The ecology and ecophysiology of Marion Island house mice, Mus Musculus L.

English: This thesis presents the results of a studv of the biotic and abiotic conditions experienced by house mice on Marion Island, their morphological and reproductional adaptations to island conditions, the seasonal changes in their diet. and of the densities and biomasses of their prey items. By establishing burrow systems and sheltered aboveground runways nuce experience a microelimate that is far less harsh than the macroelimatie regime. In terms of warmth, this extends the season of mouse activity significantly compared with what would be allowed by the macroclimate. House mice are opportunistic feeders and this plays a major role in their survival under the harsh conditions on Marion Island. The mice are primarily carnivores and impact severely on soil macroinvertebrate populations, annually removing up to several times the average instantaneous standing crop of some macroinvertebrate populations. Since macroinvertebrates are cardinal agents of ecosystem functioning by being the main mediators of nutrient cycling on the island, their predation by mice has severe ecological implications. Between 1979/80 and 1992/93 the densities and biomasses of the mouse's major invertebrate prey species have decreased. The percentage composition of the various prey types in the macroinvertebrate population has also changed. These changes have caused changes in the composition of the mouse's diet. Seasonal changes in reproductive status, sex ratio, age structure, body mass and length, kidney- and adrenal mass, and length and shape of intestines were determined, in order to provide information concerning the house mouse's response to fluctuating environmental parameters and to assess the levels of stress experienced by mice at different times of the year. Stress levels are influenced by population density, sex, reproductive status, temperature and availability of food. In 1992/93 mice had significantly larger body to tail length ratios than in 1979/80, despite the fact that the island warmed considerably in the interim. This warming has allowed a significantly longer breeding season, perhaps by as much as two months. It is suggested that this is the reason that end of season densities are now considerably higher than in 1979/80.