Does geographical origin matter? A study on the effects of soil temperature on the energy costs of burrowing in Aporrectodea caliginosa from Sweden and France

Soil temperatures are expected to increase with climate change, which will likely affect soil bioturbation by earthworms. While the ecophysiological response of earthworms to soil temperature has been studied previously, several questions remain, such as whether earthworm species from different geographical origins respond differently to environmental stress. In this study, we used A. caliginosa individuals from two contrasting European climatic zones (i.e. central Sweden and southern France), and measured their energy use (via heat dissipation using isothermal calorimetry) and their burrowing activity (i.e. burrow volume and cast volume, quantified using X-ray imaging) at five different soil temperatures (i.e. 8, 12, 16, 20 and 24 °C).

In general, heat dissipation in earthworms increased with soil temperature, and body mass-normalized heat dissipation was about 20% higher in earthworms from France. Moreover, the increase in heat dissipation with increasing temperature was stronger at high than at low temperatures. However, there was one important exception from these general trends: earthworms from Sweden showed a distinct and absolute peak in heat dissipation at intermediate temperature (16 °C). Burrow volumes created by earthworms increased with soil temperature up to 16-20 °C, after which it decreased. The high levels of heat dissipation in combination with reduced burrowing activity at 24 °C suggest high stress in such warm conditions. The volumes of burrows created by Swedish earthworms were about 50% higher than those created by French earthworms.

As a consequence of the higher heat dissipation and lower burrow volumes, the specific energy costs for burrowing (i.e., heat dissipation per unit burrow volume) was 2-3-fold higher in French earthworms than Swedish earthworms, which suggests that Swedish earthworms are more “efficient”. In general, French A. caliginosa were smaller in size and mass compared to Swedish A. caliginosa, and these differences may be a result of adaptation to distinct climates. While Swedish earthworms had a distinct activity peak at 16 °C, we could not find such an activity peak in French earthworms. Measurements with higher temperature resolution (e.g., measurements every 1 °C) might be needed.

Our data indicate that the geographical origin of earthworms plays a role in the earthworm's ecophysiological responses to environmental stressors such as soil temperature. The findings provide quantitative data on how earthworm burrowing activity is affected by soil temperature, which helps us better understand how earthworms may adapt to climate change and what the consequences on soil processes are.