Consequences of a terrestrial insect invader on stream-riparian food webs of the central Appalachians, USA

Abstract

The terrestrial invader hemlock woolly adelgid (Adelges tsugae) decimates eastern hemlock (Tsuga canadensis) stands that dominate riparian zones of Appalachian forest streams. However, the ecological consequences for linked aquatic-terrestrial ecosystems remain unknown. We measured stream-riparian trophic linkages at 21 sites of Ohio, West Virginia, and Virginia representing a chronosequence of T. canadensis decline. We measured reciprocal fluxes of basal resources (periphyton, terrestrial detritus), emerging aquatic insect flux rate and community composition, riparian orb-weaving spider density, and estimated spider trophic position and reliance on aquatically-derived energy using stable isotopes (¹³C, ¹⁵N, ²H) and Bayesian mixing models. Stream periphyton biomass was greater at uninvaded reference sites than at invaded sites and composition of the terrestrial-to-stream detritus flux changed with T. canadensis decline. Emergent aquatic insect community composition was partly explained by hemlock decline status, but the relative abundance of functional feeding groups was not. Riparian orb-weaving spider densities were highest at sites with severe hemlock decline (F = 4.27, p = 0.022), but were not linked to insect emergence flux rates (p > 0.10). Both trophic position (${\overline{x}}$ = 2.4) and relative reliance on aquatically-derived energy (${\overline{x}}$ = 83%) were comparable among spider families (Tetragnathidae, Araneidae, Pisauridae) and site decline status. Although spider δ¹³C signatures were unrelated to those of the most numerous emergent insect families, δ¹⁵N signatures of Araneidae and Pisauridae tracked emergent insect δ¹⁵N (r² = 0.42 and 0.78, respectively), suggesting a trophic linkage. Overall, the ecological consequences of this invader were clearest at lower trophic levels, with more nuanced impacts on riparian spiders.