A suite of invasive mammalian species preys on indigenous fauna in New Zealand,where bats are the only native terrestrial mammals.House mice (Mus musculus) are ubiquitous small omnivores whose importance as predators is unknown because effects of larger mammals mask their impacts.Mice are often the only mammal remaining in fenced biodiversity sanctuaries,and sometimes become abundant.Therefore,understanding the consequences of mice alone is increasing in urgency.We are assessing impacts of mice on native animals in both forest and grassland.In forest,we compared two sites fenced to exclude most mammals: +Mice,where the mouse population has been allowed to increase,and –Mice,where mice were controlled with poison.Multiple invertebrate taxa had fewer and smaller individuals in +Mice than in –Mice.Also,in +Mice,mice quickly preyed on small bird eggs in experimental ground nests.We have now reversed the experimental treatments,eradicating mice from +Mice and allowing the population to increase in –Mice.In a grassland–shrubland matrix,we used devices to detect mice,lizards and invertebrates at 49 random locations.Mouse presence coincided with few or zero detections of skinks,geckos and invertebrates.All groups were distributed across degraded,intact and mixed grass-shrub habitats,but mice were detected only in the last.This spatial distribution may change following high seed production (masting) by indigenous grasses,which leads to periodic mouse population increases.The next stage of this project is experimental comparison of mouse prey species abundance between sites with and without mice.