Habitat modeling for the presence of exotic earthworms in eastern New York forests

Temperate forests of northeastern North America have apparently not regained native earthworm populations since the last glacial period. Exotic earthworm species from Europe and Asia are now spreading throughout New York State forests, but no tracking system is yet in place to estimate current ranges or directions of spread. Logistic models were used to test whether a small number of habitat variables are sufficient to estimate the probability of presence of earthworms at the forest stand scale (ca.1). The working hypothesis was that certain forest types would be more likely to contain earthworms, using five categories in rank order: early successional>sugar maple>deciduous mixed>spruce and pine plantations>eastern hemlock. Variables examined were forest cover, soil pH, distance to surface water, aspect, elevation, and slope, with the assistance of GIS.  Sampling was conducted at 109 sites in the Helderberg range of southwest Albany County in summer 2005 and 2006.  Two species of earthworms (Lumbricus terrestris and L. rubellus) were found in all locations; two others Dendrobaena octeadra and Octolasion tyrtaeumwere never found in hemlock stands. Total abundances of all species pooled correlated with forest type in the order predicted.  Logistic regression predicted that earthworm presence/absence  was predictable using only soil pH and forest cover. This model was validated using the data from the Catskill Mountain region, with 86% accuracy rate.