Sammendrag
1. Population dynamics and interactions that vary over a species’ range are
of particular importance in the context of latitudinal clines in biological diversity.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.
2. 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.
3. Larval densities were reduced by bird predation at three lower elevations, but
not at the highest elevation, and predation rates were 1.9 times 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, although there were more eggs and
chicks at the lowest elevation. Therewere no consistent differences in predation on pupae
by elevation.
4. These results suggest that elevational variation in avian predation pressure on larvae
may help to 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.
Ecosystem exploitation hypothesis, Fennoscandia, generalist predators,
Norway, predator exclusion, pupal predation, top-down control.
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