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.
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
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: