A population irruption of the long-haired rat, Rattus villosissimus, brings rare trophic interactions to the Simpson desert, Australia

Abstract Species with irruptive population dynamics, that undergo dramatic outbreaks during which they invade normally unoccupied areas, provide a useful template with which to view rare or novel species interactions. Here we document biotic interactions during a population irruption of the long-haired rat ( Rattus villosissimus ) into the Simpson Desert, Australia. The rat is the largest extant rodent in arid Australia (average body mass: 134 g) being 3–4 times heavier than resident rodents. The study examined direct predation on and/or displacement of resident rodent species (using 21 years of monitoring data, 1999–2020) and examined if browsing caused damage and/or mortality of the sapling population of the dominant tree Acacia peuce (using 40 years of monitoring (1979–2019) of a sapling population of 146 individuals). Over 21 years, all but three records of R. villosissimus were during a period of 31 months (June 2010 to December 2012). Three distinct effects of the rats on the ecosystem were detected, specifically, potential displacement of resident rodent populations ( Notomys alexis , Pseudomys australis ) by R. villosissimus , direct predation on other rodents, and browsing impacts on A. peuce that ranged from moderate through severe damage to outright mortality in 7% of monitored saplings. The rare interactions produced by the rat outbreak did not have lasting effects on small mammals. By contrast, A. peuce sapling vigour and population growth may be negatively impacted by R. villosissimus should outbreaks occur more frequently as a result of changing climate.