October 14, 2019
Yet there are thousands of them in Death Valley, clustering for the most part around natural springs and park buildings.
Burros — known variously as donkeys, African wild ass, or Equus africanus asinus — are the largest animals in the park. They’re also relatively recent arrivals, introduced in the 1800s. And they’ve flourished to the point where they’ve been deemed a nuisance by the National Park Service, since theybulldoze through scarce water and vegetation resources, digging up riparian environments and crowding out native species.
“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 percent a year “is really the low end of what I’ve seen published.”
At first glance, the burro problem seems like a cut-and-dried example of “invasion biology,” a conservation subdiscipline that focuses on the destructive impact of non-native species — from cats and rats that wreak havoc on island ecosystems to Asian carp and zebra mussels that clog inland waterways. But for Erick Lundgren, a biologist at the University of Technology Sydney studying their ecological impact, the 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.
“I think we can get better results by focusing on protecting apex predators and landscape connectivity instead of trying to turn back time and remove wild burros,” he says.
THERE WAS A time when the burros were considered indispensable. In the late 1800s, mining towns spread throughout the Southwest, pulling mineral wealth out of places like California’s Death Valley. In the absence of internal combustion engines, something had to help grind the rocks and ferry supplies, and burros became a vital work force. But as claims ran dry and mines closed, miners released them into the desert.
In the 1930s, managers at Death Valley National Monument — the precursor to the park — began trapping and culling burros, with full-scale removal efforts ramping up whenever populations got particularly large. 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.
In 2002, the park adopted a management plan with the goal of eventually removing the entire population through partnerships with non-profits like Peaceful Valley Donkey Rescue, which trap burros and offer them for adoption. The last roundup was in 2005. According to Allison Ainsworth, a biologist with the park, burro populations in the park have since risen to an estimated 3,000.
“We’re always going to have burros,” says Ainsworth. “We don’t have fences in the park and we’re surrounded by herd management areas of BLM land, which are required by law to regulate wild horses and burro densities.
At the same time, she added, “Death Valley is a national park and we’re mandated to preserve the native species, which the burros are in competition for limited resources with.”
This kind of argument is standard in the discussion of exotic species, says Mark A. Davis, author of “Invasion Biology.” “Non-native species force you to start making decisions about what kind of ecosystem you want and what your values are,” he says. “They aren’t a change from some iron-clad [environmental] truth, and it’s not really scientific to act like they are. ‘Unhealthy’ or ‘healthy’ usually just means ‘desirable or undesirable,’ and we should be up front about that.”
From the beginning, according to Lundgren, 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 streambeds, some of which can go as deep as five feet, to get at groundwater. Photographs of these wells have been used as evidence of negative burro impact, Lundgren says. But as far as he knows, well-digging behavior and its impacts in Death Valley have never been the subject of formal scientific research.
Lundgren began noticing the well-digging behavior in 2012, and actively studying it in 2015. Initially funded by a small grant from Arizona State University, Lundgren raised a further $4,600 via crowdfunding to pay for trail cameras, which he and undergraduate students began placing around Arizona field sites like Bill Williams River. Their findings suggested that abandoned burro wells serve as germination nurseries for cottonwood and willows — keystone tree species in the Southwest — and as water resources for native amphibians, insects, and mammals.
In 2018, Lundgren turned his attention to Death Valley. Unlike the seasonal streambeds throughout the Arizona deserts, most of the water in Death Valley leaches out of hillsides of silt soil, fueling thick stands of vegetation on the slopes. The resulting springs are “strange and magical places,” he says, starkly isolated groves of densely packed willows and cattails, with game trails leading into the thickets opened up by foraging burros. So far, his cameras have caught migratory and resident birds, bobcats, coyotes, and foxes drinking from the wells.
“You go out to these sites and the only surface water are these excavations by wild burros, where they’ve dug out the soil and vegetation to make pools,” Lundgren says. “And it’s really the only water in many of these landscapes. So my first question is: What happens when you remove the animals keeping these wells open?”
One possible answer lies in the surface springs of Nevada’s Ash Meadows National Wildlife Refuge, a desert-wetlands preserve on the edge of Death Valley, where the majority of burros were removed in the 1990s. “One of the requirements for [the springs’] persistence is some level of disturbance,” says Astrid Kodric-Brown, a biologist with the University of New Mexico. “That disturbance may initially have come from Pleistocene mammals, she says, and later from management by Indian populations. “Exotic” herbivores like burros seem to be ensuring the springs stay open, she says, though it’s healthiest for the ecosystem at large if they don’t linger for too long.
According to Kodric-Brown’s research, the springs were rapidly choked by fast-growing cattails and reeds following burro removal, destroying open-water habitat for endangered native fish populations. As a result, Lundgren says, “Land managers there go and manually remove wild vegetation, doing exactly what the burros were doing for free.”