Desert Wells Dug by Wild Horses and Donkeys Keep Arizona’s Wildlife Alive

When you picture a desert, you probably imagine endless dunes, scorching sun and a silence broken only by the occasional rattlesnake. Yet, under that stillness, a surprising drama unfolds each summer in Arizona’s Sonoran Desert – a drama starring wild horses and donkeys, the very animals many consider invasive.

Back in 2014, ecologist Erick Lundgren was strolling near a dry creekbed when he saw a lone donkey poking at the cracked earth. The animal’s hooves were making little pits, and Lundgren wondered: could those pits be more than just a dusty inconvenience? Might they actually tap into the hidden groundwater that keeps the desert alive?

Curiosity turned into a research project that spanned three hot summers (2015‑2018). Lundgren and a small team set up cameras, measured water patches, and kept a close eye on four remote sites where horses and donkeys were known to dig. What they discovered was both simple and astonishing.

At one of the locations, the animal‑dug wells accounted for a whopping 74% of all the surface water that was visible on the ground. When the nearby stream ran dry, those freshly‑dug wells became the sole source of drinking water for any creature in the area. In another spot, the holes cut the average distance a thirsty animal had to travel for water by more than 800 metres – a life‑saving shortcut during the blistering peak of summer.

But the wells did more than just hold water. Camera traps captured a parade of wildlife gathering at these modest oases: migratory songbirds flitting for a sip, mule deer pausing to drink, a mountain lion stealthily approaching, and even a black bear trudging through the sand to reach the cool liquid. In total, 57 different vertebrate species were recorded using the wells, a number that matched the diversity seen at natural streams and was 64% higher than at completely dry spots.

It wasn’t just animals that benefitted. The researchers noticed something unexpected sprouting from the abandoned pits – cottonwood seedlings. Cottonwoods need moist, open soil to get a foothold, and the freshly‑dug holes provided exactly that. Some of those seedlings survived the brutal summer, pushing up to two metres tall before the next rains arrived.

These findings led the scientists to label the horses and donkeys as “ecosystem engineers,” a term usually reserved for creatures like beavers that dramatically reshape habitats. While the equids’ impact isn’t as extensive as a beaver dam, their digging does redistribute water and create micro‑habitats that other species can exploit.

It’s worth noting, however, that this isn’t a free‑pass for the animals. Wild horses and donkeys were introduced to North America centuries ago, and they do cause problems – trampling native plants, contributing to soil erosion, and competing with native grazers for food and water. The study simply adds a layer of nuance: in some dry‑land contexts, the same behaviors that cause damage can also unintentionally support other life forms when water is scarce.

Lundgren even confessed that he once sipped water straight from one of these wells. He described it as cooler and clearer than many other desert sources, though he stressed that it’s not a recommendation for humans to drink untreated desert water.

So, the next time you hear about “invasive” horses or donkeys, remember that in the harsh Sonoran landscape they sometimes double as inadvertent caretakers, digging tiny lifelines that keep a surprisingly wide array of desert life moving forward.