Horses and donkeys dig wells in dryland ecosystems, increasing water availability - and sometimes providing the only water available locally - for a wide variety of plant and animal species and ecosystem processes, researchers report. This overlooked form of ecosystem engineering shows that equids, even those that are reintroduced or feral, can buffer water availability in dryland environments, providing a service that could bolster resilience to continued aridification for a wide variety of desert species. Large terrestrial herbivores play crucial roles in their environments. However, since the late Pleistocene, megafauna have experienced drastic declines in abundance worldwide, which has led to the loss of many of their ecosystem functions. Although megafauna declines have been linked to the formation of closed forests, increased wildfires and reduced plant seed dispersal in modern temperate and tropical environments, their impact on dryland ecosystems is far less known. In dryland environments, where water is the primary limiting resource, some larger animals like wild donkeys, horses and elephants regularly dig wells - up to two meters deep - to expose subsurface water. To evaluate the impact of well-digging on the broader landscape, Erick Lundgren and colleagues surveyed several sites in the Sonoran Desert of North America. Lundgren et al. observed well-digging by the region's feral equids (horses and donkeys) and found that the equid-engineered wells increased water availability for a number of native desert species, decreased the overall distances between important water sources during dry periods, and at times provided the only water present locally. According to the authors, species richness and activity were higher at equid wells than nearby dry sites and, by mimicking natural flood disturbance, abandoned wells occasionally became nurseries for important riparian tree species.