurred in the past; they can help to determine whether natural rates of succession are preferable to more costly replanting programs; and they can provide species lists from the island’s long history if restoration replanting is desired.