America’s burros important in rangeland food web, findings suggest

Photographic evidence of cougar predation on burros. Source: Lundgren et al.
Photographic evidence of cougar predation on burros. Source: Lundgren et al.

Donkeys play a much more important role in the North American food chain in the arid western rangelands than previously thought, study findings suggest.

Erick Lundgren and his fellow researchers, reporting in the Journal of Animal Ecology, have documented crucial evidence of cougar predation in the wild. Not only that, but they found clear evidence that donkeys – usually called burros in the United States – dramatically alter their behavior in areas where cougar predation is a risk.

Many of the world’s large herbivores and predators were lost in the late Pleistocene, with North America being especially hard hit. Indeed, a second wave of decline is ongoing among the world’s surviving large herbivore species.

Conversely, several large herbivores have established thriving populations following introductions, restoring lost species richness and some important ecological traits.

However, many apex predator species, especially the largest, did not survive the late Pleistocene extinctions, and those that remain are often persecuted by humans.

This begs the question, said Lundgren and his colleagues, can surviving apex predators exert a top-down influence on introduced megafauna, given that many are larger than the native herbivores?

Existing predators are generally considered incapable of doing so, leading to unusually strong disturbances from introduced species when compared to native herbivores.

A cougar stands over its juvenile burro prey. Source: Lundgren et al.
A cougar stands over its juvenile burro prey. Source: Lundgren et al.

In their just-published paper, the study team reported the first documented predation of juvenile feral donkeys by cougars in the Mojave and Sonoran Deserts of North America.

They went on to investigate how cougar predation corresponded with differences in feral donkey behavior and associated effects on desert wetlands.

Focusing on burros living in the Death Valley National Park, the researchers used camera traps and vegetation surveys to compare donkey activity patterns and impacts between wetlands with and without cougar predation.

They found that donkeys mostly visited wetlands with cougar predation during the day, thereby avoiding the predators. However, donkeys were active throughout the day and night at sites without predation.

Donkeys were around 87% less active (measured as hours of activity a day) at wetlands with predation, they reported. Sites with predation also had reduced donkey disturbance and reduced evidence of plant consumption, including around 46% fewer access trails, 43% less trampled bare ground, and 192% more canopy cover.

The study, they said, is the first to reveal an important ecological cascade involving cougars, feral equids and vegetation.

“Cougar predation appears to rewire an ancient food web,” they said, “with diverse implications for modern ecosystems.

“Our results suggest that protecting apex predators could have important implications for the ecological effects of introduced megafauna.

“These results also suggest that removal or eradication of introduced equids and prevailing policies of predator persecution may have unintended consequences.”

The study team said wild donkeys and cougars appear to be linked in an emerging ecological network.

Donkeys, they said, were the primary recorded prey of cougars at the study sites (24 of 29 identified carcasses).

“In addition to shaping donkey ecology, cougar predation on donkeys may drive novel evolutionary trajectories in cougars and reduce predation pressure on their other prey.

“While we did not conduct an exhaustive analysis of cougar diets, the importance of donkeys as a cougar food source, and of horses as prey elsewhere, suggests that removals of feral equids could have consequences both for cougars and for their alternative prey, such as bighorn sheep, due to prey switching.”

Donkeys, like the majority of existing larger herbivores, are threatened in their native range, they noted. “This has led to calls for more inclusive conservation approaches to protect species both in their native and introduced ranges.

“Promoting protections for introduced megafauna can find productive common ground with other conservation goals, such as increasing protection and tolerance for apex predators.

“Expanding protections for apex predators may further influence the ecologies of feral donkeys and feral horses. For instance, the re-establishment of other documented equid predators, such as wolves, could increase predation risk in low ambush terrain, further shaping how these animals influence ecosystems.”

Lundgren and his fellow researchers noted that, for more than a million years, cougars co-occurred with several species of equid across North and South America.

“However, much of conservation remains rooted in recent history, with little recognition of the prehistoric and historic legacies of human-caused extinctions and range contractions.

“As such, the apparently novel effects of feral equids on desert wetlands are cited as the primary reason for their eradication and removal.”

However, megafauna influences on wetlands were likely very common from 30 to 40 million years ago, until the late Pleistocene extinctions.

“Indeed, previous donkey removals in the Death Valley region led to the extinction of several endangered and endemic fish populations due to the loss of donkey-maintained open water habitat.

“Despite this, the feral donkeys of Death Valley and surrounding lands are currently being targeted for complete removal.”

They continued: “If we had studied feral donkeys as ‘invaders’, without paying attention to predation, our data would contain a great degree of inexplicable noise.

“Instead, by studying these animals as any other wildlife, we find echoes of the late Pleistocene in a novel trophic cascade.

“Studying these interactions in light of Earth’s history can yield insight into the structure and function of both modern and prehistoric ecological communities.”

They said the introduction of equids after their 12,000-year hiatus — and predation upon them by cougars — suggests that global patterns of inadvertent rewilding can not only increase biodiversity and restore lost ecological functions, but also can rewire ancient food webs.

“Our results suggest that the conservation community should prioritize the protection of apex predators and the world’s remaining megafauna, regardless of their nativeness.”

The research team comprised Lundgren, Erik Kusch and Chris Hasselerharm, with Aarhus University in Denmark; Daniel Ramp, Eamonn Wooster and Arian Wallach, with the University of Technology in Sydney; Owen Middleton, with the University of Sussex in England; Mairin Balisi, with the La Brea Tar Pits and Museum in Los Angeles; William Ripple, with Oregan State University; Jessica Sanchez, with the University of California at Davis; and Mystyn Mills, with the University of California Riverside.

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.

The study, published under a Creative Commons License, can be read here

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One thought on “America’s burros important in rangeland food web, findings suggest

  • August 12, 2022 at 8:42 am

    Predation certainly occurs amongst feral equids in north america, that can’t and should not be ignored. The problem is expecting it to be the endgame solution to the feral equine over-population problem on American public lands. Countless occasions have shown that predation simply isn’t a significant enough factor to offset the yearly birthrate in most herds.


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