Episodic mouse plagues in Australia may be a portent of the successful eradication of rats and stoats in New Zealand.
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.
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.
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.
What's in a Name?
The familiar saying "A bird in the hand is worth two in the bush" warns about the risks that come with trying to achieve more by challenging the status quo.