Abundance, predation, and habitat associations of lemming winter nests in northern Sweden

Abstract

Spatially synchronous fluctuations of animal populations have profound ecological consequences, especially in northern latitudes. Spatially coupled fluctuations are often seen in small rodent populations, albeit with local and regional variations. While both resource limitation and predation influence rodent dynamics, their relative importance for generating spatial variation is less clear, particularly during winter. In this study, we quantify spatial variation in winter abundance of the Norwegian lemming (Lemmus lemmus) across three ecologically connected mountain areas in northern Sweden and evaluate whether the relative strength of bottom-up and top-down regulation influences such variation. Our data included observations of predated and nonpredated winter nests as well as environmental characteristics of nest locations and nest predation. While the direction of annual changes in lemming nest abundance was perfectly synchronized among the three areas, there were differences in nest abundance, potentially caused by contrasting amplitudes of temporal fluctuations in lemming winter populations. Mustelid predation was positively associated with decreasing lemming populations but did not differ in occurrence among the three areas. Lemming nests were predominantly observed in meadows, whereas areas prone to flooding and close to the tree line were underrepresented. Mustelid predation was most common close to the tree line, but not associated with geomorphological characteristics related to snow depth. We suggest that the observed differences in lemming winter abundances were caused by variations in the relative strength of bottom-up and top-down regulation in the threemountain areas. We encourage further studies evaluating how the relative strength of different processes influence local population regulation, and how such processes influence spatial variation in animal population dynamics at different spatial scales.