New research suggests that the introduced wild donkeys may benefit native species, but the National Park Service wants to remove them.
Death Valley National Park is the largest park in the lower 48 states, it contains the lowest elevation point in the country — nearly 300 feet below sea level — and has set the global heat record. It’s a place that seems utterly antithetical to life, certainly not the sort of terrain where you’d expect to find a thriving population of wild burros.
Yet there are thousands of them in Death Valley, clustering for the most part around natural springs and park buildings.
“They have no predators and no disease essentially, and so their numbers continue to increase exponentially,” says Josh Hoines, the former chief of resources at the park. A growth rate of 20% a year “is really the low end of what I’ve seen published.”
Burros of Death Valley represent a remarkable case study in resilience and potential adaptation, and are part of a far more nuanced debate about how novel ecosystems can evolve under our noses.
While a 1971 federal law forbade the hunting of mustangs and burros and set aside tracts of land under the Bureau of Land Management for both, Death Valley and other federal parks have tried to keep the populations as small as possible.
The scientific literature on Southwestern burros has assumed they are undesirable, and has emphasized the damage they cause. Burros tend to dig groups of wells in dry stream beds, some of which can go as deep as five feet, to get at groundwater.
Unforeseen consequences are common in conservation land management and the onus for environmental damage tends to be placed on non-native species rather than other factors that might be amiss in the ecosystem, like the absence of predators.
Several publications have pegged burro population growth directly to birth rates, without recording how many foals actually survive to adulthood.
In the course of the 2018 and 2019 field seasons spent monitoring Death Valley field sites in areas like Willow Canyon and Mesquite Spring and considerable evidence of predation was found: around 33 burro carcasses in the riparian vegetation, half of which he characterizes as undeniable kills from mountain lions. (The other half are less conclusive, but were found in close proximity to lion scat and tracks.)
Some mountain lion trails contain skeletons from two or three burros and the dense vegetation that springs up in the groves during the growing season may be hiding more remains. While such kills might be attributable to just a few lions, they might also be a signal that the lion population is adapting to target the largest available prey in the ecosystem, which would provide a natural check on burro populations. Some studies suggest that high levels of mountain lion predation essentially negate population growth in some wild horse herds.
IN ORDER FOR THIS kind of natural check on burros to work, lion populations have to be healthy. In some areas of Nevada where burro populations are skyrocketing, mountain lions are shot at a certain age to protect bighorn sheep or livestock meaning that cubs often don’t have exposure to more challenging prey. The young and inexperienced lions that remain have a hard time going after wild burros. In addition, burro herds in the park tend to shield themselves by clustering around areas of heavy human habitation, which lions tend to avoid. All these factors make it appear as if the burros have no predators but actually point to a more complicated dynamic.
The comparatively low resilience of the system should be measured against the gains [burros provide.] As with nearly all issues in biology, it’s very context-dependent, so they may be more helpful in some years than others.
It’s a complex issue and requires deeper study. There is no question that burros are having a direct impact on Death Valley’s ecosystems and that they are doing so under a management paradigm that sees any impact as problematic. The question is whether there are alternate methods of managing the burro population that don’t require rounding up surplus animals.