In 2015, researchers began tracking wells dug by wild horses and donkeys in Arizona’s Sonoran Desert; by 2018, they found some supplied up to 74% of available surface water and were used by 57 vertebrate species

When summer heat dries up streams in the Sonoran Desert, wild horses and donkeys can do something unexpected: dig for water. Researchers began tracking these animal-made wells in 2015, mapping their water supply across four Arizona sites over three summers. By 2018, they had found that the wells could provide up to 74% of the surface water available at one site. Cameras also recorded 57 vertebrate species, from songbirds to mountain lions, visiting wells across the Sonoran and Mojave deserts. The findings, reported by Science News and Arizona State University, add a surprising dimension to the debate over animals often labelled invasive: in some places, their digging may help other species through dry spells, even as the broader impacts of horses and donkeys remain complex.

Digging for water

The study began with an observation by ecologist Erick Lundgren. In 2014, he saw wild donkeys digging into dry ground and wondered whether the holes they made could offer water to more than just the animals that excavated them. To investigate, Lundgren and colleagues mapped the surface area of water in equid wells and groundwater-fed streams at four sites in Arizona’s Sonoran Desert. They conducted the work over three summers, from 2015 to 2018, when water conditions shifted with heat and rainfall. The results showed that the wells could make a meaningful difference to water availability. At one site, they became the only drinking-water source after a stream dried up. At another, they supplied as much as 74% of the available surface water, according to Science News’ report on the research.The holes also brought water sources closer together. On average, they reduced the distance between accessible water by 843 metres. That could matter in a landscape where animals must travel farther as streams and other sources shrink, and where competition can intensify around a few remaining water spots.

A desert watering hole for many

Representative image

The reports further suggested that once the wells were dug, other animals began using them. Researchers placed camera traps at five sites in the Sonoran and Mojave deserts, recording activity at wells, riverbanks and dry locations. Across the sites, they documented 57 vertebrate species drinking from the wells. The visitors ranged widely in size and behaviour. Migratory songbirds and mule deer appeared on camera, as did larger animals such as mountain lions. Researchers even recorded a black bear drinking from one of the wells. The number of species recorded at wells was about the same as at streams, and 64% higher than at dry spots. That comparison suggests the animal-dug holes were doing more than holding a little water for the equids themselves. When natural surface water was scarce, the wells could become busy gathering points for wildlife.Lundgren told Science News that he sometimes drank from the wells himself, describing the water as cool and cleaner than some other sources. The study was not a recommendation for people to drink untreated desert water, but his observation reflected how groundwater reaching the wells could differ from stagnant or exposed water elsewhere.

More than a water source

The benefits may extend beyond animals looking for a drink. Researchers also found that abandoned wells could create favourable conditions for cottonwood seedlings, which need moist, open ground to grow. Cottonwoods often struggle to establish themselves in crowded riverbanks, where vegetation competes for water and space. Floods can create bare areas where seedlings take root, but where dams reduce flooding, those opportunities may be less frequent. At one study site, cottonwood seedlings grew in equid-dug holes, with some surviving the summer and reaching as much as two metres tall. This finding suggests that a well can influence the landscape even after the animal that dug it has moved on. By bringing moisture to a patch of open ground, the hole may give young trees a chance to survive. Those seedlings could eventually become part of the vegetation along desert waterways.Arizona State University’s account of the research also described the animals as ecosystem engineers. The phrase refers to species that change their surroundings in ways that affect other organisms. Beavers are a familiar example because their dams alter water flow and create habitat. The equids’ effects are less dramatic, but their wells may also shape access to resources in dry country.

A complicated picture

The findings do not mean wild horses and donkeys are harmless, or that their wells benefit every ecosystem. Horses and donkeys were introduced to North America, and researchers have documented ways they can damage dryland environments. They may trample native vegetation, contribute to erosion and compete with native animals for food and water.Ecologist Jeffrey Beck, who was not involved in the study, cautioned that the effects could vary by region, as per reports. In Wyoming’s Red Desert, his research has examined how wild horses can displace pronghorn from watering places. The benefits observed in Arizona may not apply where water is harder to reach by digging or where equids put pressure on native species.That distinction matters in a debate often framed in absolutes. An animal can have harmful effects in one setting and provide a resource in another. The research offers evidence that equid wells helped wildlife at the desert sites studied, but further work is needed to understand how important those wells are to ecosystems over time.

Rethinking the role of introduced species

The study’s wider message is not that management concerns should be dismissed. Rather, it suggests that removing a species can have consequences that are not immediately obvious. If wells provide water to dozens of other species in some places, changes to equid populations could also change how water is distributed across the landscape.Researchers have argued that such interactions deserve attention when decisions are made about wild horses and donkeys. The animals may not be native, but their behaviour has become part of the ecological story in places where they now live. Understanding that role requires weighing both the costs and the benefits, rather than assuming there is only one effect.In the Sonoran Desert, the evidence began with holes in dry ground and grew into a broader picture of wildlife gathering around them. Between 2015 and 2018, researchers found that equid wells could supply a large share of available surface water and attract 57 vertebrate species. The work does not settle the debate over wild horses and donkeys, but it shows how an act of survival by one animal can sometimes create an unexpected resource for many others.