A 50 year history of rodent eradications in NZ
reviewed by Emma Waterhouse

A recent paper published in the New Zealand Journal of Ecology reviews the past half century of rodent eradications on New Zealand islands¹. The authors document development and implementation of rodent eradication technologies and look at the major challenges to expansion onto inhabited islands throughout New Zealand.

The eradication of Norway rats from Maria Island/Ruapuke was the first rodent eradication in New Zealand, and 50 years later, about a third of our islands larger than five hectares are rat –free. These islands tend to be small and uninhabited, and account for only 10% of the area of offshore islands. Looking ahead, the eradication of introduced mammalian predators from Rakiura/Stewart Island and Aotea/Great Barrier would make 50% of the offshore island estate rat-free.

Not surprisingly then, the authors acknowledge that the next challenge for eradication is engaging communities on these inhabited islands to work towards ‘a common conservation goal such as pest eradication’.

Resident human communities add complexity, and moving beyond islands considered within current capability (20,000 hectares?) will also require a change in methods to avoid logistics and costs overwhelming a project. To this end, scientific evidence of species and ecosystem recovery is increasingly in demand to justify further investment in new eradications. Other trends noted are the increasing number of eradications being achieved on mixed-tenure islands with multiple partners and agencies.

Reinvasion risks are also addressed. Following mammal eradications, Rangitoto-Motutapu Islands were considered a high-risk site for human-mediated reinvasion given their location near Auckland.  The joint agency ‘Treasure Islands ‘programme in the Hauraki Gulf has meant that rodent incursions here, and on other island, are now intercepted to the extent that very few lead to full island reinvasions. 

Going forward, the authors assert that only with ongoing improvement in eradication and barrier technology will permanent predator removal from any site in New Zealand be possible.

Notes:

¹ Russell, JC and Broome, KG. 2016. Fifty years of rodent eradications in New Zealand: another decade of advances. New Zealand Journal of Ecology (2016) 40(2): 197-204