Inspired by successful offshore island eradications, the Department of Conservation established several ‘Mainland Islands’ during 1995-96. Multiple mammal pests were targeted in these unfenced sites, some of which (Trounson, Boundary Stream, Rotoiti) are still closely managed today. Around the same time, trials with pest fencing enabled fenced ecosanctuaries such as Zealandia in Wellington and Maungatautari in the Waikato to completely eradicate key mammal pests inside the fences and keep reinvaders out.There are currently seven large ring-fenced ecosanctuaries in New Zealand and numerous smaller ones, and a similar number of fenced peninsulas, although pests can reinvade the latter around the ends of fences.
Today there are at least 80 sanctuaries around New Zealand larger than 25 ha implementing multi-species pest mammal control for ecosystem recovery objectives, and all have substantial community involvement. Most of these are unfenced.
For the past 15 years John Innes, Neil Fitzgerald and Corinne Watts have hosted an annual workshop for ecosanctuary practitioners and have maintained a database of ecosanctuary attributes, while Rachelle Binny and colleagues have collated a vast database (1 million-plus records) of biodiversity outcomes from 27 ecosanctuaries of different kinds.
So, what are the key lessons from ecosanctuaries that may be relevant to the 2050 vision of a predator free New Zealand?
- Pest-fences work. In the words of Elton Smith, manager at Orokonui Ecosanctuary near Dunedin, they “keep out most mammals most of the time”. Of course, mishaps with fences and associated gates and culverts sometimes let reinvaders in, but these are usually rapidly detected and removed.
- There is a gradient of increasing reinvasion of cleared areas by pest mammals: least on offshore islands, then progressively more on nearshore islands, ring-fenced mainland sites, fenced peninsulas, and finally unfenced ecosanctuaries. The risk of mammal reinvasion at any site is never zero, even on remote islands, as visiting boats may carry mammals as inadvertent passengers.
- Different plant or animal species respond to pest mammals in different ways. Some birds, such as North Island kōkako, robins, North Island brown kiwi and tūī, increase in unfenced reserves with control of pests by trapping and poisoning, and zero pests are not necessary for such bird populations to recover. However, sensitive species like tīeke and hihi need zero or near-zero pest mammals for their populations to recover, and then they do so only in pest-fenced or marine island ecosanctuaries. Detailed monitoring in ecosanctuaries can therefore help guide future pest control by identifying threshold pest levels above which native species will not recover.
- So-called ‘deep endemic’ bird species that have evolved for a long time in New Zealand seem to rapidly outcompete the introduced and more recent colonising bird species when mammal predators are removed. In other words, ancient New Zealand bird species are particularly vulnerable to pest mammals, but once freed from them may be more efficient foragers in New Zealand’s native environments.
- Different native species are vulnerable to different mammals, and when one mammal (such as possum) is controlled, there is often an increase in another (such as ship rat). These two factors suggest that there will be more benefit for broad biodiversity recovery if multiple mammal pests are controlled at a site, rather than just one pest.
- The mammal pest that is most likely to remain (and increase) after others are removed is the humble house mouse. Nearly all pest-fenced ecosanctuaries have struggled to remove all individuals of this small omnivore, and they rapidly become abundant when their predators and competitors (ship rats, Norway rats, stoats, weasels and cats) are removed.
- There is an increasing need for large pest-free areas on the New Zealand mainland. Takahē were recentlyestablished at a new site in Kahurangi National Park, but large areas of suitable habitat for takahē are scarce. Similarly, the recent huge breeding season of kākāpō has created a challenge to find large, pest-free sites where this iconic parrot can safely breed.
While ecosanctuaries of different kinds have enabled some insights into biodiversity recovery that can be expected when different pest control regimes are undertaken, there is no clear biodiversity vision associated with Predator Free New Zealand projects. What has been learned in ecosanctuaries can help decision-makers decide what level of pest control to implement at different sites.
Ecosanctuaries have always been made as large as possible. However, at unfenced sites, ship rats in particular demand intensive control year after year as they reinvade and breed rapidly after control. This has limited the area of sustainable mainland ecosanctuaries to about 3,000 ha, although most are smaller.
As predator-free sites, ecosanctuaries are potentially valuable research sites for predator-free studies, such as finding ways to detect and remove invaders when they are at very low density. And clearly, Predator Free New Zealand research can hugely help New Zealand mainland restoration by deriving new tools that cost-effectively control key pest mammals (stoats, possums, ship and Norway rats) over very large areas (10,000+ ha). The two very different approaches have much to offer each other.
This research is supported by Strategic Science Investment funding from the Ministry of Business, Innovation and Employment.