Local, landscape and continental scale factors controlling earthworm community structure

Past studies have praised earthworms for improving soil structure and fertility, but criticized earthworms for increasing the leaching of nutrients and soil greenhouse gas emissions. Therefore, in order to maximize the environmental benefits and reduce the environmental costs of earthworms, it is important to determine the factors controlling the structure of earthworm communities at local, landscape and continental scales. We first hypothesized that forested riparian buffer strips (FRBS) within agricultural landscapes would be a refuge for earthworms, due to higher soil moisture and organic matter compared to adjacent agricultural fields (“treatment” = FRBS vs. Field).  Within sites, we hypothesized that earthworms would be most abundant where the chemical quality of above- and belowground plant litter is high, or where soil disturbance is low. At the continental scale, we hypothesized that total summer precipitation interacts with regional and local scale factors in controlling earthworm community structure.  A field survey was conducted to quantify earthworm species abundances in FRBS and adjacent agricultural fields across Eastern Canada and Central Europe (two “bioregions” differing in rainfall). At each of 77 sites, we collected and identified earthworms from three plots within FRBS and adjacent agricultural fields, and noted the tree species, understory vegetation, drainage class, agricultural crop as well as five soil physicochemical variables (texture, pH, total C, total N and % organic matter). In each bioregion and treatment, we found proportionately more endogeic than anecic or epigeic earthworm species. In Eastern Canada there were proportionately fewer anecic and more epigeic individuals in FRBS than in fields, whereas in Central Europe there were fewer endogeic and more anecic earthworms in FRBS than in fields. We also found significant interactions between bioregion and treatment on total earthworm abundance and biomass, and on soil moisture. More specifically, in Eastern Canada we found higher earthworm abundance and biomass, soil moisture and organic matter in FRBS. Conversely, in Central Europe we found higher earthworm abundance and biomass in fields, no treatment effects on soil moisture, and higher soil organic matter in FRBS. The different earthworm distribution patterns in each bioregion were not related to the types of agricultural crops, but rather to differences in precipitation and soil moisture across bioregions. Within FRBS in Eastern Canada, earthworm abundance in deciduous and mixedwood stands were higher than in coniferous stands; in Central Europe, earthworm abundance was higher in deciduous stands only. Within FRBS in Eastern Canada, the abundance of the prominent endogeic species Apporectodia rosea was correlated with herbaceous plants, notably ferns and graminoids. Conditional regression tree analysis revealed positive relationships between earthworms and soil clay content, pH, moisture and organic matter. Our results suggest that local and landscape patterns in earthworm diversity can be predicted by soil and vegetation attributes, but the relative importance of these factors change across continual scales due to climate.  Comparing the distributions of earthworms across different scales provides insights into the potential of different species to spread into new habitats with climate change.