Relationships between spatial pattern of the endogeic earthworm Polypheretima elongata and soil heterogeneity

Spatial distribution of the endogeic earthworm Polypheretima elongata, soil texture, carbon and nitrogen contents were investigated in a 10-y-old pasture established on a vertisol, by measuring each variable at 57 points randomly located in a 25 × 60 m plot. Soil variables were measured at three depths: 0–10, 10–20 and 20–30 cm. Principal component analysis (PCA) of the data set showed the absence of relationships between density of P. elongata and environmental variables. The first principal component expressed a marked gradient of soil texture and soil organic matter (SOM). Sampling point coordinates on the first principal component axis were used as a composite variable assumed to account for the major source of soil heterogeneity at this site. The spatial pattern of this variable and three age classes of P. elongata were investigated using geostatistics. There was no evidence of any relationship between soil heterogeneity and earthworm distribution. Population of P. elongata exhibited a marked patchy distribution with opposite patterns between adults and other age classes (juveniles and cocoons). These results suggest that at the study site processes responsible for the spatial distribution of P. elongata population are of demographic rather than environmental origin.