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Feral Horses & Donkeys

Feral equids (horses and donkeys) reintroduced to desert regions in the North American southwest regularly dig wells to expose groundwater, increasing water availability — and sometimes providing the only water available locally — for a wide variety of plant and animal species and ecosystem processes, according to new research led by the University of Technology Sydney and Aarhus University.

Well digging by wild donkeys (Equus africanus asinus) in the Sonoran Desert, the United States. Image credit: Lundgren et al., doi: 10.1126/science.abd6775.

Well digging by wild donkeys (Equus africanus asinus) in the Sonoran Desert, the United States. Image credit: Lundgren et al., doi: 10.1126/science.abd6775.

“Water is the main limiting resource in dryland ecosystems. It determines species composition, food web structure, and vegetation dynamics,” said Dr. Erick Lundgren from the University of Technology Sydney and Aarhus University and his colleagues.

“Yet, the capacity for animals to enhance water availability by exposing subsurface water has received little attention.”

Wild donkeys (Equus africanus asinus) and horses (Equus ferus caballus), as well as most other equids and all elephant species, regularly dig wells of up to 2 m in depth.”

To evaluate well digging and its associated ecosystem effects, Dr. Lundgren and colleagues surveyed four groundwater-fed streams in the Sonoran Desert every 2 to 4 weeks over three summers.

They observed well-digging by the region’s feral equids (horses and donkeys) and found that the equid-engineered wells increased water availability for a number of native desert species.

“Streams were 7 to 32 km apart and were 300 to 1,800 m long,” they said.

“Like many desert streams, site hydrology was highly variable, as was the relative contribution of equid wells.”

“Equid wells were particularly important to provisioning water in midsummer as temperatures increased and water tables receded.”

“At one fully intermittent stream that lost all background water, equid wells provided 100% of surface water.”

“Even at sites which remained perennial (background water retained at headwater springs), wells provided up to 74% of surface water by accessing the water table in dry reaches.”

“Likewise, equid wells increased water density relative to background water by an average of 332% and by as much as 1,450%.”

Well-digging megafauna contribute the capacity to buffer water availability across many drylands. Image credit: Lundgren et al., doi: 10.1126/science.abd6775.

Well-digging megafauna contribute the capacity to buffer water availability across many drylands. Image credit: Lundgren et al., doi: 10.1126/science.abd6775.

To understand whether equid wells have value for other species, the scientists deployed camera traps at five sites in the Sonoran and Mojave Deserts.

Overall, they detected 59 vertebrate species at equid wells, of which 57 were recorded drinking.

“Well digging also influences vegetation,” the authors added.

“On a dammed perennial Sonoran Desert river, abandoned equid wells host numerous riparian trees, members of a small-seeded, fast-growing, flood-adapted functional group whose germination requires moist substrate without competing vegetation and whose conservation is considered a regional and global priority.”

“We show that feral equids can increase water availability in drylands, with associated effects on a variety of species and ecosystem processes,” they concluded.

“We suggest that well digging by feral equids may replace a function lost with the extinctions of large vertebrates across the world’s drylands.”

The study was published in the journal Science.

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Erick J. Lundgren et al. 2021. Equids engineer desert water availability. Science 372 (6541): 491-495; doi: 10.1126/science.abd6775

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