Despina Berdeni
Earthworms are considered to be ecosystem engineers due to their beneficial effects on the soil environment. The population size and species diversity of earthworms within a soil is a good indicator of soil biological health as earthworm communities are affected by soil management. Typically, in ‘healthy’ field conditions, earthworms are present in mixed communities which are comprised of multiple species belonging to three main ecological groups (ecotypes). The burrowing and feeding activities of earthworms are related to their ecotypes:
- Epigeic: Earthworms that live within and feed on the surface litter layer. These species tend not to make burrows.
- Endogeic: Earthworms that feed on soil and soil-associated organic matter and form horizontal burrows, which they reuse to a certain extent.
- Anecic: Earthworms that form deep vertical, permanent burrows to depths of 2m. These species feed on decomposing plant material and pull plant litter into their burrows. This group includes the largest species of earthworms in the UK.
Ecotypes and effects on ecosystem services
There is good evidence showing that increased earthworm abundance benefits ecosystem services. Earthworm diversity also has important consequences for soil ecosystem functions and health as earthworm ecotypes contribute to different soil ecosystem services. For example, the permanent burrows formed by anecics like Lumbricus terrestris generate important classes of large pores that control water infiltration and facilitate root growth to access groundwater. In turn, these contribute to reducing overland flow, storm-water runoff and soil erosion. Endogeic species promote soil aggregation and soil organic carbon stabilisation and sequestration, while epigeics are important for surface litter processing, all of which increase soil carbon and improve soil fertility via nitrogen mineralisation.
Management to promote earthworms
Earthworm populations are responsive to changes in land management. Tillage promotes decomposition of organic matter, decreases food resources for earthworms and increases earthworm predation. In general, reduced soil disturbance, increased soil cover and organic matter inputs improve the size and diversity of earthworm communities. Research has shown that in arable soils, depleted earthworm populations can recover relatively quickly with changes in management. For example, a study (Prendergast-Miller et al. 2021) of earthworm population recovery with conversion of an intensively managed arable soil to ley found that within two years, earthworm abundance in new leys was 732 ± 244 earthworms m2, similar to that in field margin soils (619 ± 355 earthworms m2) and four times higher than in adjacent arable soil (185 ± 132 earthworms m2). Similarly, results from Farm Innovation Group trials in central England (https://farmpep.net/project/deeper-rooting-yyt-fig-field-lab) found evidence of increased activity of anecic earthworms in arable soils, within a year of switching to reduced tillage with farmyard manure addition.
Checking earthworm numbers in your soil
Measuring the earthworm numbers in your soil provides a good indicator of soil biological health. The best time to do this is in autumn when soils have rewetted or in spring, once soils have warmed but are still moist. Details of a simple methodology for quantifying earthworms is available at: https://ahdb.org.uk/knowledge-library/how-to-count-earthworms. Information on how to interpret the results of earthworm counts in the context of a wider assessment of soil health is also available: https://ahdb.org.uk/knowledge-library/the-soil-health-scorecard.
Ref: Prendergast-Miller et al. 2021. Arable fields as potential reservoirs of biodiversity: Earthworm populations increase in new leys. Science of the Total Environment 789.
