Earthworms are vital ecosystem engineers, with important implications for soil structure, fertility and productivity. However, their reliance on soil water makes them vulnerable to the increasing frequency and severity of droughts predicted under climate change. A key survival strategy employed by some species is aestivation, a state of reduced metabolic activity during which earthworms coil up and seal themselves into chambers until favourable conditions return. Despite its ecological importance, the environmental thresholds that induce aestivation and the consequences for earthworm fitness and community dynamics remain poorly understood. This thesis investigates the drivers, costs and ecological implications of aestivation through controlled laboratory experiments involving Allolobophora chlorotica (Savigny, 1826), one of the most common UK earthworms, and year-long field surveys with molecular analyses. Laboratory studies demonstrated that aestivation is environmentally induced, triggered between 18-13 wt% soil moisture, with 100 % mortality below 10 wt%. Soil type modulated responses, with water potential (pF) a stronger predictor for the onset of aestivation than gravimetric moisture. While aestivation protected individuals against desiccation, repeated droughts reduced body mass, induced regression of reproductive characters, and diminished cocoon production and viability. Nonetheless, mass losses were largely transient, with rapid rehydration and compensatory growth enabling recovery once favourable conditions returned. Field surveys confirmed that aestivation was common both during dry (<25-30 vol%) and saturated (>50 vol%) conditions, highlighting its role as a general stress response. During drought, earthworm communities became less diverse and were dominated by a few aestivating, burrowing species, while litter-dwellers largely disappeared. Populations demonstrated resilience, likely through migration and drought-resistant cocoons, but recovery potential is constrained by land use and the frequency of climatic extremes. Overall, this thesis establishes soil moisture as the primary driver of earthworm aestivation, identifies thresholds critical for survival and reproduction, and demonstrates both the resilience and vulnerability of earthworm populations to climate change.
