Earthworm densities correlate with aboveground plant biomass and vegetation type across residential properties in Madison, Wisconsin, USA

Non-native earthworms have been increasing in density within cities in the USA and around the world, but how earthworms respond to variation in plant cover within these systems remains unclear. We collected earthworms and standing plant biomass from five residences in Madison, Wisconsin, USA, each possessing two plant cover types-a turfgrass lawn and a prairie garden containing flora native to the Midwestern USA-on two occasions (once each in 2007 and 2008). Soil moisture and temperature were also quantified. On average, volumetric soil moisture was 2.8% lower and standing plant biomass 384 g m−2 greater in the prairie gardens than in the turfgrass lawns. Earthworm densities increased by 0.1 and 2.7 individuals m−2 on average with each increase of 1 g m−2 in standing plant biomass and of 1% in soil moisture, respectively. Further, when the effects of plant biomass and soil moisture were factored out, earthworm densities were estimated to be 55 individuals m−2 lower in prairie gardens than in adjacent lawns. These findings suggest native-flora prairie gardens may provide less favorable habitat for non-native earthworms, all else being equal.