Regional Extent of an Ecosystem Engineer: Earthworm Invasion in Northern Hardwood Forests

Abstract

The invasion of exotic earthworms into northern temperate and boreal forests previously devoid of earthworms is an important driver of ecosystem change. Earthworm invasion can cause significant changes in soil structure and communities, nutrient cycles, and the diversity and abundance of herbaceous plants. However, the regional extent and patterns of this invasion are poorly known. We conducted a regional survey in the Chippewa and Chequamegon National Forests, in Minnesota and Wisconsin, U.S.A., respectively, to measure the extent and patterns of earthworm invasion and their relationship to potential earthworm introduction sites. We sampled earthworms, soils, and vegetation in 20 mature, sugar maple-dominated forest stands in each national forest and analyzed the relationship between the presence of five earthworm taxonomic groups, habitat variables, and distance to the nearest potential introduction site. Earthworm invasion was extensive but incomplete in the two national forests. Four of the six earthworm taxonomic groups occurred in 55-95% of transects; however 20% of all transects were invaded by only one taxonomic group that has relatively minor ecological effects. Earthworm taxonomic groups exhibited a similar sequence of invasion found in other studies: Dendrobaena > Aporrectodea = Lumbricus juveniles > L. rubellus > L. terrestris. Distance to the nearest road was the best predictor of earthworm invasion in Wisconsin while distance to the nearest cabin was the best predictor in Minnesota. These data allow us to make preliminary assessments of landscape patterns of earthworm invasion. As an example, we estimate that 82% of upland mesic hardwood stands in the Wisconsin region are likely invaded by most taxonomic groups while only 3% are unlikely to be invaded at present. Distance to roads and cabins provides a coarse-scale predictor of earthworm invasion to focus stand-level assessments that will help forest managers better understand current and potential forest conditions and identify uninvaded areas that could serve as important refugia for plant species threatened by earthworm invasion.

Invasions of non-native earthworms related to population declines of ground-nesting songbirds across a regional extent in northern hardwood forests of North America

“…1). For details about the national forests, including climate, glacial history, forest cover, and soils, see Holdsworth et al (2007b).…”

“…We stratified selection by forest district so that points were scattered across each forest, and we stratified by distance from the nearest road, because this factor provides a first-order predictor of earthworm invasion in the study area (Holdsworth et al 2007b). The selection approach was designed to provide broad spatial coverage of both national forests and to capture Lumbricus-free and Lumbricus-invaded sites.…”

“…Regional-scale studies indicate that a vast majority of northern U.S. hardwood forests are invaded by Lumbricus (Gundale et al 2005;Holdsworth et al 2007b). The broad regional extent of invasions cannot be explained solely by natural dispersal, which is slow (5-15 m/year, Marinissen and van den Bosch 1992).…”

“…Instead, invasion patterns are consistent with humanassisted dispersal. Remaining earthworm-free forests are in wilderness areas receiving minimal human use, and invasions emanate from roads, boat launches, and agricultural areas (Suarez et al 2006;Cameron et al 2007;Holdsworth et al 2007b;Cameron and Bayne 2009).…”

Earthworm, microbial biomass, and leaf litter decay responses after invasive honeysuckle shrub removal from urban woodlands.

“…Modification of the soil matrix coupled with the feeding activity of earthworms can increase litter decomposition rates (Suárez et al 2006;Holdsworth et al 2008), change the availability and retention of soil carbon, nitrogen, and phosphorus (Bohlen et al 2004a,b;Suárez et al 2004;Hale et al 2005b;Szlavecz et al 2006), alter the size and structure of soil microbial (McLean and Parkinson 2000;Groffman et al 2004;McLean et al 2006;Dempsey et al 2011) and soil microfauna communities (McLean and Parkinson 1998;Migge-Kleian et al 2006), affect the abundance of arbuscular mycorrhizal fungi populations (Lawrence et al 2003), and change the distribution of plant fine roots . Depletion of the organic horizons and leaf litter layers by exotic earthworms can negatively impact the diversity and abundance of native plant species (Gundale 2002;Hale et al 2006;Nuzzo et al 2009;Hopfensperger et al 2011), though the magnitude of these effects may vary depending on site factors and the species of earthworm involved (Bohlen et al 2004c, Hale et al 2005bHale et al 2006;Holdsworth et al 2007).…”