Making Great Mercury Island pest free began with a casual discussion between Pete Corson (DOC) and Michael Fay (Island co-owner), in the back of a ute in 2009. With the removal of kiore (Polynesian rats), ship rats and feral cats in 2014 Great Mercury/Ahuahu is now one of over 110 islands off the New Zealand coast to have achieved pest-free status. The island is currently sitting at the 8th largest but will hopefully be 9th if the Antipodes proves to be successful.
Getting to the island
This year, the Moehau Environment Group in partnership with the Department of Conservation, ran a trip to privately-owned Ahuahu as part of their summer holiday programme. For a non-boat owner, that’s the only way to get there, unless you’re a Rockstar or just mega-wealthy… the bill for an overnight stay is a cool $20,000! For logistical reasons, we explored the southern end of the island, the aim being to investigate the landscape through an ecological restoration lens.
Humans dramatically reshape the character of the landscape both intentionally as well as unintentionally. They do this through the species they deliberately introduce and other species that come along for the ride. Forests are cleared grow meat and fibre; stowaways such as rats feast on naïve native birdlife; weeds take hold on the disturbed land. This pattern has been repeated countless times on islands throughout New Zealand waters – as well as on the mainland.
At 1872 ha and only 8km off the eastern coast of the Coromandel Peninsula, Ahuahu has was colonised very early on by Maori – from around the 13th century. Thanks to the island’s strategic location and the presence of springs, seeps and streams, Pat Mizen calculated that Ahuahu could support up to 5000 Maori. As well as being the previous owner of the island, Pat was an amateur archaeologist and author of Te Korero Maori mo Ahuahu: The Maori story concerning Mercury Island (1997).
The island was purchased (along with two others nearby) by the Crown in 1858–65, with Ahuahu then passing quickly into private ownership and farmed. Today, around one-third is sheep and beef farm, one third is in pine forest and the remaining third is regenerating native forest, along with wetlands and dunes.
The Ahuahu project to remove 3 pest species was 50/50 funded by island co-owner Sir Michael Fay and the Department of Conservation. The island is now pest free – at a cost of 1.43 million NZD. In spite of the expense, the number of island eradication projects expanded rapidly in the 1990s as tools and methods have become more effective. Although slow acting poisons that eliminated rodent shyness to toxicants had been developed in the late 1970s, it wasn’t until GPS become available that toxins such as 1080 could be evenly and thoroughly distributed aerially over a whole island. From an initial focus on rodents, multi-species eradications are now gaining ground. Toxin use is contentious, but eradicating/controlling pest species in remote locations, across challenging terrain, at landscape scales and to zero or low enough densities to enable vulnerable native species to survive, is reliant on pragmatism and cost-effectiveness. A newly published book builds a lucid case for continued use, as do reports by the Parliamentary Commissioner for the Environment and Community Environmental Groups.
The threat of reinvasion looms large, particularly given the open access policy by Ahuahu owners to visiting boatees. The need for advocacy while allowing visitors to use the island’s magnificent anchorages and beaches is a trade-off. The fenced but open access sanctuary of Tawharanui on the mainland operates on a mixed model of recreation and conservation. On Ahuahu, ongoing checking of sentinel traps is the most practical means of detecting unwanted arrivals, along with educating visitors e.g., though prominent messages and guided visits like ours. For our trip, a full biosecurity check was carried out prior to leaving the mainland: all personal bags were emptied, boots scrubbed and Velcro inspected for hitchhiking seeds. Not so for boatees though . The risk however, is lower given that most of the boating public don’t leave the beach area, if they make it to dry land at all.
Rats are good swimmers, with ship rats (Rattus rattus) capable of swimming 500m, and Norway’s (Rattus norvegicus) able to swim further. Stoats (Mustela erminea) are capable of much longer distances with factors such as currents, waves and floating debris either helping or hindering their progress. While showing the group a wax tag used for monitoring, Pete mentioned that there had been over 140 rodent incursions on islands over the last decade, highlighting the need for ongoing vigilance as well as public advocacy and education.
New Zealand has over 1500 offshore islands and rock stacks. They’re spread over 2,800km from the (nearly) subtropical Raoul in the north, to Campbell in the wind-blasted sub Antarctic south. These islands are the last refuges for unique flora and fauna: burrowing seabirds (e.g., NZ storm petrels), quirky invertebrates (e.g., Mercury Is. tusked weta), rare plants (only one specimen each of the Tecomanthe speciosa vine and Pennantia baylisiana tree remain in their natural island habitat), and iconic reptiles (Tuatara and Duvaucel’s gecko).
Multi-species projects are gaining ground: Rangitoto and Motutapu (both a few short kilometres from central Auckland) lead with a total of 11 introduced mammal species eradicated. The story of Ahuahu and other islands, fenced and unfenced sanctuaries on the NZ mainland, agency, NGO and community effort show that pests can be controlled or even eradicated across large land areas. The next step is to better coordinate these diverse initiatives, improve existing technology as well as harness new technology, so that New Zealand can achieve (or at least make significant progress toward) Predator Free status by 2050. That’s a whole series of blog posts in itself.
Evaluating the use of 1080: Predators, poisons and silent forests. Parliamentary Commissioner for the Environment 2011
Fifty years of rodent eradications in New Zealand: another decade of advances James C Russell and Keith G Broome http://newzealandecology.org/nzje/3258.pdf