Posted by Mick Clout

A recent paper reveals that introduced stoats (Mustela erminea) in New Zealand have greater genetic diversity than in their native Britain, from where they were introduced in the late 1800s. The results are unusual because introducing a species to a new area is usually associated with a loss of genetic diversity, due to the small numbers released.

The current situation of stoats in Britain and New Zealand is the result of a series of ill-fated attempts at biological control of pests.

Hundreds of British stoats were introduced to New Zealand during the latter part of the 19^th century (along with weasels and domestic ferrets) in a failed attempt to control rabbit numbers. Rabbits had previously been introduced to New Zealand for food and sport, but had become agricultural pests. The stoats introduced to New Zealand were ineffective at controlling rabbits, but they spread throughout much of the country and are implicated in the decline of many native birds, including kiwi and kakapo. The swimming ability of stoats has resulted in their colonization of several offshore islands, where they flourish, especially in the presence of introduced mice. Many conservation programmes now include the control or local eradication of stoats, to allow recovery of threatened endemic birds. However stoat incursions continue on some islands from which they have been eradicated. There are now plans for the complete eradication from New Zealand of these and other mammalian predators in the long term.

Stoats have greater genetic diversity in New Zealand than in Britain, but this should not compromise attempts to control this invasive predator in New Zealand. Photo by Patrick Garvey.

Several decades after stoats were introduced to New Zealand, the native stoat population in Britain suffered a drastic decline in abundance when rabbits, their main prey there, were decimated by myxomatosis, which was introduced to Britain in the 1950s as a control measure for rabbits.

When the native stoat population in Britain collapsed in the wake of the introduction of myxomatosis, introduced stoats in New Zealand effectively conserved a reservoir of genetic diversity from the original British population.

Paradoxically, the misguided introduction of stoats to New Zealand has created an ‘invasive ark’ for genetic diversity of this species. However, this should not compromise efforts to control or eradicate stoats in New Zealand. Perhaps the British would like them back?

Professor Mick Clout is a vertebrate ecologist at the School of Biological Sciences, University of Auckland. Mick works on the conservation biology of threatened species, such as New Zealand’s native pigeon the kererū, as well as the ecology of invasive mammals, such as possums, hedgehogs, cats, mustelids and rodents. From 1993-2009 he chaired the IUCN Invasive Species Specialist Group (ISSG), a global group of scientific experts on invasive species.