The role of Allee effects in rapid synchronization of the gypsy moth
Synchronous population dynamics in invasive species can result in large scale outbreaks that reduce the landscape’s ability to buffer disturbance, challenges land managers’ ability to suppress populations and mitigate the effects, and can increases invasion rates by increasing imigrants at invasion fronts. The gypsy moth is an invasive forest defoliator in North America with synchronous periodic outbreaks at distances up to several hundred kilometers. Rates of invasion in the gypsy moth are mediated by Allee effects, defined as depressed growth rates at low population densities, which vary spatiotemporally across the gypsy moth range. At the invasion front, gypsy moth outbreaks rapidly synchronize within 10 years after colonization. This is incongruent with predictions from a theoretical model, which indicate that populations with periodic dynamics akin to those in the gypsy moth will require up to 30 years to synchronize. In this study, we test whether Allee effects could facilitate the increased rate of synchronization by adding Allee effects to an invasion model. We explore the effects of different spatiotemporal patterns of heterogeneity in the Allee effect on population synchronization at an invasion front.
How Allee effects changed the rate of synchronization depended greatly on the spatiotemporal structure of the Allee effect. Adding a spatiotemporally constant Allee effect did not alter rate of synchronization in the model. In contrast, increasing the mean strength of periodically fluctuating Allee effects increased the time to synchronization from 30 to 20 years, but fell short of matching empirical rates. Increasing the mean strength of Allee effects that fluctuated randomly in time decreased the rate of synchronization from 30 years to the empirically observed 10 year period, consistent with empirical observations. Spatial variation in Allee effects had only small effects on the rate of synchronization. These results show that Allee effects could be the mechanism for rapid synchronization at the invasion front of the gypsy moth. Moreover, in classic population models that lack an Allee effect, the rate of synchronization is negatively related to the power of periodicity. We found that adding an Allee effect reversed this relationship, where populations with high periodicity were the quickest to synchronize. Further empirical analysis of low density population dynamics of the gypsy moth at the invasion front will provide a further test of this hypothesis.