Population dynamics and interactions that vary over a species range are of particular importance in the context of latitudinal clines in biological diversity. Theory suggests that increased amplitude in population fluctuations with latitude and elevation may be related to the decreased importance of generalist natural enemies relative to specialists. Winter moth (Operophtera brumata) and autumnal moth (Epirrita autumnata) are two species of eruptive geometrids that vary widely in outbreak tendency over their range, which generally increases from south to north and with elevation in Fennoscandia. The predation pressure on geometrid larvae and pupae over an elevational gradient was tested. The effects of background larval density and bird occupancy of monitoring nest boxes on predation rates were also tested. Predation on larvae was tested through exclusion treatments at 20 replicate stations over four elevations at one site, while pupae were set out to measure predation at two elevations at three sites.
Larval densities were reduced by bird predation at three lower elevations, but not at the highest elevation, and predation rates were 1.9x higher at the lowest elevation than at the highest elevation. The rate of predation on larvae was not related to background larval density or nest box occupancy, though there were more eggs and chicks at the lowest elevation. There were no consistent differences in predation on pupae by elevation. These results suggest that elevational variation in avian predation pressure on larvae may help drive elevational differences in outbreak tendency, and that birds may play a more important role in geometrid population dynamics than the focus on invertebrate and soil predators of previous work would suggest. This work aligns with previous theory in that lower impacts of generalist predators at higher elevation may contribute to more extreme outbreaks, though from other work there is no evidence of a correspondingly greater impact of specialist natural enemies at higher elevations.