Free-Watering a Wild Population of House-Mice--A Test of an Australian Hypothesis in California

Abstract

Population declines of house-mice in South Australia in summer have been attributed to aridity. A free-watering experiment to test the generality of the hypothesis was conducted in California where breeding populations of Mus sometimes decline abruptly in summer. Though neither watered nor control population declined over summer, the watered population was larger, had higher rates of recruitment and survival and ate sixty times less fleshy petioles of Atriplex than the control population. Aridity was further implicated as a population control when reproduction increased sharply when morning dews became regular in late summer. The watered population began then to decline despite peak reproduction and few competitors. Supplemental feeding failed to arrest the continued decline which coincided with an increase in numbers and variety of predators, especially raptors. The mice became patchily distributed favoring dense cover, and the frequency of dark or sooty mice increased greatly. The main ecological differences between California and South Australia for Mus musculus appear to be lack of competition and predation in the latter.