Invasive shrub cover and tree species composition influence exotic earthworms

Abstract

Forest ecosystems are often invaded by multiple exotic species that, when combined, may amplify negative impacts on native biodiversity and ecosystem functioning. These patterns are often determined from plot-level experiments, but 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 diversity and alter litter decomposition rates. Exotic earthworms may aid this process by increasing decomposition rates and nutrient fluxes in forest soils. We hypothesized that honeysuckle increases exotic earthworm density and biomass in forest soils by providing a preferred litter resource and beneficial abiotic conditions. We established 15 transects, each 100-m long, in a stratified random design across a 245-ha deciduous forest in southwestern Ohio, USA. We measured honeysuckle shrub cover and canopy tree cover along the entire transect, and sampled earthworms and standing litter biomass at intervals along each transect. We found little evidence for Amur honeysuckle effects on earthworm density but found a strong positive effect of honeysuckle cover on earthworm biomass. We also recorded an effect of species composition of canopy trees on earthworm density and biomass. Moreover, multivariate analyses indicated a shift of earthworm community composition along an oak-elm gradient of tree species composition. Standing litter biomass was lowest in areas of high earthworm biomass and in the forest interior. Our results demonstrate that landscape-level variation in tree species composition and honeysuckle invasion drive exotic earthworm density and biomass. Our findings suggest invasion of exotic shrubs may facilitate invasion by nonnative earthworms.