Elevationally biased avian predation as a contributor to the spatial distribution of geometrid moth outbreaks in sub‐arctic mountain birch forest

Abstract

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. There were 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.

Native generalist natural enemies and an introduced specialist parasitoid together control an invasive forest insect

“…The rates of pupal mortality from predators were overall similar to those found in prior studies in British Columbia (Horgan et al, 1999; Horgan & Myers, 2004; Roland, 1988) and Nova Scotia (Macphee et al, 1988; Pearsall & Walde, 1994), but only for the lower density years following C. albicans establishment at these sites (Roland, 1994). The predation rates we recorded were similar to those reported from western Europe (Hansen et al, 2009; Heisswolf et al, 2010; Varley et al, 1973), but were higher than some other estimates of pupal mortality in northern Europe from outbreak winter moth populations or high elevation sites (Klemola et al, 2009, 2014; Pepi et al, 2017). This further supports that idea depicted in Figure 1 that, at high densities, the predator saturation point can be surpassed resulting in relatively low predation rates.…”

Insect herbivory increases from forest to alpine tundra in Arctic mountains

“…Our data from six elevational gradients, in combination with other studies addressing the effects of elevation on plant-feeding insects and insect herbivory at high latitudes (Kristensen et al, 2020;Mjaaseth et al, 2005;Pepi et al, 2017;Ruohomäki et al, 1997), indicate that an increase in herbivory with elevation is more frequent in Arctic mountains than in mountains at lower latitudes (Moreira et al, 2018). This pattern, which contrasts with both theoretical predictions and the results of the analysis of elevational changes in herbivory in tropical and temperate mountains (Galmán et al, 2018), likely emerges due to an inverse elevational gradient in temperatures at the soil surface between closed-canopy forests and open alpine habitats.…”

Forest Insects and Climate Change

“…In subarctic mountain birch forests, defoliation rates are higher during outbreaks in high elevations. Pepi et al [82] showed that predation rates on E. autumnata and O. brumata larvae were almost twice as high in low versus high elevation sites, and that release from predation pressure at high elevations can favor outbreaks in these cooler habitats. It therefore appears that top-down effects can override bottom-up effects of climate warming in some tri-trophic systems.…”