Mating failure and dispersal mortality slow gypsy moth spread in heterogeneous landscapes
Understanding how landscape heterogeneity affects patterns of range expansion is a contemporary challenge in ecology. Allee effects are thought to slow range expansion, and recent findings indicate that environmental heterogeneity can drive variations in Allee effects, but few studies have described mechanisms contributing to such variations or their impacts on spread in heterogeneous landscapes. This study combined field experiments, simulation modeling, and analysis of empirical spread patterns to investigate how landscape structure affects the spread of the gypsy moth in Virginia and West Virginia.
In experiments designed to assess mate finding in complex landscapes, we found that adult gypsy moths resisted leaving forest patches and that mate finding probabilities decayed more rapidly over distance in the non-forest matrix than in the forest. These findings informed the development individual-based simulations of gypsy moth spread in complex landscapes, which predicted rates of population persistence and spread to increase as forested area and the aggregation of forest patches also increased. These results reflect effects of landscape structure on gypsy moth mortality and mate finding, which collectively exacerbate a mating failure Allee effect. Our model predictions were evaluated against empirical patterns of gypsy moth spread, which showed relationships between spread rate and landscape structure consistent with our model predictions. These results highlight cross-scale interactions in which local processes (e.g. survivorship, mate finding) scale up to landscape-level patterns and elucidate potentially widespread mechanisms by which landscape structure may affect Allee effects and gypsy moth spread.