Earthworms

Earthworms act as important ecosystem engineers in soils, through their effects on soil aggregate stability, nutrient cycling and soil carbon dynamics. However, the relationships between earthworms, soil functions and ecosystem services are not well understood. Mechanistic models can be used to gain insights by integrating the physiological, biological and ecological mechanisms underpinning soil functions. Such models help forecast the ecological consequences of land management practices and climate changes, and extrapolate from the laboratory to the field, and between localities.

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Understanding the interactions between ecosystem engineers (earthworms), soil functions and ecosystem services will enable better forecasts about how soils respond to new and diverse land management and climate scenarios.

The diverse traits of the three ecological categories of earthworm (epigeic, anecic and endogeic) translate into different effects on soil functions and ecosystem services. These ecological differences mean that disturbed soils, such as those under conventional agriculture, are often dominated by endogeic species, which are adapted to disturbance, low soil organic matter and the removal of plant material in the litter layer. Although endogeic species do offer beneficial services to cultivated soils, more diverse earthworm populations are needed in order to optimize soil functioning under sustainable land management.

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Interactions between earthworm functional groups, soil functions and ecosystem services (modified from Keith & Robinson, 2012 Economology Journal 2: 91-99).

Alice Johnston’s earthworm research currently involves constructing, validating and applying mechanistic models of earthworm populations in a range of habitats and weather conditions. Once validated, the models can then be used to explore how management practices and extreme weather events alter ecosystem resilience through their effects on individual organisms. Published models are currently available for the epigeic ‘compost-worm’ Eisenia fetida (Johnston et al., 2014a) and the dominant endogeic earthworm found in conventionally managed agricultural fields Aporrectodea caliginosa (Johnston et al., 2014b). Excellent agreement between the A. caliginosa model with independent field data measuring the response of A. caliginosa populations to various environmental and management conditions in Johnston et al. (2015) provides ‘proof of concept’ for the approach. Models are also being developed for a more field-relevant epigeic earthworm (Lumbricus rubellus) and an important ecosystem engineer in low-disturbance habitats (e.g. zero tillage), the anecic species Lumbricus terrestris.

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Model validation against independent field data, presenting comparisons of model outputs (grey lines represent the continuous model output and the solid black line is the mean for the sample dates) and independent field data (symbols represent replicates and the dashed line is the mean) of earthworm population biomass in control conditions (left-hand panels) and in response to applications of toxic reference applications, represented by the triangle (right-hand panels). Taken from Johnston et al. (2015). 

Alice is also interested in novel methods for quantifying the economic value of earthworms as ecosystem engineers, which will allow policy makers to perform better cost-benefit analysis of land management scenarios under weather extremes and promote the use of earthworms as a natural resource to land managers.