The impacts of exotic pests are a concern among ecologists and conservationists. When these invaders target foundation species in ecological communities, a range of indirect negative effects are possible. In this study, we investigated the potential for incidental ecological effects of invasive pests on a non-target plant species. Bazzania trilobata is an abundant liverwort often found in association with eastern hemlock (Tsuga canadensis), a foundation species in New England. The future of hemlock ecosystems appears uncertain, however, with impending decline due to two exotic insects: hemlock woolly adelgid (Adelges tsugae) and elongate hemlock scale (Fiorinia externa). The replacement of evergreen T. canadensis by deciduous tree species is the most concerning ecological shift, as this will likely result in increased light exposure and temperatures, placing sensitive, hemlock-associated understory species, like B. trilobata, at risk of decline. To test the potential impact of these forest declines, we executed a multi-year transplant experiment tracking B. trilobata survival and growth under canopies of varying T. canadensis vs. deciduous tree composition. Exposing B. trilobata to a range of novel conditions relative to its natural habitat under T. canadensis enabled us to quantify the impact that invasive species-triggered canopy decline might have on this understory species.
Results/Conclusions
After two growing seasons, 33% of the 88 B. trilobata samples transplanted to our experimental site were in decline or dead, while 100% of 20 control samples that were replanted within the source population of B. trilobata under a dense canopy of T. canadensis were alive and stable. Results from single factor and multivariate logistic regressions based on the transplanted samples indicated that, among the abiotic and biotic tested as predictors of B. trilobata sample status (i.e., stable or decline), the total number of trees, and the number of T. canadensis trees surrounding each sample were the most significant correlates. These findings suggest that indirect negative effects for B. trilobata are likely with the decline of T. canadensis from A. tsugae and F. externa. This case also appears to be one of the first plant species known to be at risk from the indirect effects of T. canadensis decline due to exotic insects. Additional surveys will explore the complex niche of B. trilobata and the possible implications of changes in tree leaf-litter composition as a result of forest succession toward deciduous trees.