Earthworm Ecology

Grouping earthworms by their ecology can be useful when describing how different earthworms contribute to soil processes and ecosystem services.

Ecological Categories

A set of ecological categories were first described in the 1970s by the French scientist Marcel Bouché.  Bouché used a number of physical characteristics (such as pigmentation, body length and colour) to establish three points on a triangular scale:

• Deep-burrowing earthworms (Anecic) - Anecic earthworms make permanent vertical burrows in soil. They feed on leaves on the soil surface that they drag into their burrows. They also cast on the surface, and these casts can quite often be seen in grasslands. Some anecic earthworm species also make middens (piles of casts) around the entrance to their burrows. Anecic species are the largest species of earthworms in the UK. They are darkly coloured at the head end (red or brown) and have paler tails.
• Shallow-burrowing earthworms (Endogeic) - Endogeic earthworms live in and feed on the soil. They make horizontal burrows through the soil to move around and to feed and they will reuse these burrows to a certain extent. Endogeic earthworms are often pale colours, grey, pale pink, green or blue. Some can burrow very deeply in the soil.
• Surface-dwelling earthworms (Epigeic) - Epigeic earthworms live on the surface of the soil in leaf litter, deadwood, dung and compost. These species tend not to make burrows but live in and feed on the leaf litter. Epigeic earthworms are also often bright red or reddy-brown, and sometimes even stripy.

These ecological categories were widely accepted within the scientific community and were often used to describe the functions of earthworms in the ecosystem they inhabit- such as the perceived impacts of different ecological categories on soil processes such as decomposition, nutrient cycling, soil pore creation and soil aggregate formation. This enables scientists and policymakers to make links to ecosystem services that earthworms contribute to.

Ecological Categories Revisited

Nicolas Bottinelli and his fellow researchers highlighted that Marcel Bouché had never intended to describe a set of three definitive ecological categories and that the system described was intended to be three points on a triangular scale, with 7 ecological categories: Anecic, Endogeic and Epigeic alongside Epi-endogeic, Epi-anecic and Endo-anecic as well as an intermediate category in the middle of the triangle.

They investigated how consistently researchers were categorising individual species. They found that 15 out of 27 species were always assigned the same ecological category regardless of the study. However, 7 species were reported from 2 categories, 2 species from 3 categories and 2 species from 4 categories. This indicated that there are inconsistencies within the research community about how individual species are being categorised.

They also came up with a model for categorising earthworm species based on 13 morphological and anatomical traits, with the model determining a percentage by which each species. The hope was that by creating a system by which species could be assigned to a category, that earthworm researchers would take a more consistent approach.

Functional Groups

It is important to remember that these ecological categories are based on the traits of earthworms, rather than analysis of their behaviour. Bottinelli and Yvan Capowiez published an article in 2021 raising their concerns that ecological categories were being used more and more as a proxy for functional groups. They released a follow-up research paper in 2024 that reported on the burrowing, feeding and casting behaviour of 50 species of earthworm under laboratory conditions.

From this research, they were able to establish that European earthworms can be described as 6 functional groups when it comes to their bioturbation behaviour. Although there was some overlap with the traditional ecological categories, they demonstrated that the endogeic category should be avoided.

• Litter dwellers were described as small pigmented earthworms with very high surface activity and making a few shallow galleries in the soil. A link can be made with the epigeic ecological category.
• Burrowers were described as large pigmented earthworms with both feeding and casting activity on the surface. They make a limited number of true burrows and have higher activity close to the surface. A link can be made with the epi-anecic ecological category.
• Intense tunnelers were described as very large pigmented earthworms that also feed and cast on the surface. However, they make extensive burrow systems. A link can be made with the anecic ecological category.
• Shallow bioturbators were described as non-pigmented small earthworms that have very low surface activity and make refilled and shallow galleries in the soil. A link can be made with epi-endogeics.
• Deep bioturbators were described as non-pigmented average to large earthworms that also have low surface activity. They make refilled galleries, but these are found deeper in the soil profile. These may also be referred to as hypo-endogeics.
• A sixth category was classified as the Intermediate group. This included species without marked characteristics and included both pigmented and non-pigmented species.

Although these functional groups are very likely an improvement on the traditional ecological categories, we need to treat them with some caution. In order to fully understand our earthworm species, we need studies that cover even more species and look at different behaviours.

References

1. Bouché, M.B. (1972) Lombriciens de France. Ecologie et Systématique. Paris, INRA.
2. Keith, A. H. & Robinson, D. A. (2012) Earthworms as Natural Capital:  Ecosystem Service Providers in Agricultural Soils. Economology Journal, 2, 91-99.
3. Bottinelli, N., Hedde, M., Jouquet, P. & Capowiez, Y (2020) An explicit definition of earthworm ecological categories – Marcel Bouché’s triangle revisited. Geoderma, 372, 114361. DOI: 10.1016/j.geoderma.2020.114361
4. Capowiez, Y., Marchan, D., Decaëns, T., Hedde, M. & Bottinelli, N. (2024) Let earthworms be functional – Definition of new functional groups based on their bioturbation behaviour. Soil Biology and Biochemistry, 188, 109209. DOI: 10.1016/j.soilbio.2023.109209