Understanding human activities and global w“o”rming and its impacts on soil properties and temperature in a Minnesota hardwood forest

Abstract

Invasive earthworms create widespread ecological changes after they are introduced.Exotic earthworms are transported mainly through anthropogenically-mediated activities, including fishing, agriculture, horticulture, and development. Here, we use a conceptual framework to review the ways in which exotic earthworms are transported to new environments. This conceptual framework involves invasion filters (human activities filter and climate & edaphic filter) that constrain which exotic earthworm species can be transported within specific contexts. Differences in earthworms’ ecological behaviors, life cycle, and physiological tolerance of environmental conditions influence which species are transported and which regions can successfully be invaded. Within the human activities filter, we utilize the pathways of invasion laid out by Hulme et al. 2008, including release, escape, contaminant, stowaway, corridor, and unaided. These pathways follow a continuum of human intention; five of these pathways are associated with human activity: with release, escape, and contaminant pathways related to commodities, and stowaway plus corridor related to transport infrastructure. We find that the major human activities that transport exotic earthworms include discarding of fishing bait, agriculture, composting and horticulture, and development (e.g. the construction of roads, trails, houses, or campgrounds). Our review finds that although a diverse number of human activities transport exotic earthworms, the magnitude that specific activities transport earthworms is vastly understudied. We conclude that more research needs to be conducted to understand the methods that transport exotic earthworms in order to slow their spread. In the Upper Midwest, temperate hardwood forests have been heavily altered followingthe introduction of invasive earthworms of European, and more recently, Asian origin. Earthworms significantly modify the biological, chemical, and physical composition of soils in these ecosystems by mixing overlying organic horizons with underlying mineral soil layers. The recent invasion of these forests dominated by invasive European earthworms (Lumbricus terrestris, Lumbricus rubellus, Aporrectodea spp.) by ‘jumping worms’ (Amynthas agrestis and Amynthas tokioensis) has created profound and distinct changes to the soil that are not well understood. We surveyed forests at the University of Minnesota Landscape Arboretum in 2020 and 2021 and established transects with discrete areas of European or jumping worm dominance. We found that these discrete areas had distinct soil features and that jumping worm populations appear to replace Lumbricus spp. populations. Soils in areas dominated by jumping worms had a loose, granular casting layer near the surface, decreased bulk density, increased soil organic matter, increased pH, and higher leaf litter mass than soils in sites dominated by European earthworms. We also measured lower monthly average soil temperature, lower maximum soil temperature, and lower soil temperature variability in our jumping worm dominated sites. Our soil temperature results indicate that Amynthas spp. will not be limited by soil temperatures in advancing much further north than central Minnesota. It remains to be seen how invasive earthworm populations and soil properties will evolve with the continued invasion of jumping worms in the long-term.