Abstract: Geographic variation in density-dependent dynamics impacts the synchronizing effect of dispersal and regional stochasticity

Abstract

Explanations for the ubiquitous presence of spatially synchronous population dynamics have assumed that density-dependent processes governing the dynamics of local populations are identical among disjunct populations, and low levels of dispersal or small amounts of regionalized stochasticity ("Moran effect") can act to synchronize populations. In this study we used historical spatially-referenced data on gypsy moth, Lymantria dispar, outbreaks to document that density-dependent process can vary substantially across geographical landscapes. This variation may be due in part to geographical variation in the habitat (e.g., variation in forest composition). We then used a 2nd order log-linear stochastic model to explore how among-population variation in density-dependent processes affects synchronization via either synchronous stochastic forcing or dispersal. We found that geographical variation in direct density dependence (1st order) greatly diminishes synchrony caused by stochasticity but only slightly decreases synchronization via dispersal. Variation in delayed density dependence (2nd order) diluted synchrony caused by regional stochasticity to a lesser extent than first order variation, but it did not have any influence on synchrony caused by dispersal. In general, synchronization caused by dispersal was primarily dependent upon the instability of populations and only weakly, if at all, affected by similarity in density dependence among populations. We conclude that studies of synchrony should carefully consider both the nature of the synchronizing agents and the pattern of local density-dependent processes including how these vary geographically.

Keywords: Spatial synchrony, Moran effect, gypsy moth, population cycle, stability, Lymantria dispar, outbreak