In the Mojave and Sonoran deserts of North America, donkeys roam wild. Also called burros, feral donkeys travel in herds, seeking out water sources in small wetland oases, often trampling vegetation in the process. These critical wetlands include freshwater springs and seeps that many animals rely on to survive in the otherwise bone-dry desert environment.
The National Park Service considers the donkeys an invasive species, and many conservationists advocate for killing or relocating them as they are assumed to lack predators that could keep their populations in check. But in a recent study, a team of ecologists found that these desert-dwelling donkeys have become a favorite snack for hungry cougars.
“We have imagery of a donkey group going by and then a cougar right behind them, like literally walking in their footsteps,” says Erick Lundgren, ecologist at Aarhus University and lead author on the study.
The scientists monitored donkey and cougar activity in wetlands throughout Death Valley National Park in California and published their findings last month in the Journal of Animal Ecology. They found that donkeys spent much less time in wetlands where cougars had previously preyed on other donkeys. The vegetation was also less trampled at sites where cougars killed donkeys. These findings highlight how important apex predators like cougars can influence the stability of an ecosystem.
To study this previously unknown predator-prey interaction, the researchers surveyed 14 different wetlands using cameras that are triggered to record videos whenever an animal walks by. Using these cameras, they were able to identify eight wetlands with clear evidence of cougars killing donkeys. They also visited each site in person to look for donkey carcasses, which cougars will often cache and return to later on. “Some of these cache sites were used over and over again, so that every time we went, there would be three or four fresh kills,” says Lundgren.
In the sites that lacked cougars, donkeys were frequently caught on camera during the day and night. But in sites with active cougar predation, the donkeys were only seen during the day. At night, the donkeys avoided any wetlands where cougars had recently killed other donkeys.
The donkey presence in desert wetlands was associated with striking changes to the local plant life. “These sites where they're there all day, they're trampling and eating the vegetation,” says Lundgren. “It really strongly leads to a lot of bare open ground, a lot of dung and then a really strong reduction in plant cover.”Wetlands where cougars were observed killing donkeys had greater canopy cover, more vegetation around water sources and less bare ground than sites that lacked donkey predation.
This type of indirect interaction where organisms at the top of the food chain affect those at the bottom, or vice versa, is called a trophic cascade—named for the effects that cascade throughout the rungs of the food chain.
According to Jerrold Belant, a wildlife ecologist at Michigan State University who was not involved in this research, studying trophic cascades is not easy to do. In part, this challenge stems from the many complex relationships that shape an ecosystem. “There's invariably many ecological processes occurring simultaneously across the landscape,” he says. “They all have their relative contributions and interact in myriad ways, many of which we may not understand or even be aware of at this time.”
Julie K. Young, a wildlife ecologist at Utah State University who was not involved in this research, says that documenting the beneficial role of cougars in the environment could help boost their image. “It could elevate that social tolerance for them on the landscape,” she says. Especially among ranchers who worry about their cattle’s safety, Young explains, highlighting the benefits of keeping cougars around might go a long way in helping to conserve this important apex predator.
While Lundgren also hopes these findings will help more humans see cougars in a positive light, he also cautions against viewing the donkeys in a uniformly negative one. The ancestors of horses and donkeys evolved in North America millions of years ago, and only went extinct on this continent within the past 12,000 years, Lundgren explains. “Since 35 million years ago, we've had big animals,” he says, “and it was only a heartbeat ago that these big animals disappeared, almost certainly from human hunting.” In Lundgren’s view, donkeys in North America are not so much recent invaders as replacements for the ancient animals that have been lost.
On a practical level, Lundgren worries that if conservation managers remove donkeys from the desert ecosystem, it could have unintended consequences. “Those cougars are going to eat something,” he says. Lundgren explains that if the donkeys disappear, the cougars in Death Valley may simply switch to feeding on bighorn sheep or other native wildlife.
Mathias Pires, an ecologist at the University of Campinas who was not involved in this study, agrees that it is critical to carefully consider how the management of an introduced species could also affect many other species either directly or indirectly. “Unless we have very good information on how things are connected to each other and affect each other,” he says, “we might make bad decisions.”