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Feral horses, donkeys dig desert holes to hydrate biodiversity

Wednesday, May 5th 2021, 10:06 pm - The wild donkeys and horses, hazards to local biodiversity, can actually boost desert ecosystems by digging equid wells and providing much-needed water.

When it comes to finding water and staying hydrated in desert environments, animals will take all the help they can get. This includes from other wildlife, even ones that pose threats to natural ecosystems.

Researchers have found that feral donkeys and horses have been digging equid wells in search of water, not only for themselves, but for other species, too. The findings of the study were recently published in Science.

SEE ALSO: Ontario officials watch invasive pig numbers so they don't go wild

The study found that equid-dug wells increased water availability and were used by more than 57 other species, including many birds, other herbivores such as mule deer and mountain lions. As well, the abandoned holes decreased distance between water sources and increased germination in critical riparian tree species.

On average, it was discovered that the number of species was 51 per cent higher at the wells than what was observed in nearby dry areas during the same time periods.

Wild horse/Erick. J. Lundgren Wild horse. (Erick Lundgren).

This is despite the fact the two feral species are believed to be a threat to biodiversity and have been subject to widespread eradication and control programs in Australia, for example.


In North America deserts, the feral animals were responsible for creating wells up to two metres in depth in order to access groundwater. It was noted in the study that most other equids and all elephant species dig wells, too. The equids increased the density of water features and was the only source of water at times.

"Vertebrate richness and activity were higher at equid wells than at adjacent dry sites, and by mimicking flood disturbance, equid wells became nurseries for riparian trees," the study said. "Our results suggest that equids, even those that are introduced or feral, are able to buffer water availability, which may increase resilience to ongoing human-caused aridification."

In the review, the authors stated that water is the main resource in dryland ecosystems, in limited supply. It determined species composition, food web structure and vegetation dynamics.

Donkey/Erick. J. Lundgren Feral donkey. (Erick Lundgren)

"At one fully intermittent stream that lost all background water, equid wells provided 100 per cent of surface water. Even at sites [that] remained perennial (background water retained at headwater springs), wells provided up to 74 per cent of surface water by accessing the water table in dry reaches," the study outlined.

As part of the research, the team surveyed four Sonoran Desert groundwater-fed streams in Arizona every two to four weeks over three summers. Equid wells were "particularly important" to supplying water in midsummer as temperatures increased and water tables receded.

"It’s a very hot, dry desert and you’ll get these pretty magical spots where suddenly there is surface water," said Erick Lundgren, a field ecologist at Denmark's Aarhus University and one of the study's co-authors, in an interview with NewScientist.

The well-digging also had a positive influence on vegetation. For example, on a dammed perennial Sonoran Desert river, several riparian trees were uncovered in abandoned equid wells. The trees are part of a small-seeded, fast-growing, flood-adapted functional group, whose germination requires moist layers without competing vegetation.

Thumbnail courtesy of Erick Lundgren.

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