Abstract
Feral house mice (Mus musculus) are well established throughout New Zealand. They are a potential pest species about which comparatively little is known, especially with respect to invertebrate populations where we might expect impact through predation to be highest. In this study I sought to model the potential local impact of feral house mouse on naturally occurring invertebrate populations. This was achieved by estimating consumption rates needed to meet daily energy demand, as determined in laboratory trials on both summer and winter-acclimatised mice captured upon coastal sand dunes just south of Dunedin, New Zealand.
Mouse density was estimated four times a year between Dec 1996 and Dec 1997 using mark recapture techniques. Estimates varied from 7 mice.ha-1 (Dec, 1996) to 16 mice.ha-1 (Sept, 1997). Males predominated during live-trapping and sex-related differences in hind foot and tail length were found. Most mice ceased breeding in winter and estimated mean litter size was comparatively small (5.1 ± 1.7) although postpartum fertilisation was evident. Nocturnal activity prevailed and range lengths averaged 57 .6 m ± 10.3 m with pregnant and/or lactating mice being over-represented among those mice making movements of greater than 150 m.
Stomach analysis revealed that mice were omnivorous although diet was biased towards invertebrates. More stomachs contained invertebrate material in summer whereas plant material was encountered significantly more often in winter. Lepidopteran (Noctuidae) and Coleopteran (Elateridae) larvae were important dietary constitutes but this importance varied significantly with season. The greater occurrence of Araneae remains in stomachs of reproductively active female mice in summer suggest that they might be an important food during female reproduction .
Winter acclimatisation involved changes in several thermoregulatory physiological parameters (basal metabolic rate and minimum thermal conductance and possibly body mass). Such adjustments are consistent with genotypic or developmental adaptation to cold and constitute energy saving mechanisms is response to high energy demand under conditions of low or mediocre food availability. Average daily metabolic rates varied little between seasons (summer, 2.69 ml.min-1; winter, 2.60 ml.min-1) while temporal changes in oxygen demand mimicked activity patterns of free-living mice. Energy assimilation efficiency of sunflower kernels (Helianthus annuus) was high and daily energy expenditure of winter-acclimatised mice was comparatively low (29.6 kJ.d-1).
Invertebrate consumption per mouse per day at Ocean View Reserve was estimated to be 4.37 g.d-1 (dry mass) in summer (Dec-May) and 4.23 g.d-1 (dry mass) in winter (Jun-Nov) which corresponds to an annual population consumption of 19.39 kg.ha-1.y-1 (dry mass). The results of this study clearly illustrate the potential significance of mouse predation upon invertebrate populations in coastal habitats in New Zealand. They highlight the need for detailed research into the largely ignored potential impact of mice on endemic invertebrate populations in New Zealand. Lepidopteran larval species appear to be particularly susceptible to mouse predation.