From a bird perspective undoubtedly the most serious animal pests to have been introduced to New Zealand are the mustelids: stoat, ferret and weasel.
These are slender bodied carnivorous mammals that naturally occur throughout Asia, Europe and North America. While these do not eat native plants they, along with possums, kill native bird species that are vital for seed dispersal and/or pollination of many native plant species.
By the mid 1870s, rabbits were becoming an agricultural pest in New Zealand. Farmers demanded that mustelids be introduced to control rabbit numbers. Despite warnings from scientist like Walter Buller (the Bittish ornithologist) that stoats represented a significant threat to our native birds, many of which were flightless, the government of the day approved the release of stoats. Up and down the country ferrets, stoats and weasels were released in pastoral areas and by the mid 1890s they had spread into forests west of Lake Manapouri. It took till 1936 for all legal protection of mustelids to be removed. Mustelids are now regarded as a significant pest species in New Zealand.
Some research estimates that stoats kill 40 North Island brown kiwi chicks per day on average. Ferrets mainly feed on rabbits but ground nesting birds are also easy prey. Weasels have been implicated in the demise of lizard, invertebrate and bird populations.
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