Mountain lion predation on wild donkeys rewires an ancient food web

This blog post is provided by Erick Lundgren and tells the #StoryBehindThePaper for the paper “A novel trophic cascade between cougars and feral donkeys shapes desert wetlands“, which was recently published in the Journal of Animal Ecology. Contrary to popular belief, they find that mountain lions are capable of hunting wild donkeys, positively affecting wetlands due to changes in donkey activity levels.

Around 12,000 years ago, several species of equid roamed North and South America. These animals were ubiquitous over the last 35 million years but went extinct during the late Pleistocene, most likely due to hunting by recently arrived humans. More recently, wild horses and donkeys, relatives of these extinct animals, have established thriving wild populations on these same continents. As introduced organisms, these animals are considered ‘‘invasive’ by many conservationists. In part, this is due to long standing claims that these animals have no natural predators, leading to high population growth rates and strong ecological effects relative to native herbivores. As such, government agencies in the USA and elsewhere commit significant resources to eradicating or regulating introduced wild equid populations.

Wild donkeys in Death Valley National Park. © Michael Alfuso

We too, despite our interest in whether introduced equids restore lost ecological functions, had accepted this claim. We took for granted that mountain lions, the only extant predator to substantially overlap with wild equids, were too small to effectively hunt wild donkeys and horses. We were wrong. Quite by chance, we captured two predation events of wild donkeys by mountain lions using camera traps. It quickly became obvious that predation of wild donkeys by mountain lions was not only possible but was extremely common. Many wetlands we visited while studying the effects of donkeys on wetland vegetation and water availability had several fresh donkey kills every month, suggesting that lions were killing a donkey a week. Indeed, other researchers recently found that mountain lions in the deserts of Nevada have become specialized in hunting wild horses.

Predation of wild donkeys by mountain lions, captured in the Sonoran Desert of Arizona (a-b) and Death Valley National Park (c-d). ©The Authors

While predation can influence population size, it also changes prey behavior. Prey tend to be more cautious in areas of high predation risk, creating a “landscape of fear”. These behavioral changes can modify how prey populations affect their environment: areas of high predation risk tend to be used more sparingly, leading to reduced impacts on vegetation, soil, and so on. 

To find out if this was happening with wild donkeys we installed camera traps at desert wetlands in Death Valley National Park, comparing sites with mountain lion predation of donkeys to sites without predation. At sites that lacked predation, donkeys were active an average of ~5.5 hours a day on hot days and were active day and night. However, at nearby sites with predation, donkeys were only active ~40 minutes a day and avoided visiting at night, which is when mountain lions are most active and most effective at ambush predation. 

We then conducted vegetation surveys to understand if these behavioral changes corresponded with altered donkey effects on these ecologically important wetlands. We found that sites with predation had significantly less trampled ground, fewer trails, and more vegetation cover. 

Sites without mountain lion predation (left) have significantly reduced canopy cover, more trampled bare ground, and higher trail density than sites with mountain lion predation (right) ©Authors

Mountain lions and several equid species cooccurred for several million years. However, mountain lions are largely considered to have been too small to hunt equids, which were instead preyed upon by larger now-extinct predators, like sabertooth cats and dire wolves. Our results suggest that mountain lions are filling in these lost roles and rewiring an ancient food web. Similar results have been found in many novel ecosystems where extant predators rapidly adjust to consume novel prey. As such, our study adds to evidence that ecosystems are more dynamic and resilient than typically considered.

Detail from “Trapped in Time”, ©Mark Hallett, courtesy La Brea Tar Pits

Many conservationists have long sought the removal of wild equids in order to return ecosystems to a semblance of how they were when first described. This aim has converged with that of the meat industry, who also wish to remove feral equids in order to increase livestock stocking rates. Meanwhile, other state and federal agencies kill mountain lions to protect livestock or to boost populations of popular game species, like bighorn sheep. Instead of chasing ideals of ecological purity, our results suggest that increasing protections for predators offers a new, and pragmatic, vision of flourishing and dynamic wild landscapes in North America and beyond.

Read the full paper here: Lundgren, E. J., Ramp, D., Middleton, O. S., Wooster, E. I. F., Kusch, E., Balisi, M., Ripple, W. J., Hasselerharm, C. D., Sanchez, J. N., Mills, M., & Wallach, A. D. (2022). A novel trophic cascade between cougars and feral donkeys shapes desert wetlands. Journal of Animal Ecology, 00, 1–10. https://doi.org/10.1111/1365-2656.13766

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