Feral horses and wild donkeys in the American southwest have been observed digging desert wells with their hooves in the soft sand of riverbeds, resulting in a network of additional fresh water outlets for the area’s native creatures.

This discovery has thrown a wrench in the conventional wisdom that wild equids, which were introduced by the Spanish, are pests that should be eradicated, as the scientist behind the study believes they could be performing a critical role once played by now-extinct Pleistocene mammals.

In modern conservation, a species that appears in a place where it didn’t exist a few hundred years ago and thrives is called invasive. Invasive species such as foxes, cats, goats, rodents, dogs, pigs, cane toads, rats, carp, and others have wreaked havoc on fragile habitats in Madagascar, the Galapagos Islands, and Australia, to name a few.

95,000 wild horses and donkeys live in the Sonoran and Mojave Desert habitats, which are considered invasive pests that outcompete other native herbivores and suppress or trample native plants.

Conservation doctrine would dictate that they be eradicated or relocated, but life is more complicated than that, and Erick Lundgren of the University of Aarhus in Denmark has shown that the desert fauna’s eagerness to drink from these equine wells should be taken into account before making any decisions regarding the species’ fate.

Lundgren discovered that the water holes were frequented by 59 different species, and that the species diversity around them was 64 percent greater than the ecosystem’s average square-mile diversity.

Engineers Of landscape

“Equid wells strongly reduced the isolation of water features, reducing average nearest-neighbor distances between water features by an average of 65%, and at most by 99%,” wrote Lundgren and his co-authors in the paper they published in Science.

From 2015 to 2017, Lundgren monitored four separate locations in the Sonoran and Mojave Deserts, sampling over 3,258 trap nights and finding bobcats, javelina, mule deer, scrub jays, and 55 other vertebrates drinking.

“There was a cacophony of organisms,” he told New Scientist.

Digging for water is a popular activity among large mammals all over the world, and elephants’ water wells in Africa are a gift to nearby animals. Elephants, beavers, and bison, for example, are known as “ecosystem engineers” because they form their landscape so dramatically that the flora and fauna rely on and anticipate their effects, and have adapted to accommodate or exploit it.

Lundgren claims in his paper that American wild equids should be classified as ecosystem engineers.

“By changing the abiotic environment around them, certain organisms can really strongly facilitate other species and processes,” Lundgren said in a recent interview with Science. “The most notable aspect of deserts is the scarcity of water, and these animals can really enhance the availability of it through drought and in the hot summers where natural sources of water tend to dry up.”

There’s a part to play

In mammal conservation, questions such as whether the existence of wild equids has changed the environment in a positive way, what constitutes an invasive species and how far back is it calculated, and whether our job is to actively try to protect what exists now, knowing that 99 percent of all species have gone extinct and that Earth’s past has seen constant change, are common.

A huge number of worldwide megafauna has gone extinct in several different kinds of habitats since the Pleistocene, according to another of Lundgren’s reports. The services, or engineering, that those organisms provided to the benefit of many animals and plants that still exist today is largely unknown.

Nonetheless, introduced megafauna have helped to recover about 15% of the world’s recorded Pleistocene megafauna populations.

North America, for example, was home to not only extinct pachyderms like the mastodon, but also hyenas, sprinting cougars, the world’s largest bear, and, more surprisingly, many species of wild horse.

“Recent and ancient extinctions and range contractions of megafauna, and the loss of their distinct ecological functions, has led to highly modified modern landscapes,” he writes. “Although introduced megafauna have primarily been studied as threats to conservation goals, growing evidence suggests that they present a countercurrent to ancient losses, and may replace lost ecological functions.”

Might the desert animals observed in Lundgren’s study be responding to a role that their forefathers, the donkeys and horses of today, performed tens of thousands of years ago on the landscape? It’s a provocative topic, one that Smithsonian reports has divided the nation, with some opting to stick with the current orthodoxy and others reconsidering the status of wild American equids as pests.

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