Abstract
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 Darwin's 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 both total and ecotype-specific 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 use of earthworms as soil quality indicators must therefore be carried out with care, ensuring that sufficient samples are taken within field to take account of variability in earthworm populations that is unrelated to soil chemical and physical properties.
Highlights
The study of the spatial variation in a species abundance has often resulted in significant insights into their biology and ecology, e.g. [1]
As with other studies populations are dominated by juvenile and juvenile endogeic earthworms e.g. [45,46,47,48,49], while among adult populations, endogeic earthworms, notably, Allolobophora chlorotica and Apporectodea caliginosa often dominate with Lumbricus rubellus and/or Lumbricus castaneus being the most abundant epigeic species; Aporrectodea longa and L. terrestris are the only two UK anecic species e.g. [46,47,48,49,50,51]
Darwin’s insights were based on sparse quantitative data it seems that his essential intuition was correct: at the field scale, the spatial variation in earthworm abundance is controlled or at least heavily influenced by some factor other than soil properties, most likely biotic, giving the appearance of a spatially random distribution of ~30 - ~100 m patches of high earthworm abundance
Summary
The study of the spatial variation in a species abundance has often resulted in significant insights into their biology and ecology, e.g. [1]. [13,14] and other, environmental variables, for example plant species present [15] This raises the prospect that spatially resolved measures of earthworm diversity and abundance may provide a good approximation both to soil health/quality and the ecosystem services delivered by them. The existing work on controls on earthworm distributions suggest to us that earthworm distributions are undoubtedly integrators of soil properties and that earthworms are responsible for soil processes that give rise to ecosystem services, their use as soil quality indicators is still far from straightforward To address this issue, we conducted a spatially nested survey of earthworm abundances in pasture fields managed at varying levels of intensity across Britain, at farms located across the dominant national climatological gradients (temperature and rainfall). We used our data to test the hypotheses that at a field scale the spatial variation of earthworm abundance is random whilst at a national scale the variation in earthworm abundance is driven by climate-related gradients in temperature and rainfall that will be reflected in soil temperature and soil moisture values
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