Interspecies variation in survival of soil fauna in flooded soil

While many studies have examined the effects of flooding on earthworm population distributions, few studies have investigated physiological and behavioural responses of earthworms to the low oxygen conditions caused by flooding. An earthworm’s skin is its oxygen exchange organ, allowing earthworms to survive in flooded environments provided that the water contains sufficient dissolved oxygen. Individuals of three species of earthworm, the anecic Lumbricus terrestris (Linneaus, 1758), the green morph of the endogeic Allolobophora. chlorotica (Savigny, 1826) and the epigeic Lumbricus castaneus (Savigny, 1826) were placed in reconstituted groundwater that was either kept aerated or kept in a sealed container so that dissolved oxygen was gradually consumed as the earthworm respired. Oxygen saturation of the water was measured over time in sacrificial triplicate replicates from each treatment at discrete time points, with earthworm death recorded. Before treatments, oxygen levels in all treatment tubes were 9.53 (± 0.64) mg O2 L-1. L. terrestris, a large species which emerges at night to forage at the soil surface died when oxygen levels reached 0.82 (± 0.46) mg O2 L-1 after approximately 36 hours. L. castaneus, a smaller species which lives on the soil surface, died when oxygen levels reached 3.60 (± 2.01) mg O2 L-1 after approximately 168 hours. A. chlorotica, which is similar in size to L. castaneus, lives in the upper 20 cm of soil and is known to aestivate during the summer, did not die, even when oxygen levels reached 1.49 (± 0.40) mg O2 L-1 after 280 hours. The results suggest that earthworm respiration is closely linked to both body size and to behavioural ecotype. These findings suggest that if flooding increases in frequency resulting in episodic reductions in soil oxygen levels, the species composition of earthworm communities may change, with an increased presence of endogeic earthworms which show a responsive plasticity to flooding events.