An invasive insect, hemlock woolly adelgid, indirectly impacts Louisiana Waterthrush nest site selection and nest survival in the southern Appalachians

ABSTRACT Eastern hemlock (Tsuga canadensis) is declining throughout the eastern United States due to hemlock woolly adelgid (Adelges tsugae Annand), an invasive insect from Asia. In the southern Appalachians, hemlock is concentrated in moist ravines and its decline threatens riparian ecosystems. Previous research on this invasion has focused on adelgid control and how hemlock decline affects community composition or forest processes; few studies have evaluated the consequences for demography of obligate riparian species. The Louisiana Waterthrush (Parkesia motacilla) is an obligate riparian species that could be sensitive to hemlock condition in this region, but how individuals respond to decline is currently unknown. To address this knowledge gap, we leveraged recent adelgid treatment efforts in Great Smoky Mountains National Park to evaluate the relationship(s) between hemlock decline and waterthrush habitat selection (foraging and nest site) and vital rates (nest and adult survival). We found that hemlock decline was unrelated to foraging habitat selection and apparent adult survival, but was related to nest site selection through an interaction with percent ground cover of exposed live tree roots: birds selected for nest sites in areas with more exposed live roots but only when hemlock was in poor condition. Nest survival was lower in areas where deciduous species (vs. evergreen species) dominated the understory, suggesting that adelgid invasion could indirectly impact waterthrush fitness depending on how vegetative succession proceeds following hemlock decline. Our results suggest that the short-term consequences of adelgid invasion on this riparian avian species are minimal in this area, but these relationships are likely dynamic and dependent on local habitat features and the predator community response to hemlock decline. LAY SUMMARY Although eastern hemlock decline caused by the invasive hemlock woolly adelgid is altering riparian habitats in the southern Appalachians, Louisiana Waterthrush (Parkesia motacilla) was minimally impacted, at least in the short term. Understanding the impacts of hemlock decline is necessary for strategic conservation efforts and management guidance for waterthrush and other species that rely on riparian habitats experiencing adelgid invasion and hemlock mortality. We predicted that, as an obligate riparian species, waterthrush would be sensitive to hemlock condition with respect to their habitat selection (foraging and nest site) and vital rates (nest and adult survival). Contrary to our predictions, hemlock condition was unrelated to foraging habitat selection and apparent adult survival and was only indirectly related to nest site selection and nest survival. Our results suggest that the short-term consequences of hemlock decline on waterthrush are minimal, but these relationships are dynamic and likely dependent on local habitat features, forest succession following hemlock decline, and predator community response to hemlock decline, so further monitoring is warranted.