Abstract
Summary 1. Sporadically eruptive house mouse populations provide an opportunity to test ideas about how intrinsic and extrinsic factors interact to limit rodent populations. New Zealand house mouse populations inhabiting beech forests and associated mosaics erupt when autumn beech seedfall is high. We analysed 25 years of data on mouse density and beech seedfall from a beech and podocarp/hardwood forest mosaic in New Zealand to test the effects of mouse density, beech seedfall and rat density on rates of change in mouse abundance, estimated as the rate of change in a density index for sequential seasonal transitions (r). 2. For autumn to winter and winter to spring transitions, r was related to the magnitude of autumn seedfall and mouse density at the start of each transition, regardless of whether forest types were considered separately or together. For spring to summer and summer to autumn transitions, r was related to mouse density alone. Density dependence could reflect the influence of intrinsic social factors, intraspecific competition for food, or the effects of parasites, diseases or predators. 3. We modified a conceptual model of variation in rodent reproductive effort to derive a general model of how numerical responses to density‐independent increases in food availability, modified by density‐dependent mechanisms and extrinsic precursors to population increase, drive sporadic eruptions in the density of some rodent populations. For house mice in New Zealand, the numerical response to seedfall was modified by some unidentified density‐dependent mechanism(s) to generate sporadic eruptions. Extrinsic precursors to population increase appear to be important in other eruptive house mouse populations. 4. House mouse density in beech forest was more volatile than in podocarp/hardwood forest, maximum rates of increase and decrease being higher in the former. Density tended to be higher in podocarp/hardwood forest when overall density was low, and higher in beech forest when overall density was high. This suggests that between eruptions, mice persist in podocarp/hardwood forest at higher densities than in adjoining beech forest.
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