Quantifying regional-scale impacts of hemlock woolly adelgid invasion on North American forest bird communities

Humans significantly influence geographic patterns of biological invasions, creating conditions for species to overcome biogeographic barriers and colonize new areas. In the eastern United States, forested landscapes containing eastern hemlock (Tsuga canadensis) are under threat by the invasive hemlock woolly adelgid (Adelges tsugae). Although several studies have shown the negative effects of adelgid invasion in local bird communities, its regional impacts have not yet been quantified. Using broad-scale spatial (entire eastern US range of hemlock) and temporal (>40 years of bird monitoring data) databases, we built spatial auto-regressive generalized mixed linear models to estimate immediate and long-term population effects of adelgid infestation on population trends of fourteen bird species. We also determined how winter temperature interacted with adelgid infestation to affect population trends. We selected the best models using WAIC, and validated model performance and power using Monte Carlo simulation, permutation tests, and sensitivity analysis. For all but one species, the best model included the effects of adelgid infestation on abundance trends. We observed a > 30% decline for two hemlock-associates species after infestation: the Blackburnian warbler, and the Hermit thrush. Declines were greater in the warmest part of their ranges. In contrast, no control species showed similar declines. Our results demonstrate that birds locally associated with hemlock habitat, when evaluated at a broad spatiotemporal scale, also decline in abundance following infestation. At the same time, declines are not universal and are dampened as compared to local studies suggesting that spatial heterogeneity buffers demographic decline. We also found that cold winters are associated with smaller bird population declines, suggesting that rising winter temperatures due to climate change might remove this environmental barrier. Despite the difficulties of understanding and connecting landscape-scale processes with those at finer scales, it is critical to evaluate processes that govern biodiversity distribution from a regional perspective.