Synchronous population fluctuations among spatially separate populations have been widely observed in many taxa, at distances up to thousands of kilometers, often due to synchronous environmental fluctuations (i.e., Moran effects). However, geographic patterns in population synchrony are rarely studied in greater detail than the distance-decay of statistical correlation among populations. This study evaluates evidence for a biogeography of synchrony in gypsy moth outbreaks in the Northeastern US, 1975-2014. We address three mechanisms that may underlie spatial structure in population synchrony: spatial pattern in a synchronizing environmental driver; differences in density-dependent population regulation; and changes in the synchronizing environmental variable owing to differences in the limiting factor for population growth. Our approach combines graph theory with synchrony metrics based on wavelet analysis.
Results/Conclusions
There is a biogeography of gypsy moth defoliation synchrony. Clustering of defoliation timeseries based on wavelet transforms indicated three dominant patterns: synchronized oscillations with a 4-5 year period, synchronized oscillations with an 8-10 year period, and a combination of the 4-5 and 8-10 year cycles. Considering populations as a network with edge weight equal to the wavelet coherence between population pairs, the left skew of the weighted degree distribution suggests that most populations are relatively synchronous with each other. Within-cluster coherence was on average 22% higher than between-cluster coherence. Spatial patterns in synchrony were positively related to the spatial pattern in synchrony of weather (coeff. = 0.139, p = 0.044), while controlling for the effect of proximity. Patterns of synchrony were also related to forest type and synchrony of forest biomass production. These findings suggest that biogeography of gypsy moth defoliation synchrony is a complex phenomenon resulting from multiple mechanisms.