Seabirds on Pokohinu: The ecosystem engineers
In October this year, Curator of Land Vertebrates Dr Matt Rayner joined a team of researchers and staff from Auckland Council and the University of Auckland on an expedition to Pokohinu (Burgess Island). He explains the importance of seabirds in the island's wildlife recovery, and how historic specimens can help us learn about changes in the ecosystem over the last century.
Burgess Island is the second largest of the Mokohinau Islands, an island group perched on the edge of the continental shelf some 50 km off the coast of Pakiri Beach. To Ngāti Rehua, the island is known as Pokohinu, or oil of the muttonbird, in relation to the seasonal harvest of “Oi” – northern muttonbird or grey-faced petrel (Pterdroma macroptera).
Unfortunately, after a century as a maritime lighthouse station and Second World War military observation post – with the associated introduction of livestock and rats – the island saw a major decline in flora and fauna. But since the lighthouse was de-manned, the livestock removed, and the eradication of rats in 1990, our team has seen a recovery of wildlife on the island.
Home to endangered and rare species
The Mokohinaus are home to a number of New Zealand’s endangered species including the undescribed Mokohinau gecko (Dactylocnemis sp), Mokohinau skink (Oligosoma townsii), an endemic stag beetle (Geodorcus ithaginis), and range of other rare reptiles including moco skink (Oligosoma moco) and Suter’s skink (Oligosoma suteri). We have seen these populations rebounding.
The islands' birdlife is also recovering, with a recent publication by our team indicating a doubling in bird species from 24 to 46 species since the removal of rats 24 years ago. Native species that are uncommon or absent on the mainland – such as Kakariki (red-crowned parakeet) and Bellbird (Anthornis melanura) – are now common on Pokohinu.
Seabirds drive recovery
Driving this recovery are the seabirds, which are what we scientists call ‘ecosystem engineers’. Why is this you might say? Well, seabirds bring nutrients ashore in the form of their guano and turn the soil with their burrowing. This ‘nutrient energy’ travels up the food chain, enhancing the growth of plants, thus the abundance of insects that feed on this plant material, and then to other vertebrates (such as reptiles and birds) that feed on both. Nice one, eh!
Luckily for the Mokohinaus, it has burrowing seabirds aplenty, with our work confirming the islands as a New Zealand seabird hotspot. There are seven species, including the grey-faced petrel, fluttering shearwater (Puffinus gavia), sooty shearwater (Puffinus griseus), North Island little shearwater (Puffinus assimilis haurakiensis), common diving petrel (Pelecanoides urinatrix), white-faced storm petrel (Pelagodroma marina maoriana), and black-winged petrel (Pterodroma nigripennis).
Bitten by a shearwater
On this expedition, my primary goal was to recover geolocation tracking devices (geolocators) from fluttering shearwaters. These beautiful little seabirds are one of the gulf's most common species yet, amazingly, we know very little about their biology, which this research is trying to address.
Thus, I find myself on a rainy day on Burgess Island, shoving my hand down a small dark hole in the ground, hoping to be bitten. My luck holds out and a sharp beak latches onto my finger and enables me to extract the resident from its burrow, where it was sitting on an egg. I emit a little cry of jubilation – common for biologists who spend too much time on small islands – as this shearwater has a geolocator that was attached to its leg over a year ago. Just what I was looking for.
Geolocators are tiny tracking devices that twice a day report the location of the animal to which it is attached. These devices are revolutionising our understanding of the biology of seabirds by allowing us to follow their lives at sea, which we could not follow before.
Seabird diet affects island habitat
Downloading of the device reveals our little friend (X2B1587 to be unromantically but scientifically precise) has been busy and, although considered an Auckland local, took a winter holiday in Australia over two thousand kilometres away. I also collect feather and blood samples from the bird. These samples will then be compared against historic specimens held in the Auckland Museum Land Vertebrate Collection, providing insight into how the diet of this species (ascertained from chemical signatures in the tissues) has changed over the past century in response to changes in the Hauraki Gulf ecosystem.
This is important information given that changes at sea affecting seabird populations on land can have big implications for the island habitats in which they breed.
With thanks to Ngāti Rehua, for permission to conduct this research on the Mokohinaus, and to Dr Todd Landers (Auckland Council), Dr Brendon Dunphy, Dr Louis Ranjard, Rachael Sagar and Megan Friesen (University of Auckland), Graeme Taylor (New Zealand Department of Conservation) and Chris Gaskin for their field support and fine company.
Post by: Matt Rayner
Matt Rayner is a conservation biologist who specialises in the study of avian behaviour, ecology and evolution. With a particular interest in the Pacific seabirds, he works on closely with conservation and advocacy groups in Australasia and the Pacific through his role as Curator of Land Vertebrates and as a research associate of the University of Auckland. Read Matt's profile.
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