Ecological consequences of exotic invaders: Interactions involving European earthworms and native plant communities in hardwood forests

During the last few decades, European earthworm species have been invading previously worm free hardwood forest ecosystems in the cold temperate regions of North America. Anecdotal observations suggesting that the abundance and diversity of native plant species and tree seedlings declines following earthworm invasion raised concerns about the potential widespread loss of native forest plant species and the stability of hardwood-forest ecosystems. Incipient earthworm invasions, in the form of visible leading edges, exist in many hardwood forests in northern Minnesota, providing a rare opportunity to document the dynamics of earthworm invasions as they are occurring. Dissertation research objectives were to: (1) Use field studies to describe the rates and patterns of change in earthworm populations across leading edges of invasion and associated changes in the forest floor and upper soil horizons, herbaceous understory vegetation and seedling demography; (2) Use experimental mesocosms to examine the differential effects that the 3 widely distributed European earthworm species have on the forest floor, upper soil horizons and understory vegetation. Visible leading edges advanced up to 30 meters over 4 years and were associated with a succession of earthworm species and increasing total biomass leading to rapid declines in forest floor thickness, development of a thick A horizon, increased soil bulk density and declines in fine root biomass, nutrient availability, tree seedling density herbaceous plant diversity. Understory plant composition shifted from one of high diversity to a community dominated by Carex pennsylvanica and Arisaema triphyllum with rare occurrences of other native plant species. In the greenhouse mesocosm experiment individual earthworm species led to increased forest floor mass loss and A horizon thickness, but a 3 species assemblage led to increased nutrient availability and plant mortality with decreases in total fine root and total plant biomass. Individualistic responses among plant species to earthworm treatments also occurred, suggesting that this will be an important factor determining the trajectory of compositional changes in hardwood forests following earthworm invasion.