Drivers of Mouse Plagues: Exceptional Rainfall Following Prolonged Drought Triggers Mouse Plagues in Southeastern...

Population outbreaks of house mice (Mus musculus L.) (= mouse plague) are a periodic feature of southeastern Australia's grain belt, causing significant crop damage and social-economic impacts. From 1904 to 1980, a plague occurred once every 5-7 years. Since 1980, there has been an increasing frequency observed in all regions except South Australia. Numerous hypotheses have been proposed to describe the phenomenon, but the specific combination of rainfall patterns in generating conditions for a mouse plague is not clear. Monthly rainfall data over 125 years were collected from three locations, and a measure of 6-monthly rainfall anomaly was calculated to determine trends in dry (deficit) and wet (surplus) periods. These rainfall patterns were then analyzed in relation to the timing of known mouse plague events. A mouse plague that occurred after a particular sequence of dry weather was observed (well below 6-month accumulated long-term average rainfall anomaly of -45 to 65 mm over 25-31 months), followed by an exceptional wet period (well above long-term 6-month accumulated average of 55 to 90 mm over 17-20 months). The combination of the magnitude and duration of rainfall deficit, followed by the magnitude and duration of the rainfall surplus, was required; anything less did not lead to a plague. The dry conditions appear to reset the system in some way, with mice responding to increased food supply following exceptional post-drought rainfall. A conceptual model of factors driving mouse plagues is proposed. There remain questions about the underlying biological mechanisms that drive these plagues, including density-dependent effects and the role of post-traumatic stress through inherited traits.