Irruption and collapse of a population of pale field-rat (Rattus tunneyi) at Heirisson Prong, Shark Bay, Western Australia
Pale field-rats have long disappeared from Australia’s arid and semiarid zones, other than for some Pilbara islands and a single mainland population of indeterminate status and extent identified at Shark Bay in 1968. Hence, it was noteworthy when a field-rat was first caught at Heirisson Prong in 1994, 40 km north-east of the previous location at Shark Bay. Further individuals were caught regularly from late 1995. The population peaked in July–October 2000 (with captures of ~190 individuals per month) and had collapsed by July 2001 (with only the occasional animal caught thereafter). None were caught beyond 2006, despite regular trapping to 2013. This irruption and collapse was beyond the established range of the species and was in atypical habitat. Widespread trapping after the collapse suggested that the population inhabited few localised ‘source’ areas and a broad area of ‘sink’ habitat, with the latter occupied only after extraordinarily high rainfall events leading to higher grass cover. A return to dry years and the consequent loss of cover (aided by an abundant rabbit population) and strong growth in predator numbers (feral cats and small birds of prey) in response to the high number of field-rats appears to have facilitated the collapse.