Non-native invasive earthworms (Lumbricus spp.) substantially change previously earthworm-free hardwood forests of North America by consuming the leaf litter layer, reducing cover and richness of herbaceous plants, and increasing dominance of sedges and grasses. These changes have been associated with reduced density of Ovenbirds (Seiurus aurocapilla) and Hermit Thrushes (Catharus guttatus) in 10-20 ha forest stands, and with reduced Ovenbird nesting success. Whether earthworms reduce songbird populations across a regional extent is unclear. We investigated relationships among Lumbricus, vegetation structure, landscape patterns of forest cover, and density of four ground-nesting songbird species at points scattered across the Chequamegon-Nicolet (Wisconsin) and Chippewa (Minnesota) National Forests, USA. In both national forests, Ovenbird density was significantly lower at invaded points than Lumbricus-free points, but only in sugar maple (Acer saccharum) and sugar maple/basswood (Tilia americana) (hereafter, maple-basswood) woodlands. Density of the Hermit Thrush, Black-and-white Warbler (Mniotilta varia), and Veery (Catharus fuscescens) did not differ in relation to Lumbricus. In maple-basswood forests, Lumbricus biomass was the best predictor of Ovenbird density, with greater biomass associated with reduced density. Vegetation structure and landscape pattern variables received weak support as density predictors. Across all forest types, Ovenbird density was most strongly related to forest cover within 500 and 1,000 m radii. Our results suggest that earthworm invasions may pose a regional threat to Ovenbirds within maple-basswood forests of the U. S. northern Midwest.
Bibliographical noteFunding Information:
Acknowledgments Research was funded by the American Museum of Natural History, Bell Museum of Natural History, Dayton-Wilkie Foundation, The Explorers Club, Minnesota Ornithologists’ Union, U.S. Department of Agriculture (Chequamegon-Nicolet National Forest), and Wisconsin Society for Ornithology. S.R.L. was supported by a University of Minnesota Graduate School Fellowship and National Science Foundation Interdisciplinary Graduate Education and Research Traineeship: Risk Analysis for Introduced Species/Genotypes (NSF-DGE-0653827). We thank C. Hakseth, L. Raab, and K. Bennett for earthworm sampling assistance, J. Bednar, P. Dolan-Linne, A. Bracey, J. Bailley, C. Lapin, D. Ostrowski, H. Seeland, and J. Smith for conducting bird surveys, and A. Grinde for compiling vegetation and bird data. We thank L.E. Frelich, D.E. Andersen, P. Bolstad, A. Holdsworth, and C. Hale for guiding the study’s development. Work was conducted with appropriate permits and approval by the University of Minnesota Institutional Animal Care and Use Committee.
- Hermit Thrush
- Invasive earthworms
- Minnesota, USA
- Wisconsin, USA