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

Forested landscapes containing eastern hemlock (Tsuga canadensis) are under threat by the invasive hemlock woolly adelgid (Adelges tsugae) in eastern North America. 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 (eastern US hemlock range) and temporal (> 40 years of bird data) databases, we built spatial auto-regressive generalized mixed linear models to estimate the effects of adelgid infestation on fourteen bird species’ demographic trends. We also evaluated how winter temperature mediates the relationship between infestation and bird trends. We selected the best models using WAIC and validated model performance and power using Monte Carlo simulation, permutation tests, and sensitivity analysis. The best model for all but one species included the effects of adelgid infestation on abundance trends. We observed a > 30% decline for two hemlock-associate species after infestation: the Blackburnian warbler, and the Hermit thrush. In contrast, no control species showed similar declines. When evaluated at a broad spatiotemporal scale, hemlock associates also decline in abundance following infestation. At the same time, declines are not as universal compared to local studies, suggesting that spatial heterogeneity might buffer them. Declines were greater in the warmest part of species ranges. Rising temperatures due to climate change will likely affect bird persistence even further by lowering adelgid overwinter mortality. Despite the difficulties of understanding and connecting landscape-scale processes with those at finer scales, it is critical to evaluate biodiversity distribution from a regional perspective.