Spatial Synchrony in Forest Insect Outbreaks: Roles of Regional Stochasticity

Spatial synchrony, that is, correlated population fluctuation over wide geographical areas, has been detected in diverse taxa and over various geographical scales. The most commonly suggested mechanisms to explain spatial synchrony include dispersal and regional stochasticity (i.e., “the Moran effect”). We analyzed landscape-scale historical outbreak data for six forest insect species: spruce budworm (Choristoneura fumiferana), western spruce budworm (C. occidentalis), larch bud moth (Zeiraphera diniana), forest tent caterpillar (Malacosoma disstria), mountain pine beetle (Dendroctonus ponderosae), and gypsy moth (Lymantria dispar). We used a recently developed statistical method (the nonparametric covariance function) for quantifying the magnitude and spatial range of synchrony in both outbreak and corresponding weather data. The varying dispersal capabilities of the species enabled us to speculate on the relative importance of dispersal vs. the Moran effect as potential mechanisms behind the observed patterns. Our results indicated that spatial synchrony was not directly associated with dispersal capabilities at the spatial scales considered. In contrast, the spatial correlation in weather variables was high enough to account for the levels of synchrony observed in the outbreak data. Therefore, the Moran effect appeared to be the more dominant process affecting the spatial dynamics of these species at the landscape scale. In general, however, the synchrony in outbreaks declined more steeply with geographical distance than the correlation in the weather variables – breaking with the predictions of Moran’s theorem. A more detailed analysis of gypsy moth outbreak data showed that local dynamics varied considerably in a spatially dependent manner. The existence of such variation violates one of the assumptions of the Moran’s theorem, namely that the dynamic properties of disjunct populations are identical. We used a simple theoretical model to demonstrate that such geographical variation in local population dynamics dynamics may indeed force synchrony to decline more rapidly with distance than the correlation in the environment.