The Vascular Flora of Rakitū (Arid) Island
JACK WARDEN
In 2020 Rakitū Island was declared predator free following two aerial toxin drops by the Department of Conservation-Te Papa Atawhai in 2018. The removal of ship rats and kiore from the island is predicted to have a major impact on restoration of the flora and fauna of the island, but describing and measuring that change means there is the need to survey and record the current ecological status of Rakitū Island. Over the period 2017-2019 Jack Warden and colleagues surveyed the vascular flora of Rakitū and here shares with us his initial findings.
The ‘Threatened – Nationally Vulnerable’, koru (Lobelia physaloides) photographed on Rakitū Island March 2018. Rakitū Island is the species known southern limit in New Zealand. The species is absent from Great Barrier Island and Little Barrier Island (Photo: Jack Warden)
Introduction
Rakitū Island (Arid Island) located 2.5 km off the east coast of Aotea/Great Barrier Island is the third largest island in the Aotea Group at ~328 hectares in size with two highpoints which fall towards the centre of the island to form a series of streams which eventually discharge in the cove at the north-western aspect of the island. The name ‘Arid Island’ was given by Captain James Cook when the Endeavour passed close to the island in November 1769, probably in response to the apparent bare appearance of the island because of its underlying geology as an eroded remnant of a complex rhyolitic volcano(1). It was later noted that ‘Arid Island certainly does not deserve the name bestowed on it by Captain Cook’(1). Rakitū is a culturally significant landscape to the Ngāti Rehua Ngātwai ki Aotea, as it was home to one of their founding ancestors, Rehua.
Human disturbance and impact
The island has had a long history of anthropogenic disturbance starting with Māori in the in the 13th Century followed by European settlement in the late 1800s. Māori used fire to clear the land for occupation and cultivation of crops, and brought with them kiore and kuri. Archaelogical surveys have identified sites of pa, terraces and pits; cultivation sites; obsidian and basalt adzes (toki); and burial sites (urupa)(2). European purchase of the island from Māori in the mid 1880s resulted in further clearance of vegetation and the introduction of grazing livestock. After several changes in ownership the Island was purchased by the Crown in 1993 from the Rope family and is now managed as a Scenic Reserve by the Department of Conservation in partnership with Ngāti Rehua Ngātwai ki Aotea.
Earlier botanical surveys
The first known record of the vegetation on Rakitū Island is a short account by Frederick Hutton and Thomas Kirk(3), who visited the island for just a few hours in December 1867. They noted that most of the open land had been burnt just a few days before they landed by a party of Māori muttonbird (tītī) hunters, a practice to more easily access the bird burrows. They recorded 41 native and 2 adventive vascular plants for the island (Table 1). They also noted “sheltered valleys covered with luxuriant ferns and bush”
Hutton and Kirk concluded their short account with a statement on the need for a more comprehensive survey of the flora on the island.
“The note just read must be considered as merely a contribution to the botany of Arid Island. We venture to express the hope that some member of the Institute may visit the island under more favorable circumstances, than fell to our lot, and be able, at least, to make a compete catalogue of its phaenogamic plants and ferns: not only on account of the interesting nature of the locality; but because of the positive value possessed by an exact and minute knowledge of the local distribution of plants, as an element in the ultimate circumscription of their specific limits.”
It was a further 115 years before a more comprehensive description of the flora of Rakitū Island was reported by Cameron & Wright in 19824. They recorded 241 native and 80 adventive species on the island (Table 1). The very large number of adventive species presumably reflected the considerable disturbance of the vegetation from farming. A further survey in 2005 by Cameron and Bellingham(5) recorded 90 adventive species with little change in the number of native species at 242 (Table 1).
The ‘At Risk – Relict’, mawhai (Sicyos mawhai) photographed on Rakitū Island March 2018 (Photo: Jack Warden)
Current botanical survey
With the proposed eradication of kiore (Rattus exulans) and ship rats (Rattus rattus) in 2017 and confirmation that no surveys of the flora of Rakitu were to be undertaken I took the opportunity to assemble a volunteer group to survey the vascular flora of the island. The primary focus of the time spent making various trips to the island between 2017-2019 was to record what species could be confirmed to be present to assist future management decisions on the preservation and restoration of the flora of the island.
Table 1: Total of vascular flora species recorded on Rakitū Island over time (1867 – 2019)
From the field trips undertaken between 2017-2019, analysing past reports from the previous studies and New Zealand Herbarium Data we have added ~65 additional species to the vascular flora of Rakitū Island of which 47 are native and 18 are adventive(6). Of the 289 native species records, 24 are listed as ‘Nationally Threatened’(7) which accounts for 8% of the native species recorded on the Island. Exotic species have increased over time and many of these species are a direct result of past agricultural activities and those that have naturalised around the old settlement area within the cove. Of the 108 adventive records at least 15 are desirable to control and manage long-term due to their potential impacts on natural regeneration and displacement of native species.
It is anticipated that the full findings of the various field trips will be published in the near future to include field collection data, a full description of the islands vascular flora making up the various ecosystem types, changes in the island’s vegetation overtime, proposed future changes and potential management techniques for threatened plant species(6).
Conclusion
Rakitū Island in the Hauraki Gulf Marine Park is now pest-free (Source: Stuff Limited)
Rakitū Island for its size harbours a high number of native species, many of which are of regional and/or national importance and are not found on Aotea/Great Barrier Island or nearby Hauturu/Little Barrier Island. Like many offshore islands, Rakitū Island is an example of a ‘lifeboat’ in relation to the ‘sinking ship’ that is New Zealand’s native vascular flora. Although the declaration of Rakitū Island as ‘Mammalian Pest Free’ is an outstanding success in adding an island to the list of successful eradication projects, no surety can be provided to the future trajectory of the Islands vascular flora and decisions that need to be made in relation to what species may need immediate and/or future management priority. Some of the future threats include fire, pathogens and weed invasion.
Although the opportunity has not yet been missed, conservationist and the public alike need to be presented with facts on the successes and failures of such projects to drive future management decisions especially if we are to consider the vision of a Predator Free 2050. Baseline knowledge of the vascular flora of Rakitū Island will be vitally important in determining whether passive or active restoration methods are employed in the future management of the island by the Department of Conservation in partnership with Ngāti Rehua Ngātwai ki Aotea. It is also important that this information is shared with the community and that they get the opportunity to visit and see for themselves how the ecology of the island responds in the absence of predators.
Acknowledgements:
I would like to thank the people who have joined me in the field or on trips to the island including Jeremy Warden, Cameron Kilgour, Leon Perrie, Lara Shepherd and Jordan Scarlett.
Herbarium staff at Auckland War Memorial Museum, Museum of New Zealand Te papa Tongarewa and Allan Herbarium for sharing collection information, data and allowing me to view specimens.
Ewen Cameron for reviewing draft species lists, helping with collections, old specimens and identifications.
The support from Auckland Council and Department of Conservation on the trip in June 2019 is acknowledged.
References:
Hayward, B. W. 1982. Offshore Islands research group trip to Rakitū (Arid) Island, northeast New Zealand, New Year 1980-81. Tane 28: 79-84.
Hayward, B. W., Matthews, P. J., & Moore, P. R. 1982. Archaeological sites on Rakitū (Arid) Island, North-East New Zealand. Tane 28: 175-186.
Hutton, F.W. & Kirk, T. 1868: Description of Arid Island, Hauraki Gulf. Transactions an Proceedings of the New Zealand Institute 1:108-112.
Cameron, E.K. & Wright, A.E. 1982. The vegetation and flora of Rakitū (Arid) Island, northern New Zealand. Tane 28: 85-124.
Cameron, E.K. & Bellingham, P.J. 2005. Vascular flora of Rakitū (Arid Island), additions and comments. Auckland Botanical Society Journal 60 (2): 102-107.
Warden, J.O.; Warden, J.P. & Kilgour. C.D. 2019. The vascular flora of Rakitū Island (Arid), updated (2017-19). Unpublished.
de Lange, P.J.; Rolfe, J.R.; Barkla, J.W.; Courtney, S.P.; Champion, P.D.; Perrie, L.R.; Beadel, S.M.; Ford, K.A.; Breitwieser,I.; Schönberger, I.; Hindmarsh-Walls, R.; Heenan, P.B.; Ladley, K. 2018: Conservation status of New Zealand indigenous vascular plants, 2017. New Zealand Threat Classification Series 22. Department of Conservation, Wellington. 82 p. Retrieved from https://www.doc.govt.nz/documents/science-and-technical/nztcs22entire.pdf