Population biology of small mammals in Pureora Forest Park .2. The feral house mouse (Mus musculus)

Over five years from November 1982 to November 1987, we examined 395 mice collected from unlogged and logged native forest and from exotic forest at Pureora Forest Park, in the central North Island of New Zealand. Sex ratio, litter size, and breeding effort (pregnancy rate in females, proportion of males with visible tubules) were similar in all samples. By contrast, both density (captures per 100 trap-nights = C/ 100TN) and recruitment (proportion of young mice of age classes 1-3) were higher in densely vegetated habitats (along the road edge or in a young exotic plantation) than in the forest interior, whether logged or not. The age structures of the road edge and interior forest samples were significantly different (road edge, 33-35% young; interior, 10-11% young, means adjusted for sex, season and year by GLM). Mice of a given age caught in summer were larger, especially the females, implying that young mice grew faster in summer than at other seasons, and that older mice, especially females, also put on extra weight in summer. Most pregnant mice were found in spring and summer, but there was no winter quiescence in mature mice of either sex, and three of 29 pregnant females were collected in August. In five of 29 litters of embryos, at least one embryo was resorbing, totalling 12 of 161 embryos (7.4%). Litter size (viable embryos only) ranged from 5 to 8 (average 6) in 23 spring and summer pregnancies, but only 1-5 in four autumn and winter pregnancies. At high densities during 1984 in the young plantation (41.1 C/100TN in May) mice were significantly smaller in autumn, though somewhat larger in spring, and fewer young were recruited in 1984 and 1985. In these years we found significantly fewer males fertile, litters smaller and pregnancy rates lower, both in the plantation and in other habitats. The population Peak was much higher than most apparently similar post-seedfall Peaks in beech forest documented by the same methods, but it was different because (1) it developed very suddenly in autumn rather than building up slowly over winter and spring and Peaking in summer; (2) it was not preceded by winter breeding; and (3) it was made up mostly of mice born in the previous summer, whereas Peak populations in beech forests are usually made up of mice born during the previous winter and spring.