Global earthworm distribution and activity windows based on soil hydromechanical constraints

Abstract

Earthworm activity modifies soil structure and promotes important hydrological ecosystem functions for agricultural systems. Earthworms use their flexible hydroskeleton to burrow and expand biopores. Hence, their activity is constrained by soil hydromechanical conditions that permit deformation at earthworm's maximal hydroskeletal pressure (≈200kPa). A mechanistic biophysical model is developed here to link the biomechanical limits of earthworm burrowing with soil moisture and texture to predict soil conditions that permit bioturbation across biomes. We include additional constraints that exclude earthworm activity such as freezing temperatures, low soil pH, and high sand content to develop the first predictive global map of earthworm habitats in good agreement with observed earthworm occurrence patterns. Earthworm activity is strongly constrained by seasonal dynamics that vary across latitudes largely due to soil hydromechanical status. The mechanistic model delineates the potential for earthworm migration via connectivity of hospitable sites and highlights regions sensitive to climate.

Fig. 1. Earthworm bioturbation activity in structured soil.

a Subterranean bioturbation relies on earthworms’ ability to mechanically penetrate and deform the soil using their flexible hydroskeleton, which is b modeled considering penetration and cavity expansion transverse to the earthworm body where radial stresses ${\sigma }_r$ exerted by the earthworm from the local cavity of size $r_c$. Yielding soil material is bounded by a remote elastic zone at a distance $R_P$ from the center of the cavity is dependent on c soil hydromechanical conditions that enable their hydroskeleton to form cavities. d Soil hydromechanical soil states can be mapped globally depending on soil water content and soil type, enabling inferences to earthworm distributions.

Fig. 2. Global map of earthworm hospitable zones.

a Green regions indicate that annual average pressures required for cavity expansion are below the earthworm’s hydrostatic pressure limit (200 kPa). Pressures are truncated to values below 400 kPa for visualization (dark red) and permafrost regions were removed (gray). b Other factors that may impede earthworm activity. Cyan regions indicate the sub-zero mean annual temperature (MAT), magenta regions mark soil pH < 4.5, yellow regions indicate coarse soil texture (sand content > 80%), and orange regions indicate that there are fewer than two consecutive months during which the soil mechanical properties permit cavity expansion. Regions of different limiting factors may overlap and were ordered for visibility. c Overlap in the area (Jaccard index) that is considered hospitable based on pressure below 200 kPa (P₂₀₀) compared with other variables. d Latitudinal distribution of terrestrial area that is excluded by considering each variable independently (colored lines) and the fully constrained habitat area (black).

Fig. 3. Comparison of predicted hospitable zones and reported earthworm distribution.

a Potential earthworm habitats (green) including soil hydromechanical limitations for Australia. Locations with reported presence of earthworms from two datasets are displayed; GBIF (blue points) and Abbott (orange points). Regional limitation of earthworm activity is delineated by 400 mm yr⁻¹ of mean annual precipitation (cyan contour) as previously reported. b Predicted earthworm habitats for North America. Observed occurrences (Global Biodiversity Information Facility, GBIF) are in good agreement with regional extents of earthworm communities (redrawn from Hendrix and Bohlen, red). c Regions in East Eurasia and Northern Africa that could support earthworm soil bioturbation. d Global distribution of earthworm occurrence. Made with Natural Earth. Free vector and raster map data @ naturalearthdata.com.

Fig. 4. Temporal windows of potential earthworm burrowing activity.

a Global map of temporal hydromechanical variations (coefficient of variation of limiting pressures). b Median earthworm limit pressures across latitudes for a climatic year. c Time series comparison of modeled cavity expansion limiting pressure (red) with measured earthworm abundance (black). Earthworm abundance was measured monthly in the New Forest, Hampshire UK (5.9°N, −1.6°E,) over six consecutive years. d, e Median climatic limiting pressures (boxes indicate central 50 and 90% of values) required to burrow through the soil are associated with mean daily precipitation (blue line and shading; 30 days running median and central 50 and 90% of values) for (d), a grassland (g: 9.55°N, 14.65°E) and (e), a desert (d: −22.95°N, 132.95°E) as indicated in (a). The maximum radial earthworm pressures P_w (dashed line) are shown. Soil limit pressures are reported for the topsoil (0–7 cm) and are assumed to represent the driest part of the soil profile. f Habitat fragmentation based on habitable regions is plotted in comparison with species richness results for different latitudes. The maximum radial earthworm pressures P_w (dashed line) are shown. Soil limit pressures are reported for the topsoil (0–7 cm) and are assumed to represent the driest part of the soil profile.