Editorial: Twenty Years of Gaining Knowledge - The Ever-Changing Ecosystems of Aotea

JOHN OGDEN

John Ogden is the founding Chair and current Science Advisor, and committee member of the OME project. He is the author of Birds of Aotea, a twenty year study of the island’s avifauna. He was editor of Environmental News from 2012 to 2015 He was formerly Associate Professor of Ecology at the University of Auckland and has lived on Aotea since 1990. 

Reflecting on 20 years of writing for the Environmental News it is the ‘bird’ issues that stand out for me, so I’ll try to summarise for you the Environmental Trust’s 20 years of information gathering and spreading awareness of Aotea’s birds, from bitterns to black petrels, and what all this data tells us about the future. 

About the Birds: Getting Specific

Between 2006 and 2008 the Trust first began 5-minute bird counts at six locations, spread over different seasons. Fenella Christian, Judy Gilbert and Sue Daly organised the teams and their instructions. I wrote five detailed reports for the Department of Conservation (DoC) and accounts in Environmental News(1). The transect lines (where people counted birds) were similar or identical to those set up for the longer-term Aotea Bird Count beginning in 2019, allowing comparisons of bird species that were ‘conspicuous’ over more than a decade(2).  

We also organised monitoring (with Amelia Geary, then at DoC)(3), of the rare Australasian bittern and undertook Boxing Day kākā counts. These surveys established that part of the kākā population moved seasonally from Aotea to the Auckland Isthmus (and elsewhere) and provided the first supportable estimates of kākā numbers on the Island(4). A strong seasonal migration pattern for kōtare/kingfisher was also established as well as information on nest-hole sites(5). A colony of grey-faced petrel/ōi was located nesting in burrows above the cliffs at Awana, and subsequent monitoring and protection – most recently by Barry Scott – indicates increasing numbers, probably spill-over from the growing colony on rat-free Cuvier Island. The Trust has also participated in monitoring black petrel on Hirakimatā and advocated for their protection(6). 

The importance of the Whangapoua estuary, both for local Aotea waders, and for international migrants, was first emphasised by Colin Ogle in 1980(7). I set up annual Aotea-wide NZ dotterel monitoring and published the first account of the population size and movements of this species on Aotea(8). It was clear that dotterels, variable oystercatchers, wrybill, banded dotterel, bar-tailed godwit and Pacific golden plover, spend some of their non-breeding period feeding on the mudflats of the Whangapoua estuary and hang out on the Okiwi spit at high tide, when they can be counted (annual counts were made by Environmental Trust members during the 2000 – 2010 period). 

Trustees were involved in DoC’s annual pāteke monitoring and restoration programme(9). Although pāteke have continued to decline on Aotea, there are some signs of recent improvement at Oruawharo, where a community group was formed as a result of the Local Board’s Ecology Vision (Oruawharo Medlands EcoVision, or OME). Wetland restoration is underway to provide better habitat for them, and flock numbers of up to 40 have been seen this year. 

Windy Hill: proof of damage being done by rats to Aotea’s birds

In parallel, an enormous amount of work was being carried out by the team at Windy Hill Sanctuary, managed by founding AGBET trustee, Judy Gilbert. The Windy Hill bird counts were done every December at up to 46 points in the sanctuary (where there was rat control) and at 12 points in the unmanaged area with six replicate counts at each site, spread over a week. Spreading them out like that allows for the occasional wet or windy day when the smaller birds are reluctant to sit up and sing for it is their songs and calls that mainly identify their presence. A consequence has been that it is no longer necessary to keep monitoring but if done it should be every few years rather than annually. A week’s work for a field team are now resources which can be spent on other conservation activities.  

Big money may be required to achieve final island-wide eradication of pests, but the benefits are clear from eradications on many islands worldwide, and from 20-plus years of work at Windy Hill. 

The Tū Mai Taonga project has secured some big money for feral cat and rodent eradication and is now underway in Te Paparahi. As it expands southwards, it will eventually link up with the Oruawharo Medlands Ecovision (OME) project and Windy Hill Sanctuary, so putting resources into the linking of OME and Windy Hill Sanctuary now, represents a logical step in resource allocation. 

Conclusions from a review of Windy Hill data collected over a ten year period:

1. There was a consistent statistically significant difference in bird abundance between the managed and unmanaged areas. This could be attributed to greater breeding success for most species in the managed area – where rat numbers were demonstrably reduced compared to the unmanaged areas. Alternatively one could say that more birds (eggs or nestlings) were being predated in the unmanaged area. The loss amounted to 4 or 5 birds for every hectare. This might not seem like much, but it amounts to saving c. 3500 birds every year in the 770 ha. Sanctuary. This has huge implications for the remainder of the island’s (unmanaged) forest cover. An extrapolation of the data obtained from the Windy Hill monitoring would suggest that c. 85,000 birds are lost to rat predation every year across the Island11! Even assuming, arbitrarily, that these mortality figures are 10´ too high, we still get 8500 birds lost per annum. The true figure must be far higher, and this explains why birds ‘bounce back’ so quickly when rats are eradicated from is-lands. 

2. There was a gradual increase in abundance in the 5-minute count data for most bird species in the rat managed area (or no decrease). Not only were the larger and more con-spicuous birds such as tūī increasing in the Sanctuary, they were also increasing in the unmanaged sites and even outside the area. This suggests that the extra birds are mov-ing from the protected area to the rest of the island. The potential movement of birds about the island, and a justification for re-introductions of lost species, is exemplified by the movement of two banded toutouwai/robins, one from Windy Hill, and another from Glenfern, to Hirakimatā, where there is now a small breeding population. 

But what about the forest? Changes are also occurring in the Aotea forest. At Windy Hill and Glenfern, change in forest composition is mainly due to native conifers and broadleaf species spreading out from the remnant patches that survived in gullies or damp south-facing slopes and which are establishing seedlings under the tall, dying kānuka. In 201012 and 201513 work by AGBET trustee George Perry and others, demonstrated that the future vegetation cover could be predicted by identifying the potential replacement of any tree species by counting the established seedlings of all tree species close to it. So, some tree species will decrease in abundance, while others will increase. 

Moreover, modelling showed that this successional forest process is speeded up if rats (important consumers of tree seed and seedlings) are reduced. 

From the 2006 – 2008 five-minute bird counts, we know that the kānuka forest is preferred habitat for mainly insectivorous birds, such as fantail and grey warbler, while the broadleaf forest is preferred by the larger frugivorous species such as tūī and kererū. So, looking to the longer term, and assuming that fire can be excluded from most of the forest, the bird fauna will potentially shift as the forest changes. Introduced species, such as finches and thrushes, are not likely to invade the native forest much, but will become more restricted to the exotic vegetation of gardens and farmland. 

CONCLUSION

Both the vegetation and the birds – and by inference, all components of the native forest eco-systems – are changing. They will shift to become more stable and predictable than at present, but with the ever-changing climate we should not strive for some primaeval ideal, because there never was one. Our goal should be simply to create areas of Aotea where native species of plants and animals flourish in the absence of alien competitors and predators. That is not a remarkable conclusion, but it is based on two decades of work by many people, not least the Aotea Great Barrier Environmental Trust and their supporters, and the dedicated field-workers at Windy Hill. 

References:

1. Four reports are summarised in: Ogden, J. 2009. Great Barrier Island Charitable Trust Final Report on Birds of Great Barrier Island 2006-2008. Department of Conservation Biodiversity Advice Fund AV 207. March 2009. See also reports in Environmental News, issues: 7, 8, 9 and 11. 

2. Comparisons between ABC 2019 bird-count results, the 2006-07 GBICT bird counts, and 2018 Windy Hill data. Report by: John Ogden for Great Barrier Island Environmental Trust, June 2020. See also: Aotea Bird Count Results, December 2019, 2020, 2021 and 2022. Aotea Great Barrier Environmental Trust. 

3. Geary, A., Corin, S. & Ogden, J. Monitoring Report. Australasian Bittern, Great Barrier Island 2012. Department of Conservation Te Papa Atawhai. GRBAO – 22380. See also: Environmental News # 8, 23, 29, 30, 44 

4. Ogden, J. 2011. Boxing day kākā count – and some conclusions. Environmental News # 24: 7 – 11. 

5. Ogden, J. 2007. Third Bird Count. Environmental News #9: 3-5. 

6. Waterhouse K. 2011. Black petrel – hanging on, just. Environmental News # 27: 4-9. See also items by same author in Environmental News # 30, 32, 33, 35, & 40. 

7. Ogle, C.C. 1980. Wildlife and wildlife habitats of Great Barrier Island. Fauna Survey Unit Report No. 24. Wellington: Department of Internal Affairs. See also; Ogden, J. 2016. Central Bird Exchange. Environmental News #35 (Summer 2016): 4-6. 

8. Ogden J. and Dowding, J. E. 2013. Population estimates and conservation of the New Zealand dotterel (Charadrius obscurus) on Great Barrier Island New Zealand. Notornis 60: 210-223. 

9. See, for example, Ogden, J. 2016. Pāteke population trends and the impact of predator control, Great Barrier Island, Environmental News #35: 11-14. 

10. www.omeaotea.co.nz 

11. Note that using logic and data published by Innes et.al. (2010) in New Zealand Journal of Ecology 34(1):86-114. I reached a numerically similar conclusion.

12. Perry, G.L. W., Ogden. J., Enright, N. J. & Davy, L. V. 2010. Vegetation patterns and trajectories in disturbed landscapes, Great Barrier Island, Northern New Zealand. New Zealand Journal of Ecology 34(3):311-323. 

13. Perry G.L.W.; Wilmshurst, J.M.; Ogden, J.; Enright, N.J. 2015. Exotic mammals and invasive plants alter fire-related thresholds in Southern Temperate forested landscapes. Ecosystems. DOI: 10.1007/s10021-015-9898-1 Springer Science.