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"People just didn't think it was worthy of scientific attention," said the scientist, who is now at the University of Technology Sydney. Lundgren had read about African elephants digging wells that were the only source of water for other animals during the dry season, and wanted to know if horses and donkeys might play a similar role in America. The idea was intriguing, "especially since donkeys and horses are considered agents of biodiversity harm" as they are not native species in the region, he said.

Over the course of three summers, he and his team surveyed sites in the Sonoran Desert that stretches across Arizona and California. They found that wells dug by the "equids" to depths of up to six feet (two meters) increased water availability for many native desert species, and decreased the distances between important water sources during dry periods. The wells were especially important during the hottest and driest parts of summer, when they provided the only available water source at some sites.

Lundgren said the horses and donkeys acted as "buffers" against the extreme variability of desert streams from year to year. "The donkey wells kept water in the system. And these features were used by pretty much every species you could picture, including some surprising ones like black bears, that we didn't expect to see in the desert," he said. Other species that flocked to the wells and were caught on camera included mule deer, bobcats, Woodhouse's scrub jay and javelinas.

Horses and donkeys were introduced to the Americas by Europeans to assist with the colonization of the continent, but their use declined with the advent of the internal combustion engine. Since then, they have been studied as "invasion biology," said Lundgren, which does not consider them to be a part of the local wildlife. But this thinking is too tunnel-visioned and has prevented scientists from having a more nuanced understanding of their effects on their ecosystems, he argued.