Description of mustelids and feral cats

Ferrets are common throughout Canterbury in pasture, scrublands and braided riverbeds. Their distribution generally matches that of rabbits in pasture and semi-improved grassland. Their population responds to changes in the rabbit density, and can range from 2.8 to 8.4 per square kilometre in tussock grassland. Stoats and weasels are slightly smaller than ferrets, and are widely distributed but more common in forested areas. Their numbers are lower in pasture and tussock grasslands, but can range between 2.2 and 70 per square kilometre in beech forest, and their population dynamics are linked to fluctuations in their primary prey – birds and small mammals in forested areas. Both stoat and ferret populations have the potential to increase rapidly and reinvade areas following control or changes in their prey density. Ferrets rely heavily on rabbits as their primary prey species, but they also feed on a variety of indigenous wildlife as secondary prey. Large-scale changes in rabbit numbers, such as following control operations, can cause substantial changes in ferret diets.

Stoats are specialised predators of small mammals and birds, although they do predate on rabbits in riverbeds and tussock grassland. In forested areas their numbers fluctuate in response to beech seeding years.

Weasels are found occasionally in Canterbury. Because of their patchy distribution and scarcity, their impacts on biodiversity, soil and water quality and indigenous species are largely unknown compared with either ferrets or stoats. However where they are present it is considered they do pose a threat to indigenous wildlife. They are not discussed further in this strategy.

Feral cats, largely resulting from human settlement activities, behave in a similar manner to mustelids. 

Adverse effects of mustelids and feral cats

Ferrets and stoats are capable of having a serious impact on native fauna through direct predation.  Ferrets and stoats have been known to prey on brown kiwi, weka, white-flippered penguin and pigeons in the forest/scrub/pasture mosaic.  Ferret predation is the main cause of death for black stilts.  Other vulnerable riverbed species include the wrybill, black fronted tern and caspian terns.  Ferrets also pose a threat to several species of skinks and geckos as well as the native invertebrates such as giant wetas and grasshoppers. Stoats are considered a serious conservation pest because they threaten the long-term viability of several species of birds through predation, and in Canterbury populations of yellowhead appear particularly vulnerable.  It is now well documented that ferrets can carry bovine tuberculosis and general agreement amongst the scientific community that they play  a role in its transmission to cattle and deer.  They are considered a major vector of bovine Tb in Canterbury. 

Anecdotal evidence supports feral cats causing similar affects to those of mustelids. 

Environment Canterbury's role

Mustelids Ferret (Mustela furo), Stoat (Mustela erminea), Weasel (Mustela nivalis) and Feral cats (Felis catus) are included in the Canterbury Regional Pest Management Strategy (RPMS) under the Biodiversity Protection Programmes where they are only able to be managed in high-value environmental areas.  This means that mustelid and feral cat control is primarily the responsibility of the land owner/occupier.  There are no enforceable rules around the requirement to control mustelids or feral cats. 

Environment Canterbury can facilitate and assist community and land occupier selfhelp programmes to destroy these pests particularly if adjacent to areas of high environmental value to complement control operations, or in other areas if there is community support for control operations.  This would be done through the 'Biodiversity Protection Programmes'.

NOTE: Rodents such as mice and rats are not included in the RPMS as at the time of review it was decided that individual action is effective in preventing serious adverse effects.