Exotic earthworm effects on hardwood forest floor, nutrient availability and native plants: a mesocosm study

European earthworms are invading previously worm-free hardwood forests across Minnesota and the Great Lakes region. In many of these forests, earthworm invasions have been associated with the loss of a previously thick forest floor. The ability of earthworms to alter and control ecosystem processes has been demonstrated in agricultural systems, but the dynamics and impact of these invasions in native forest ecosystems is largely unknown. The impacts of earthworm invasion are expected to be related to the size and species composition of the earthworm population because different species have different habitat and feeding preferences. We identified four sugar maple dominated forests in north central Minnesota in the Chippewa National Forest with active earthworm invasion. In each site a sample grid of 45 points (30 x 150 m) 10 m apart in three parallel transects with 15 points each was established that spanned a visible leading edge of invasion. Over four years earthworm populations and forest floor thickness were sampled across all transects, thus providing both a space-for-time assessment of decadal scale successional dynamics and a four-year window into shorter time changes. We found a succession of earthworm species across the visible leading edge due to different patterns of colonization by different earthworm species. Marked increases in space and time in earthworm biomass were associated with the development of discrete transition zones where forest floor thickness decreases to zero in as little as 75 m from areas that have forest floor layers up to 10 cm thick with advancement of the visible leading edge of up to 30 m in four years at three of the study sites. The epi-endogeic species Lumbricus rubellus led to the most rapid removal of forest floor material during initial invasion. Epigeic and epi-endogeic species of earthworms may facilitate the establishment of other species of earthworms leading to the establishment of stable populations of endogeic and anecic species, which prevent recovery of the forest floor