Wild donkeys and horses engineer water holes that help other species

Often cast as invasive pests, the equids may actually benefit some ecosystems

a kulan digging a hole in the ground

Donkeys and other equids are known to dig wells in dryland areas in search of water, like this kulan in central Asia. In the American southwest, new research suggests that wells dug by feral donkeys and horses can benefit the whole ecosystem by increasing water availability during dry times.

© Petra Kaczensky

Water drives the rhythms of desert life, but animals aren’t always helpless against the whims of weather.

In the American Southwest, wild donkeys and horses often dig into the dusty sediment to reach cool, crystal clear groundwater to quench their thirst. New research shows this equid ingenuity has far reaching benefits for the ecosystem.

Equid wells can act as desert oases, providing a major source of water during dry times that benefits a whole host of desert animals and keystone trees, researchers report in the April 30 Science.

Introduced to North America in the last 500 years or so, wild donkeys and horses are often cast as villains in the West. These species can trample native vegetation, erode creek beds and outcompete native animals. But when Erick Lundgren, a field ecologist at Aarhus University in Denmark, first observed wild donkeys digging wells in 2014, he wondered whether these holes might benefit ecosystems, similar to the way elephant-built water holes can sustain a community in the African savanna.

a doe and fawn leaning down to drink water out of a hole in the ground
Wells dug by wild donkeys and horses are used by numerous other species. A camera trap caught this doe and fawn mule deer drinking from a donkey well in the Sonoran Desert.E. Lundgren

“Because of the way we value [feral] horses and donkeys, the orthodoxy tends to focus on how they harm ecosystems,” he says. “We wanted to see whether these holes provided a resource when water is scarce.” 

First, Lundgren and his colleagues had to see whether these holes actually increase accessible water. Over the course of three summers from 2015 to 2018, they mapped out the surface area of water in wells and groundwater-fed streams at four sites in Arizona’s Sonoran Desert. 

Water availability was highly variable among sites, but equid wells generally increased accessible water, especially as temperatures rose. At one site, wells were the only source of drinking water once the stream completely dried up. Elsewhere, wells provided up to 74 percent of available surface water. Wells also decreased the distance between water sources by an average of 843 meters, making this essential resource more accessible and easing tensions that can escalate among drinkers at isolated water holes, Lundgren says.

Once wells were dug, other animals came. In droves.

Researchers set up cameras at five sites in the Sonoran and Mojave deserts, staking out wells, riverbanks and dry spots. They documented 57 vertebrate species, from migratory songbirds to mountain lions, slurping at the wells, which is about equal to the number of species seen at streams and 64 percent higher than dry spots.

“We even caught a black bear drinking from a well,” says Lundgren, who also takes swigs from the wells from time to time. “The water is quite cool, and cleaner than other sources.”

a photo of a small tree growing in a hole
Donkey wells can act as nurseries for tree seedlings by providing easier access to water away from the competition of established plants on the riverbank. Here, a several-year-old cottonwood grows from an abandoned donkey well.© Michael Lundgren

Wells can also be nurseries for cottonwood seedlings that require moist, open areas to grow. These fast-growing seedlings struggle to break through the vegetation-stuffed riverbanks, and instead rely on floods for their first sips of water. But at one site, researchers found seedlings thriving in equid holes. Many survived the summer, growing as tall as 2 meters. In areas where dams reduce flooding, equid wells could be fulfilling an important ecosystem service for these iconic tree species, the researchers say.

The study “clearly shows that equids can alter these ecosystems in ways that can benefit other species,” says Clive Jones, an ecologist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y., who wasn’t involved in the study. Such hydrological engineering isn’t unheard of — beavers, for example, have an outsized ability to engineer ecosystems (SN: 11/28/18). Whether equid wells play a similarly crucial role remains to be seen, Jones says. “More data is needed to say exactly how important wells are in terms of the functioning of these ecosystems.”

Though the benefits of wells are clear in this study, it’s too early to conclude that feral donkeys and horses are good for ecosystems, notes Jeffrey Beck, a restoration ecologist at the University of Wyoming in Laramie.

“There’s a whole body of research documenting the detrimental effects these animals can have on drylands around the world,” Beck says. In Wyoming’s Red Desert, for instance, he’s studied how wild horses often drive antelope-like pronghorn from watering holes. Additionally, “the benefits [the equids] demonstrate in this study might be limited to this area,” he says, since surface water in other areas may not be as accessible by digging.

Still, the researchers hope this study can chip away at the notion that introduced species are wholly bad for ecosystems. In some areas, feral equids “are being killed by the hundreds of thousands in the name of purifying nature,” says study author Arian Wallach, an ecologist at the University of Technology, Sydney. To her, this study shows “donkeys [and horses] are part of nature too,” and that eradication efforts might ripple throughout an ecosystem in unforeseen and unfortunate ways.

Jonathan Lambert is a former staff writer for biological sciences, covering everything from the origin of species to microbial ecology. He has a master’s degree in evolutionary biology from Cornell University.

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