Ecology of the rare but irruptive Pilliga mouse ('Pseudomys pilligaensis'): I. Population, fluctuation and breeding season

During a 4-year study in the Pilliga Scrub, trappable densities of the Pilliga mouse, 'Pseudomys pilligaensis' (Muridae), were low before a wildfire in November 1997, higher in late 1999 and February 2000 (5–30 mice ha⁻¹) and very high (up to 83 mice ha⁻¹) in April 2000; however, the densities fell sharply by July 2000, remaining low (0–5 mice ha⁻¹) until trapping ended in October 2001. Site-specific densities and their fluctuations differed among the four trapping sites, although fluctuations were broadly synchronised by the irruption peak. Within-site distribution changed as density fluctuated, from sparse to almost ubiquitous and back to sparse, and within-grid pre-irruption distributions did not predict those after the irruption. After the population decline, mice virtually disappeared from three of the four sites. The species' breeding season spanned at least October–April; some females bred repeatedly within a season. Prolonged good rains soon after the wildfire may have facilitated the irruption. The study suggested that 'P. pilligaensis' is distributed in disjunct patches of (refuge) habitat within its range except when environmental conditions are favourable, and that it is able to irrupt and become briefly ubiquitous before suddenly declining to a low density and sparse distribution. We suggest approaches for monitoring of this rare species.