We tested the hypothesis that pupal predation by shrews underlies the altitudinal distribution pattern of the geometrid moth species Operophtera brumata L. (winter moth) and Epirrita autumnata Bkh (autumnal moth) in a sub-arctic birch forest in northern Fennoscandia. In particular, we predicted more intense pupal predation at low altitudes where the two moth species normally do not reach outbreak densities. Predation of pupae of both moth species was estimated along 10 parallel altitudinal transects, spanning from sea level to the altitudinal tree-limit in a coastal birch forest in northern Norway. Shrew abundance and the abundance and population growth rate of the two moth species were assessed in the same transects. Our study provided no support for the hypothesis that pupal predation by shrews can account for the altitudinal distribution of the two moth species. Despite high densities of common shrews (Sorex araneus L.) and an observed predation rate of ∼90%, there was no difference in the rate of pupal predation either between the two geometrid species or between the various altitudes. These results narrow down the range of possible explanations for the altitudinal distribution pattern of these insects in northern birch forests.
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