Viewpoint

Ferrets and Tb, Rabbits and RCD

Craig Morley

Until recently ferrets (Mustela putorius furo) have had a fairly low profile in New Zealand, but awareness has escalated with the mounting evidence that ferrets may spread Tb to livestock. As a result, ferrets (and cats) are now the target of a massive control programme.

Rabbit numbers are reasonably low and stable throughout most of lowland New Zealand, except for the semi-arid lands of South Island. The main stabilising factors influencing rabbit numbers in the lowlands are hypothesised to be rainfall and natural predation.

With the removal of predators such as ferrets and cats from lowland areas, the question arises -- will rabbit numbers increase once their natural predators are removed? Any increase in rabbit numbers represents a cost to landholders that needs to be considered in an analysis of the true costs and benefits of controlling predators.

Ideally it would be preferable to simultaneously control both rabbits and predators that spread Tb. However, to do so would cost a small fortune. Priorities for control costs will differ from one situation to the next. In areas where Tb is negligible (such as on a farm with sheep only), a farmer may target rabbits and leave the predators; in an area where Tb is a problem (on a deer or cattle farm), ferrets and cats may be the main focus of control. As most farms in lowland New Zealand have combinations of cattle, deer and sheep, the choice is not easy.

One option being currently considered to help control rabbits is the biological control agent Rabbit Calicivirus Disease, or RCD. RCD is touted as the "new weapon" in the war on rabbits, but it too has costs and benefits. Modelling predicts that in high-density rabbit populations, such as in the South Island high country, rabbit numbers may decline by up to 95% after the initial outbreak of the disease, but in the stable lowland rabbit areas, the models are ambiguous concerning whether the effective control rate of rabbits will be achieved. Problems with potential vectors, transmission rates and climatic conditions mean that RCD may not be as successful as is hoped.

Even if RCD is successful in reducing rabbit numbers in the lowlands, it is likely that there will be refuge areas in which rabbits survive. If rabbit predators are controlled when RCD is released, the overall impact of RCD in lowland areas may actually be reduced, because the rabbits living in refuge areas could quickly recover and expand. Essentially, fewer predators could hasten the recovery rate of rabbits after RCD outbreaks, especially given that young rabbits under two months of age (the main prey of ferrets and cats) seem to be immune to RCD. Furthermore, ferrets could be a useful tool in spreading RCD from one rabbit burrow to another, thus aiding in reducing rabbit numbers even further after the initial release of RCD.

One hypothesis being considered is that a reduction of rabbits will mean a reduction of ferrets and cats, and thus Tb, because the predators' main food source will be depleted. Unfortunately, some predators are flexible and could switch to other food sources such as black stilts, grebes, kiwi, reptiles and insect fauna. Furthermore, a reduction in ferret and cat numbers may mean an increase in rat and stoat numbers. From a conservation perspective, these pests could pose an even greater threat to our threatened and endangered native fauna.

Many may argue that rabbit numbers in stable lowland areas of New Zealand are "not worth worrying about" and this may be true for many landholders. However, if the aim of introducing RCD into New Zealand is to reduce the rabbit population to almost nil, then the rabbits tucked away in the lowland refuge areas should also be targeted.

History shows that the stable lowland areas are the hardest and most costly to control because of the time and effort required to get rid of that last elusive rabbit.

Therefore the question arises: do we leave the rabbits' natural predators after the introduction of RCD (even knowing they may be spreading Tb) because they may be an additional tool in reducing rabbit numbers, or do we target rabbits as the greater evil and hope that the economy (i.e. Tb risk) and other wildlife can endure a period of transition when RCD arrives?

Craig Morley is in the Department of Zoology at the University of Canterbury.