The Humble House Mousesmall, effective, resilient by Jo Ritchie

Environmental News Issue 13

Mice – How many of us have started off with a couple as pets and ended up with many many more than 2! Their passion for breeding and their ability to survive and thrive in most environments are the secrets to their success – their present world distribution is probably more extensive than that of any other mammal apart from us.

Part of my work is as an animal pest eradication planner for conservation projects. Commonly the projects I work on involve the removal of multiple species of animal pests at the same time. There are a couple of reasons for this. One is operational and cost efficiency - resources for these projects are limited. Removing multiple species of pests at the same time as opposed to one at a time is obviously more time and money efficient and we know it works.

The other reason is because of the interrelationships between pest species. Simply put there is a hierarchy of dominance. For example take out stoats, weasels (mustelids) and cats and there are no natural predators for rodents. Rodent nirvana – no predators, few competitors for food so their numbers increase – more mouths to feed leads to more predation on native species. A similar thing happens if you remove the rodents and not their prey species – there are many examples of rodent numbers being significantly reduced but not the numbers of stoats and weasels who then increase their consumption of native species.

As if this is not enough rodents have their own hierarchies. There are 4 species of rodents in New Zealand – the Norway rat (impressive physique can be up to 400grams), the ship rat (smaller, more agile, great swimmers and unrivalled tally sheet for effects on native species), the kiore (not as widespread as shippies but just as effective) and the humble house mouse (may be the smallest but fierce for its effect on native invertebrates). Size matters in the rodent world – Norway rats dominate over ship rats and kiore who in turn dominate over mice for both habitat and food. Remove one and the numbers of the others often increase. Note that only kiore, mice and ships are present on GBI.

On Kaikoura Island kiore and mice have yet to be detected but do not be fooled — absence of evidence is not evidence of absence. Ship rats are likely to be in such high densities that they have suppressed kiore and mice.

The evidence of how effective this suppression can be was highlighted to me during the animal pest eradication programme at Tawharanui Regional Park. Tawharanui is a 588 hectare park administered by the Auckland Regional Council. It has a 2.5km predator fence on its western end. Prior to installing the fence and doing the eradication programme the ARC had been running tracking tunnels for a number of years. Mouse numbers were low given the amount of grass seed, invertebrates and forest food that was around. Rats (all 3 species) along with mustelids, hedgehogs, possums and cats were all present.

An aerial baiting programme using Pestoff brodifacoum pellets and two bait drops was undertaken in late 2005. This formulation has been proven to be highly effective for rodent eradication programmes. The combination of the aerial baiting and a ground based trapping and bait station programme initially appeared to have been really effective. No mustelids, hedgehogs, possums, cats, rats or mice. We were nervously excited, the first four months of tracking tunnel results after the drops showed no mice prints and then the B@#$%^&* moment!

Mice prints started to show up on a few of the tracking tunnels. We assembled the troops and tried to quash the enemy. No mean feat given the average mouse home range is 10m2 – I can tell you that the enjoyment level of placing a trap or bait station every 10 metres in tight Manuka scrub on a hillside or waist deep kikuyu is way below zero!

Nothing tells the story of the mastery of the ability of mice to recover better than the following graph: