Mouse Plagues

Episodic mouse plagues in Australia may be a portent of the successful eradication of rats and stoats in New Zealand.

Australia

News from the world's smallest continent regularly remind us that Australia truly is a land of extremes. The physical forces of nature regularly demonstrate their destructive power but nature's biological forces can also flex their muscles, none more so than when mice reach plaque proportions.

Australia is the driest inhabited landmass on earth so it is no surprise that drought constantly stalks the continent. In response to this, mobile Australian wildlife have developed nomadic migration where they move to all points of the compass when drought strikes. Some birds keep going eventually arriving in New Zealand. As a result, Grey Teal, Welcome Swallow, Spur-winged Plover and White-faced Heron are now self-introduced native species in Aotearoa whereas Glossy Ibis are not infrequent visitors.

Glossy ibis courtesy of Charles J. Sharp

Bush fire courtesy of CSIRO

The terrible twin of drought is fire. Once forest vegetation is sufficiently dry, natural events, such as lightening storms, can ignite conflagrations that consume everything in their path. With haunting names such as "Black Saturday" and "Ash Wednesday", massive wildfires are both human tragedies and a necessary part of the natural cycle of Australian forests. They also exert a terrible toll on wildlife with conservative estimates of more than a billion native animals perishing in the unprecedented 2019/20 fire storms.

The flip-side of drought and fire is flood and the Australian landscape is no stranger to extreme rainfall. Stretching from the temperate south to the tropical north, huge monsoonal deluges periodically flood the world's 13th largest lake in northern South Australia. Kati Thanda–Lake Eyre covers 9,500 sq.km but is mostly bone dry and only completely fills a few times each century when floodwaters from outback Queensland migrate southwards through the "Channel Country" to the lowest point in Australia.

Lake Eyre courtesy of Marian Deschain

Dingo courtesy of Quartl

​Australia's native mammal fauna is dominated by marsupials into which invasive placental mammals have been introduced.  The most charismatic of these is the dingo, a medium sized dog that arrived in Australia maybe 8,000 years ago. Since then, the familiar passengers of colonisation have also been introduced including rats, mice, cats, pigs, horses and even camels. However, it is the house mouse that possibly has the most interesting story to tell.

Australia and China are the two countries where mouse plagues are known to occur and in Australia it is the grain growing areas of the east and south that are most affected. Approximately once every four years somewhere will experience a mouse plague and when food is plentiful mice can reach a density of 3,000 animals per hectare, or one mouse every three square meters.

This boom and bust cycle has been recorded from as early as the 1870's and indications are it is getting worse as time passes.

Mouse plague courtesy of CSIRO

The current plague began in 2020 after years of crippling drought ended in the 2019/20 bush fires that were followed by substantial rain. Agricultural production of grain and hay boomed in the ideal conditions and mouse numbers tracked upwards as a result. By autumn 2021 the swarms of mice were unstoppable and were destroying stored products, damaging houses and buildings and crippling infrastructure. The damage bill is expected to exceed A$1 billion.

New Zealand

So is there a message here for New Zealand? What if Aotearoa is successful with its aspiration to eradicate rats, stoats and possums? What might happen to mice and what might result from unchecked mouse population growth in New Zealand forests? Could these changes result in mouse plagues?

We know mouse numbers in New Zealand are regulated by rat and mustelid predation and we know that mast years in New Zealand forest result in massive seed fall from forest trees, especially beech trees. We also know that during mast years rat populations explode and stoat numbers significantly increase as a result. The autumn 2019 mega mast was the largest of these events for 40 years and climate change may make mast seeding both more common and larger.

During the 2019 event news reports from Arthurs Pass described mouse numbers as the 'biggest mice plague in five years' and that happened with rats and stoats still being present. What might have happened if rats and stoats were not present to exert downward pressure on mouse numbers and what might have been the result?

Mice are predominantly herbivores eating plant material such as flowers and seeds, but also insects and other invertebrates. Accordingly, in native habitats they are food competitors of native wildlife rather than being significant predators of native wildlife. However, taking rats and stoats out of the equation may drive two changes - a significant reduction in predation of wildlife and a significant increase in food competition for wildlife as mouse population levels move upwards with the removal of a key limiting factor on their own population. From this higher population level we could very well see mouse populations peak at plaque levels during mast seeding events that will continue to occur.

If elevated mouse populations do have the potential to increase food competition for native wildlife then they also have the potential to gradually change the structure of New Zealand forests as they impact the viability of seed dispersal and germination of favoured food species. Being "predator free" is be a noble aspiration but we should be careful what we wish for. Being selective about those predators we seek to eradicate may in turn create the conservation equivalent of the sword of Damocles.