Spatial synchrony in fluctuations of population abundance is normally attributed to synchronous environmental perturbations (e.g., weather) or dispersal. There is also theoretical evidence and limited empirical evidence that spatial synchrony in one species due to Moran effects may be transferred to other species through tropic interactions. Evaluating this hypothesis in nature is difficult due to a lack of both data and statistical methods. Previous simulation modeling of the spatiotemporal dynamics of gypsy moth populations in North America, their chief predator (the white-footed mouse), and the primary winter food source for the mouse (red oak acorns) suggests the synchrony of acorn masting propagates to the mouse and then to the gypsy moth via a chain of trophic interactions. Previous analyses of defoliation, weather, and acorn masting data indicated relationships between the synchrony of precipitation and the synchrony of both defoliation and acorn masting, but empirical evidence for the propagation of synchrony from acorns to gypsy moth populations was lacking. Here, we evaluate the possibility of trophic transfers of spatial synchrony among species based on spatially explicit data on forest tree species composition, and 40 years (1975-2014) of annual defoliation and weather data. We quantify geographic variation in the synchrony of gypsy moth defoliation and weather with multivariate local indicators of spatial association. We then evaluate the role of red oak in the local synchrony of gypsy moth outbreaks while accounting for the local synchrony of weather using multiple regression.
Several hotspots in the synchrony of gypsy moth defoliation were identified, with areas of strong local synchrony occurring primarily in the New England states. Hotspots of strong local synchrony in weather variables (represented by principal components axes) were more widely scattered across the study region. The local synchrony of defoliation increased significantly with red oak abundance and the local synchrony of precipitation variables but there was no relationship between the synchrony of defoliation and the synchrony of temperature variables. The results provide empirical support for the hypothesis that spatial synchrony can be propagated among species through chains of trophic interactions.