Darwin was right, in any given field, the spatial variability of earthworm communities in pastures isnt driven by measurable soil properties.
Abundance and distribution of earthworms in agricultural fields is frequently proposed as a measure of soil quality assuming that observed patterns of abundance are in response to improved or degraded environmental conditions. However, it is not clear that earthworm abundances can be directly related to their edaphic environment, as noted in Darwins final publication, perhaps limiting or restricting their value as indicators of ecological quality in any given field. We present results from a spatially explicit intensive survey of pastures within United Kingdom farms, looking for the main drivers of earthworm density at a range of scales. When describing spatial variability of earthworm abundance within any given field, the best predictor was earthworm abundance itself within 20 - 30 m of the sampling point; there were no consistent environmental correlates with earthworm numbers, suggesting that biological factors (e.g. colonisation rate, competition, predation, parasitism) drive or at least significantly modify earthworm distributions at this spatial level. However, at the national scale, earthworm abundance is well predicted by soil nitrate levels, density, temperature and moisture content, albeit not in a simple linear fashion. This suggests that although land can be managed at the farm scale to promote earthworm abundance and the resulting soil processes that deliver ecosystem services, within a field, earthworm distributions will remain patchy. The divergence in the interpretative value of earthworm abundance as an ecological indicator is a function of spatial scale, corresponding to species specific biological factors as well as a response to environmental pressures. Species abundance can effectively be used as ecological indicators, even if, at first, distributions seem random. However, care must be exercised, in the sampling design for the indicator species, if its abundance is to be used as a proxy for environmental quality at a particular scale (e.g. a management scale such as field scale).