It is known that the biodiversity of insects and other invertebrates is a central component of ecosystem function, not only because they constitute a great part of the animal biomass, but they also act as ecosystem engineers. However, the synergistic effects of novel disturbances, such as tree pathogens, and the resulting changes in environmental variables on invertebrate biodiversity in forested habitats are not well understood. We have begun a replicated, long-term, field experiment designed to test the impact of the hemlock woolly adelgid (Adelges tsugae) on the structural features of and invertebrate biodiversity in hemlock-hardwood forests in Maine. The relationships between invertebrate community structure, vegetation patterns, and environmental variables were measured in twenty, 40 m diameter, circular plots. In each plot we measured ant and earthworm species richness and abundance (dependent variables), plus the following biotic and abiotic environmental variables (independent variables): overstory and ground layer plant composition, soil moisture, soil texture, photosynthetically active radiation (PAR, μmol/m2), soil organic matter, leaf litter type and depth, and the presence, size, and decay class of fine and coarse woody debris. Ants were collected using leaf litter samples, hand-collecting and pitfall trapping methods. Earthworms were collected in 30 x 30 cm subplots using a mustard-extraction method. Data were analyzed using a stepwise multiple regression to determine which independent variables explained a significant component of ant and earthworm species richness and abundance.
Results/Conclusions
We found that ant species richness was significantly associated with vegetation composition and amount of woody debris, but ant abundance was not. Hemlock basal area of the plots, and leaf litter type and depth were not significantly associated with species richness or abundance, although ant richness was significantly associated with PAR and soil moisture. Earthworm abundance was found to be positively related to soil organic matter, clay content, and woody debris, but negatively related to litter depth and PAR. Our study shows vegetation composition and the presence of woody debris are particularly influential in maintaining both ant and earthworm richness and abundance in a forested ecosystem. These findings suggest that both vegetation and woody debris loads must be considered in forest management plans that seek to support invertebrate diversity. These results will guide the next phase of our investigations that will experimentally manipulate these variables in order to mimic the potential effects of the hemlock woolly adelgid in northern forests that have yet to be infested with the insect.
