Julie Leven and her husband Des brought their camper up to visit their son in northern New South Wales, Australia, just before Christmas last year. They noticed masses of white dots moving over the black road surface as they drove back to their house in Gilgandra, some 430 kilometers northwest of Sydney, at night. They immediately discovered the dots were mice.
When the Levens arrived at their home, they saw a scene of rodent carnage. Mice had infested their house in such large quantities that it had become uninhabitable.
The critters had gnawed their way into the pantry and destroyed everything they could reach. Their feces and foul urine were strewn across the soft furnishings and beds from one end of the house to the other. The rats had even eaten the insulation surrounding the engine wire in two tractors, destroying the hay bundles that had been collected.
In Australia, outbreaks of this invasive species are not uncommon. "We have data dating back to the 1900s essentially demonstrating that there are mouse plagues somewhere in Australia every four or five years, and within any given location, it might be every seven or ten years," says Peter Brown, an ecologist at CSIRO in Canberra who studies vertebrate pests. But, on the other hand, farmers tell Brown that this is the greatest mouse epidemic they've ever seen.
The major cause is considered an excess of food due to a wet and rainy summer. Following many years of severe drought, which culminated in the terrible bushfires of 2019-2020, eastern Australia had abundant rainfall during much of 2020, notably in agricultural regions. Mice are opportunistic feeders who consume practically everything, but cereal crops offer the most appealing bounty.
According to Brown, one of the most important predictors of mouse populations is rainfall. "Our models rely on rainfall as a proxy for food availability because if it's a good year for producing crops, it's a good year for all sorts of other things that mice eat," he adds.
Population surveys from September to November 2020-springtime in the Southern Hemisphere-showed that mouse activity was quickly increasing. Trapping-in particular, inspecting captured female mice for scars from past pregnancies, which indicate fecundity-and mouse chew cards, which are tiny squares of paper soaked in canola oil that researchers distribute for mice to gnaw on-are used to track this. The higher the mouse activity at a certain place, the more chewing there is when the cards are gathered.
"We were keeping an eye out for what was going on in different regions in spring and summer last year, and northern and western New South Wales was starting to emerge as a bit of a hot zone of activity," Brown says.
Throughout the summer, mouse populations exploded, causing significant agricultural damage that the state government pledged AU$150 million to assist farmers in dealing with the assault.
The house mouse (Mus musculus) has arrived in Australia with British colonists in the late 1700s, swiftly establishing itself in local ecosystems and posing a threat to native species. "In Australia, competition with introduced rodents is one of the risks to native rodents," says Emily Roycroft, an evolutionary biologist at the Australian National University in Canberra. "It's generally pretty difficult for the ecosystem to maintain more than one species that are comparable in that manner when creatures consume similar foods and have similar body sizes," Roycroft adds.
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