Geographically partitioned spatial synchrony among cyclic moth populations

Abstract

Many species of forest lepidopterans exhibit regular population cycles, which culminate in outbreak densities at approximately ten‐year intervals. Population peaks and mass outbreaks typically occur synchronously and may lead to extensive forest damages over large geographic areas. Here, we report patterns of spatial synchrony among cyclic autumnal moth (Epirrita autumnata) populations across Fennoscandia, as inferred from 24 long‐term (10–33 years) data sets. The study provides the first formal analysis of spatial synchrony of this pest species which damages mountain birch (Betula pubescens ssp. czerepanovii) forests in the sub Arctic. We detected positive cross‐correlations in population growth rates between the time series, indicating overall spatial synchrony. However, we found the strongest degree of synchrony within geographically and climatically distinct regional clusters, into which time series were partitioned using cluster analyses. Within regional clusters, moth populations were exposed to the synchronizing effects of common, spatially autocorrelated environmental conditions, i.e. a Moran effect. Consequently, we conclude that a geographically and climatically restricted Moran effect, perhaps interacting with dispersal, is the most likely explanation for the regionally partitioned pattern of synchrony among autumnal moth populations in Fennoscandia. Our results emphasize that high amounts of environmental variation may result in a clear structuring of spatial synchrony at unexpectedly small scales.

Spatially mismatched trophic dynamics: cyclically outbreaking geometrids and their larval parasitoids

“…This also regards previous studies of population dynamics of the two focal geometrid species (e.g. Ims et al 2004;Klemola et al 2006;Tenow et al 2007). Previous studies of impact of parasitoids in these moths have invariably been conducted on relatively small spatial scales, and even such local studies have not proved that larval parasitoids are able to terminate outbreaks (Ruohomäki et al 2000).…”

Phase-dependent outbreak dynamics of geometrid moth linked to host plant phenology

“…Like most other studies addressing the spatial dimensions of cyclic outbreak dynamics, previous analyses of birch forest moths have focused solely on the issue of regional synchrony across the temporal extent of the study (Klemola et al 2006;Tenow et al 2007), thus without attempting to partition out phase-or scaledependent patterns and processes as we have done here. Nevertheless, the previous studies have concluded that although dispersal could not be ruled out as a contributing synchronizing mechanism (Ims et al 2004), a regionalized climatic Moran effect is more likely.…”

“…birch) have recently been highlighted for the insecthost plant system in question (Stenseth & Mysterud 2002;van Asch & Visser 2007;Hagen et al 2008). As phenological patterns are observed to be structured according to latitudinal and altitudinal gradients , as well as topographical features, such a phenological mismatchdriven Moran effect could also be responsible for the more complex spatial dynamics recently reported for birch forest moths (Klemola et al 2006;Tenow et al 2007). Yet, other studies have hinted that dispersal (either in the moths or their enemies) may be important, at least under some circumstances (Ims et al 2004).…”

“…The cyclicity of the outbreaks is documented in qualitative historical records as far back as the 1860s (Tenow 1972;Nilssen et al 2007). Individual outbreaks vary greatly in amplitude, duration and spatial extent (Tenow 1972;Ruohomäki et al 2000;Klemola et al 2006;Tenow et al 2007). The outbreaks can be massive and have dramatic effects, with severe defoliation over vast areas and occasionally death of the forest (Tenow 1972;Kallio & Lehtonen 1973;Lehtonen & Heikkinen 1995;Tenow & Bylund 2000).…”

“…Yet, other studies have hinted that dispersal (either in the moths or their enemies) may be important, at least under some circumstances (Ims et al 2004). Previous studies of spatio-temporal dynamics of birch forest geometrids have been analysed according to the population synchrony approach (Klemola et al 2006;Tenow et al 2007;Hagen et al 2008).…”