Relations among Invasions of Non-native Earthworms, Forest Floor Habitat, and Populations of Ground-nesting Songbirds in North Temperate Hardwood Forests

In north temperate and boreal North America, European earthworms ( Lumbricus spp.) have invaded previously earthworm-free forests, substantially changing soil structure, removing the litter layer, and reducing richness and cover of plants. Whether these changes affect ground-nesting and ground-foraging songbirds remains unknown. I sampled earthworm populations and surveyed for, monitored nests of, and measured attributes of habitat of Ovenbirds ( Seiurus aurocapillus) and Hermit Thrushes (Catharus guttatus ), both ground-nesting songbirds, in the Chequamegon-Nicolet National Forest, Wisconsin. In another study at survey points scattered across 2 national forests, the Chequamegon-Nicolet and the Chippewa in north-central Minnesota, I investigated relations between Lumbricus biomass, point and landscape-scale habitat, and abundance of 4 ground-nesting songbird species.;In the first study, bird surveys indicated significantly lower densities of Ovenbirds and Hermit Thrushes in relation to Lumbricus invasions. Moreover, Ovenbirds experienced greater nest predation rates---possibly due to reduced nest concealment---in association with increased sedge cover and decreased litter depth, habitat characteristics strongly related to Lumbricus invasions. In the second study, the Ovenbird was the only species to display significantly reduced density in relation to Lumbricus, with this relation evident only in sugar maple (Acer saccharum)-basswood (Tilia americana ) forests. Lumbricus biomass was the variable most strongly related to Ovenbird density, displaying a significant inverse relation. These results provide strong evidence that earthworm invasions contribute to local---and potentially regional---population declines of ground-dwelling songbirds, and especially of Ovenbirds.;Quantifying earthworm invasions is necessary for understanding earthworm impacts and for identifying earthworm-free areas for targeting prevention measures. Current earthworm sampling methods are effort intensive, making it economically impractical to quantify invasions across broad spatial scales. I developed a method for rapidly assessing earthworm invasion severity by testing how accurately a 5-stage system of visual classification, based on soil and forest floor characteristics, reflected true invasion stage. I found that the visual system successfully approximated previously proposed stages of earthworm invasion. Biomass of earthworm species differed as expected among the 5 categories. I conclude that the visual method provided an efficient and accurate approach for rapidly assessing severity of earthworm invasions in hardwood forests of the northern Midwest region.