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Wild Donkeys and Horses Dig Wells That Provide Water for a Host of Desert Species

A new study finds these equine wells attracted 59 other vertebrate species, boasting 64 percent more species than the surrounding landscape

A donkey digging a well in the Sonoran Desert of Arizona. (E. Lundgren)
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Wild horses and donkeys are often considered a problem in the American West, but new research suggests their penchant for digging wells with their hooves offers benefits to the ecosystems they inhabit, reports Douglas Main for National Geographic.

The study, published this week in the journal Science, shows that when wild or feral horses and donkeys dig wells, they increase the availability of water for other species living in the parched desert landscape. These wells can be up to six feet deep and provide access to groundwater to species including badgers, mountain lions, deer and birds.

Donkeys and horses were introduced to North America roughly 500 years ago, and the Bureau of Land Management currently estimates there are more than 95,000 wild donkeys and horses roaming the West. That figure is more than triple what land managers say the landscape can sustain, and the growing population can “trample native vegetation, erode creek beds and outcompete native animals,” writes Jonathan Lambert for Science News.

In 2014, however, Erick Lundgren, a field ecologist at Aarhus University in Denmark, saw wild donkeys digging wells and wondered if other animals in the environment might use them in much the same way that animals from far and wide take advantage of elephant-dug watering holes in the African savannah, per Science News.

To investigate, Lundgren and his co-authors kept an eye on four dried-up streams in the Sonoran Desert of Arizona. The team monitored the streams during the summers of 2015, 2016 and 2018 and made note of any new wells dug by horses and donkeys.

Researchers found that the wells drew 59 other vertebrate species, 57 of which were seen drinking from the equine waters. Some wells even appeared to provide a boost to desert trees such as willows and cottonwoods, which researchers observed germinating from the moistened soil.

“These resources are in fact used by all other animals—there was a cacophony of organisms,” Lundgren tells Karina Shah of the New Scientist.

In fact, the team found that the average number of species around a horse or donkey well was 64 percent higher than in dry surrounding areas observed at the same time periods.

By mapping sources of open water around their four study sites, the researchers also calculated that the horses and donkeys increased the density of accessible surface water up to 14-times over, according to National Geographic.

Lundgren tells National Geographic that these wells are such boons for desert flora and fauna alike that it places wild horses and donkeys in the same category of so-called “ecosystem engineers” as beavers, which alter their environment in ways that can be beneficial by building dams.

Clive Jones, an ecologist at the Cary Institute of Ecosystem Studies who wasn’t involved in the study, tells Science News that the study “clearly shows that equids can alter these ecosystems in ways that can benefit other species.” However, he adds “more data is needed to say exactly how important wells are in terms of the functioning of these ecosystems.”

Other researchers are even more hesitant to ascribe ecosystem benefits to the nearly 100,000 horses and donkeys roaming the West. “Some research from the western United States has shown that feral horses exclude native wildlife from water sources in deserts,” Lucas Hall, a wildlife ecologist at California State University, Bakersfield who was not involved in the study, tells New Scientist. “The benefit they may provide by creating new water sources will likely be offset by their high populations and exclusionary effects on other wildlife.”

If nothing else, the new study offers a reason for scientists and land managers to take a fresh look at the potential of horses and donkeys to make contributions to environments they’ve traditionally been seen as despoiling.

“Even though they are introduced, they are performing a really important ecological function,” Wayne Linklater, a wildlife biologist at California State University, Sacramento, tells National Geographic. “This paper is very challenging to those traditional conservationists who want to see all introduced species as somehow invasive and alien.”

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