US-IALE 2018

AUTHORS: Gwendolyn Lloyd*, Michael Mahon, Thomas O. Crist – Miami University

ABSTRACT: Natural ecosystems are often invaded by multiple exotic species that when combined, may exacerbate negative impacts on native biodiversity and ecosystem functioning, but these are often determined from plot-level experiments and it is less clear how interactions among native and exotic species vary across landscapes. Amur honeysuckle (Lonicera maackii) is an invasive understory shrub in eastern forests, observed to reduce understory plant abundance and diversity, and exotic European earthworms may aid this process by decreasing nutrient availability and increasing decomposition rates in forest soils. We hypothesized that honeysuckle increases abundance of exotic earthworms in forest soils by providing a preferred litter resource and facilitating overall litter decomposition. We predicted that areas of high honeysuckle cover in the landscape would have less standing litter biomass and greater earthworm abundance compared to areas with lower honeysuckle invasion. To test this hypothesis, we established 15 transects, each 100-m long, in a stratified random design across a 150-ha forest within the Miami University Natural Areas in southwestern Ohio, USA. We measured honeysuckle cover and forest canopy cover of different tree species along the entire transect, and then sampled earthworms, soil moisture, and standing litter biomass at three evenly spaced points along each transect. Structural equation models showed there was no direct effect of honeysuckle cover on exotic earthworm abundance, however, honeysuckle cover directly increased soil moisture, a factor that was positively related to earthworm abundance. Higher earthworm abundance, in turn, resulted in reduced leaf litter biomass, which was also influenced by tree species composition in the forest canopy. Our findings suggest that landscape-level variation in the indirect effects of soils and tree composition influence the outcome of interactions among honeysuckle, exotic earthworms, and leaf-litter biomass.