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Donkeys and wild horses craft water holes that help other species

Water drives the rhythms of desert life, but animals are not always helpless against the vagaries of time.

In the American Southwest, wild donkeys and horses often dig in the dusty sediment to reach fresh, crystalline groundwater to quench their thirst. New research shows that this equine ingenuity has benefits for the ecosystem.

Equine wells can act as a desert oasis, providing an important source of water during dry seasons that benefits a number of desert animals and key trees, the researchers reported on April 30th.

Introduced into North America in the last five hundred years or so, wild donkeys and horses are often thrown as villains in the West. These species can tread on native vegetation, erode the beds of streams and outnumber native animals. But when Erick Lundgren, a field ecologist at the University of Aarhus in Denmark, first observed wild donkeys digging wells in 2014, he wondered if these holes could benefit ecosystems, similar to the way water holes built by elephants can sustain a community in the African savannah.

Ponds dug by donkeys and wild horses are used by many other species. A camera trap caught this deer mule deer drinking from a donkey pit in the Sonoran Desert.E. Lundgren

“Because of the way we value (wild) horses and donkeys, orthodoxy tends to focus on how ecosystems are damaged,” he says. "We wanted to see if these holes provided a resource when water is scarce."

First, Lundgren and colleagues had to see if these holes actually increase accessible water. Over three summers from 2015 to 2018, they mapped the surface of the water in wells and streams fed with groundwater at four sites in the Sonoran Desert, Arizona.

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

Once the wells were dug, other animals came. In mass.

The researchers installed cameras at five sites in the Sonoran and Mojave Deserts, spreading wells, riverbanks and dry spots. They documented 57 species of vertebrates, from migratory songbirds to mountain lions, crawling in wells, which is almost equal to the number of species seen in streams and 64 percent higher than dry spots.

“We even caught a black bear drinking from a well,” says Lundgren, who also occasionally carries twigs from the wells. "The water is quite fresh and cleaner than other sources."

a photo of a small tree growing in a holeDonkey wells can act as nurseries for tree seedlings by facilitating access to water away from the competition of plants established on the river bank. Here, a multi-year-old cotton grows from an abandoned donkey pit.© Michael Lundgren

Wells can also be nurseries for cotton seedlings that require open, moist areas to grow. These fast-growing seedlings struggle to cross the banks full of vegetation and instead rely on floods for their first sips of water. But at one site, researchers found seedlings thriving in equine holes. Many survived the summer, growing up to 2 meters. Researchers say that in areas where dams reduce flooding, equine wells could play an important ecosystem service for these iconic tree species.

The study “clearly shows that equidae can alter these ecosystems in a way that they can benefit other species,” says Clive Jones, an ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York, who did not participate in the study. This hydrological engineering is not unheard of: beavers, for example, have an excessive capacity to engineer ecosystems (SN: 28/11/18). Jones is looking at whether equine wells play an equally crucial role. "More data is needed to say exactly how important wells are in terms of how these ecosystems work."

Although the benefits of wells are clear in this study, it is still early to conclude that wild donkeys and horses are good for ecosystems, says Jeffrey Beck, a restoration ecologist at the University of Wyoming in Laramie.

“There’s a whole bunch of research documenting the harmful effects these animals can have on dry areas around the world,” Beck says. In the red desert of Wyoming, for example, he studied how wild horses often lead to a prongorn resembling a watering can antelope. In addition, “the benefits (equidae) demonstrated in this study may be limited to this area,” he says, as surface water in other areas may not be as accessible as digging.

Still, researchers hope this study can dispel the notion that introduced species are totally bad for ecosystems. In some areas, wild equines "are being killed by the hundreds of thousands in the name of purifying nature," says study author Arian Wallach, an ecologist at Sydney University of Technology. For her, this study shows that “donkeys (and horses) are also part of nature” and that eradication efforts can be traced back to an ecosystem in an unpredictable and unfortunate way.

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