Within-field spatial distribution of earthworm populations related to species interactions and soil apparent electrical conductivity

Within-field spatial distribution of ecosystem engineers such as earthworms determine the spatial patterns of important ecosystem processes at the field scale. But the driving factors that shape the within-field spatial variability of earthworm populations remain largely unclear. The aim of this study was to describe the earthworm distribution patterns in a tilled arable field and to explain earthworm spatial variability as a function of biotic interactions within populations and of abiotic soil heterogeneity measured as the soil apparent electrical conductivity (ECa). Earthworms were sampled at 100 locations within an area of (105x75)m² in a harvested wheat field on a loess soil in central Belgium. The soil ECa was measured using a mobile electromagnetic induction (EMI) sensor as a proxy for soil textural composition. Maps of earthworm density and soil ECa were produced by variogram modeling and ordinary kriging. Two approaches were followed in the data analysis: (i) a pixel-by-pixel comparison of the spatial patterns based on categorical maps derived from a fuzzy k-means clustering, and (ii) causal modeling based on point-by-point Mantel tests. The endogeic species Aporrectodea caliginosa and A. rosea inhabited similarly sized and overlapping patches, which were neither related to the spatial occurrence of the deep-burrowing species Lumbricus terrestris and A. longa, nor to the measured soil ECa variability. Endogeic adults and juveniles lived closely associated in the same spatial clusters. The segregated field distributions of both deep-burrowing species were largely determined by the subsoil textural properties (as measured by ECa) and not by competition. A. longa individuals lived in field areas with high ECav values (related to relatively higher clay content) while L. terrestris juveniles occupied regions with low ECav values. Anecic juveniles were found in larger and spatially differing clusters than adults, suggesting the dispersal of juveniles from parental clusters into neighbouring areas. L. terrestris adults were spatially organized in distinct patches of ~15m diameter and it is hypothesized that the particular mating behaviour of this species requires such intimate distributions. The rapid, easy and non-invasive geo-referenced soil characterization by means of EMI-based measurements proved to be a useful tool for determining and understanding the within-field spatial distributions of earthworms but requires further testing in a variety of (agro-)ecosystems.