Earthworms consume organic material, produce biologically active casts and create channels through the soil. Those channels can improve aeration and provide pathways for water and roots. Earthworms also perform something machinery struggles to achieve without disturbance: they continually incorporate organic matter while maintaining the soil's living structure [REF].
Their abundance generally tells us that several conditions are being met simultaneously: there is food; there is sufficient moisture; there is oxygen; and the environment is capable of supporting biological activity.
“When earthworms flourish, the landscape is telling us something about the conditions we have created.”