Wild horses and burros are ravaging the public lands of the West. According to the Bureau of Land Management (BLM), there are currently an estimated 82,384 wild horses and burros on public lands in the western United States. Meanwhile, the agency says that a healthy and balanced population in relation to wild animals and domestic livestock would be only 27 thousand individuals. Overpopulation of wild horses and skunks is resulting in less native vegetation and more invasive weeds, more wild horses and potholes on highways and causing damage to private property, and starvation not only of wild horses and skunks, but also of wild animals. Each year, the BLM conducts large-scale roundups of wildlife to remove them from the landscape. Recently, researchers discovered that a wild predator may also be helping the cause.
In Death Valley National Park, researchers captured the first images of mountain lions preying on wild grubs, also known as donkeys. Ecologists and park staff had suspected mountain lions were preying on wild animals, but until recently, there was no evidence. Lead researcher Erick Lundgren of Aarhus University captured the first image of a mountain lion killing a wild donkey in 2019 on a trail camera – a mountain lion in the middle of a fight with a wild donkey on the ground.
Lundgren and his team captured another predator example in 2020, which showed the entire kill, including an image of the mountain lion standing over its dead quarry. “Getting that kill on camera was a great moment of validation,” Lundgren said science. Lundgren and his fellow researchers then investigated the “trophic cascades” that can result from predation. According to Brittanica, a trophic cascade is an “ecological phenomenon caused by the addition or removal of top predators and involving reciprocal changes in the relative populations of predators and prey through a food chain, often resulting in dramatic changes in the ecosystem”. In other words, an apex predator not only affects the species it preys on, but can also affect other parts of the ecosystem.
Lundgren’s team found that in parts of Death Valley National Park, which straddles the California-Nevada border, where mountain lions were not present, wild donkeys would hang around one site for an average of 5.5 hours. Meanwhile, in places where mountain lions were present, they only stayed an average of 40 minutes. In those places the vegetation was in better shape and there were fewer signs of trampling. Mountain lions hunting wild donkeys were improving ecosystem health.
“This [research] adds to growing evidence that ecologically important predator-prey interactions can emerge rapidly in novel ecosystems,” Lundgren and his fellow researchers write in a study published in Journal of Animal Ecology. “These results also suggest that removals or eradications introduced [donkeys and horses] and prevailing policies of persecuting predators may have unintended consequences.”
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Death Valley National Park staff are currently working to remove all wild donkeys from the park. Lundgren says they should reconsider this plan given the results of the study. He adds that mountain lions are likely to enter bighorn sheep if wild donkeys are no longer available as a food source. However, a spokesman for Death Valley National Park said science that the study will not affect the NPS plan to remove the pits from the park.