06/01/2026
Routine ecosystem maintenance can look like causing damage if you don't look at the big picture. What else might you be missing if your focus is too narrow? I'd love to hear your thoughts in the comments.
A feral b***o in the Sonoran Desert digs a hole in a dry riverbed with its front hooves until it hits groundwater six feet down. It drinks. It leaves. By the next morning, a badger, a mule deer, two javelinas, a bobcat, a Woodhouse's scrub jay, and an elf owl have all used the same hole.
The animal accused of destroying desert ecosystems is building the only water source some of those animals have access to in the hottest months of the year.
Erick Lundgren, a field ecologist at Aarhus University in Denmark, spent three summers from 2015 to 2018 studying feral horse and b***o wells across four sites in the Sonoran Desert of western Arizona and one in the Mojave Desert near Baker, California. His team mapped water availability at each site every two to four weeks, deployed camera traps at the wells and at natural water features, and recorded every species that visited.
The results, published in Science in April 2021, showed that b***o and horse wells increased the density of accessible surface water at every study site. At one site, where the seasonal stream dried up completely in midsummer, b***o wells provided one hundred percent of the available drinking water. At other sites, the wells contributed up to seventy-four percent of surface water during peak heat. The wells were deepest and most critical during the exact window when natural water disappeared. The b***os were not supplementing the water supply. They were replacing it.
Fifty-seven vertebrate species were documented using the wells. Camera traps recorded mule deer, American badgers, black bears, javelinas, bobcats, mountain lions, coyotes, kit foxes, desert cottontails, jackrabbits, Gambel's quail, scrub jays, and elf owls drinking from holes that b***os had excavated in dry sand. Several of the bird species documented at the wells are in decline, including the elf owl, which has been disappearing from its historic range across the low desert. The wells did not just provide water. They concentrated wildlife. Lundgren described the sites as hotbeds of animal activity.
The mechanics are simple. A b***o walks into a dry wash where groundwater sits a few feet below the surface. The sand and gravel are loose enough to excavate with hooves. The b***o paws at the ground, digging with its front feet the way a horse digs at a frozen water trough. It keeps going until water seeps into the hole. The well fills. The b***o drinks and moves on. The hole persists for days or weeks depending on temperature, evaporation rate, and how frequently other animals visit. Some wells are re-excavated by the same or different b***os after they start to fill in.
The behavior is not unique to North American b***os. Wild equids across the world do it. Mountain zebras dig wells in Namibia. Grevy's zebras dig in Kenya. Kulans dig in the Gobi Desert. Feral horses dig in the Australian outback. African elephants have been documented digging wells that sustain entire savanna wildlife communities during drought. Lundgren's argument is that well-digging by large herbivores is a global ecological function that has been operating for millions of years, and that feral equids in North America are performing a version of the same service that native horses provided on this continent before they went extinct roughly twelve thousand years ago.
That last point is the one that starts arguments. The modern feral b***o is descended from the African wild ass, brought to North America by Spanish colonizers in the 1500s. It is not a native species. It is classified as feral and invasive by every federal land management agency in the United States. The Bureau of Land Management and the National Park Service manage b***o populations through roundups, removals, and fertility control because the animals damage native vegetation, compete with desert bighorn sheep for food and water, trample riparian areas, and accelerate erosion. All of that is documented and real.
But the well-digging is also documented and real, and it complicates the narrative. A b***o standing in a dry wash digging a hole that a declining elf owl will drink from tomorrow is simultaneously an invasive species damaging the desert and an ecosystem engineer providing a critical resource that no native animal in the current landscape is providing. Both things are true at the same time. The ecological debate around feral equids in the American West cannot be reduced to good or bad because the animal is doing both.
Lundgren put it directly. Because of the way we value feral horses and donkeys, the orthodoxy tends to focus on how they harm ecosystems. We wanted to see whether these holes provided a resource when water is scarce. They do. At one site, without the b***o wells, there would have been no surface water at all during the hottest weeks of the summer. Every animal that drank from those wells during that period owed its hydration to a feral donkey that dug a hole and walked away.
Source: Lundgren, E.J. et al. (2021). "Equids engineer desert water availability." Science, 372(6541). / National Geographic / The Wildlife Society.
Image is for illustration purposes only
