The Desert’s Most Loved Invasive Species

The American Southwest is home to thousands of feral donkeys, mostly descending from the mules used by miners in the 1800s. They’ve since become among the most ubiquitous and controversial large animals of the desert.

National Park Photo

I still vividly remember the first time I saw a wild burro. I was driving through western Death Valley, on an seemingly endless stretch of empty road, when what looked like a miniature horse popped out of nowhere. I pulled over, expecting to see its owner clamoring it to come back, but no one came. No horse trailers, no green ranch nearby, not even a single car in sight. Only a completely unbothered burro blindingly staring at me, as if I were the one out of place.

It wasn’t alone. The population of feral burros in the American desert is well into the thousands, with estimates ranging from a conservative 9,000, up to 20,000, making them more numerous than human inhabitants in some remote valleys. These animals are living reminders of the region’s mining history, when donkeys were the primary beast of burden for transporting supplies, running mills, and even hauling raw ore from inside the mines.

The American burro is the domesticated form of the African Wild Ass, a relative of the wild horse native to the eastern-most stretches of the Sahara desert. They were first domesticated around 5,000–7,000 years ago, and have since become among the most common beasts of burden, present in every continent except for Antarctica. Donkeys have been used for farming, construction, warfare, and travel ever since, and remain commonplace in much of the developing world.

The first burros in North America were brought by the Spanish in the 1500s, but it would take until the 1800s for large numbers of the animals to arrive in the Southwestern deserts. They are perhaps best remembered for the mule teams used to transport borax from Death Valley, which is still sold today as “20 mule team borax” in grocery stores across the country. Burros began to be replaced by diesel and gasoline engines by the late 19th and early 20th centuries, when many of the animals would simply be left in the desert to fend for themselves, where they established permanent populations.

Prospectors pack a burro, circa 1898. From Wikipedia

On face value, burros are the quintessential invasive species. They mostly live around sensitive desert springs, where they easily outcompete native species, trample on rare plant species, and pollute waterways. A single burro eats around 6,000 pounds of forage a year, and adults can weigh over 500 pounds. Worst still, unchecked burros populations grow at a rate of 20% a year, and can completely dominate riparian ecosystems within a matter of years. Particularly at risk are the desert bighorn sheep, which can be pushed out of rare desert springs by the much larger burro.

Burros also frequently come into conflict with human inhabitants. They’ve been blamed for overgrazing land reserved for cattle, damaging property, and causing countless car accidents due to their stubborn tendency to stand directly on the road. Burros have even caused the military to abort test flights, due to their presence on remote landing strips. Perhaps as a consequence of these conflicts, “burro murders” have become unfortunately common in the region, from Death Valley to Arizona.

But on closer inspection, their record is somewhat more mixed.

During times of drought, burros are known to dig large holes in dry stream beds up to 6 feet deep, which have been seen to be used by 59 different desert species, which might otherwise not be able to access any water. These wells are also helpful in establishing new populations of rare desert trees, such as cottonwoods. Further, despite the common belief that burros have no natural predators, mountain lions have been seen to hunt the large herbivore, often taking down individuals far larger than themselves. Even the burro droppings can be beneficial, as they provide nutrients and foster plant growth.

The role of the burro in riparian desert ecosystems was finally put to the test when the Ash Meadows National Wildlife Refuge eradicated the animals from their preserve, with hopes to protect the wetlands. The plan backfired spectacularly. As soon as the burro were gone, thick groves of cattails and phragmites took over much of the area, dropping the water table, causing the disappearance of multiple pools, and even leading to the extinction of endangered fish species from some locations. Park managers now have to routinely remove excess plants by hand, a task that was once done for free by burros.

Even the basic claim of burros being non-native is only half-true. Equines actually originally evolved in North America, and only spread to the Old World around a million years ago. Native horses roamed the American west until as recently as 10,000 years ago, and burros today fill an evolutionary niche left by the disappearance of the Pronghorn Antelope from the Mojave in the early 20th century.

There is a certain irony surrounding the debate over burros in the American West. They may technically be an invasive species, but they’re extremely useful in combating other, more insidious invasive species. Neither the cattails nor phragmites are originally from North America, and many of the native herbivores that might feed on them have since been driven out of the region.

I for one welcome the curious furry animals in America’s deserts. Burros are a remarkably docile species, giving the rugged landscape a reassuring breath of congeniality. For all the talk of donkeys being a stubborn and unintelligent species, there is something deeply human about their tendency to make eye contact and stare inquisitively at passersby. That, and they’re pretty darn cute.

burro in Death Valley. Original photo