In the Mojave and Sonoran deserts of North America, burros roam wild. Also called donkeys, wild donkeys travel in herds, searching for water sources in small wetland oases, often trampling vegetation in the process. These critical wetlands include freshwater springs and seeps which many animals rely on to survive in the otherwise bone dry desert environment.
The National Park Service considers burros invasive species, and many conservationists advocate killing or relocating them, as they are presumed to lack predators that can keep their populations in check. But in a recent study, a team of environmentalists found that these desert-dwelling donkeys have become a favorite snack for hungry cougars.
“We have footage of a group of donkeys passing by and then a puma right behind them, kind of literally walking in their footsteps,” he says. Eric Lundgrenecologist at Aarhus University and lead author of the study.
Scientists monitored burro and mountain lion activity in the wetlands of California’s Death Valley National Park and published their findings last month in the Journal of Animal Ecology. They found that donkeys spent much less time in wetlands where pumas had previously preyed on other donkeys. Vegetation was also less trampled at sites where pumas 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 studied 14 different wetlands using cameras that trigger to record video whenever an animal passes by. Using these cameras, they were able to identify eight wetlands with clear evidence of pumas killing donkeys. They also visited each site in person to look for donkey carcasses, which pumas often hide and return to later. “Some of these cache sites were used over and over again, so every time we went, there were three or four new deaths,” says Lundgren.
At sites lacking pumas, donkeys were frequently caught on camera during the day and at night. But at sites with active puma predation, donkeys were only seen during the day. At night, the burros avoided wetlands where pumas had recently killed other burros.
The presence of donkeys in wet desert lands was associated with surprising changes in local plant life. “These sites where they are there all day, they are trampling and eating the vegetation,” says Lundgren. “It really leads strongly to a lot of bare open ground, a lot of manure, and then a very strong reduction in vegetative cover.”
Wetlands where pumas were observed killing donkeys had greater canopy cover, more vegetation around water sources, and less bare soil than sites lacking burro predation.
This type of indirect interaction in which organisms at the top of the food chain affect those at the bottom, or vice versa, is called an interaction. trophic cascade— named for the effects that cascade down 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 arises from the many complex relationships that shape an ecosystem. “There are 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 documenting cougars’ beneficial role in the environment could help improve their image. “It could raise that social tolerance for them in the landscape,” she says. Especially among ranchers concerned about the safety of their livestock, Young explains, highlighting the benefits of keeping cougars could go a long way toward helping conserve this important predator.
While Lundgren also hopes these findings will help more humans view cougars in a positive light, he also cautions against viewing donkeys in a uniformly negative light. the ancestors of horses and donkeys it evolved in North America millions of years ago, and only became extinct on this continent in the last 12,000 years, Lundgren explains. “For 35 million years, we’ve had large animals,” she says, “and it was only a second ago that these large animals disappeared, almost certainly due to human hunting.” In Lundgren’s view, donkeys in North America are not so much recent invaders as replacements for 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,” she says. Lundgren explains that if the donkeys disappear, the cougars in Death Valley may simply move on to feed on bighorn sheep or other native wildlife.
matthew pires, an ecologist at the University of Campinas who was not involved in this study, agrees that it is essential 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 about how things are connected to each other and how they affect each other,” he says, “we could make bad decisions.”