Managing invasive and non-native species in the National Park System is already tough, but what happens when some of the biggest culprits are actually visitors' favorites?
By Susan J. Tweit
Inside the historic Grand Canyon Lodge at Bright angel Point on the North Rim stands a bronze statue of Brighty, a burro who roamed the canyon’s trails as a prospector’s companion and Park Service mascot between the 1890s and 1922. The plucky donkey won the hearts of generations of readers in Marguerite Henry’s best-selling novel, Brighty of the Grand Canyon, published in 1953.
But the thud of burros’ hooves and their loud “hee-haw” braying no longer echo from the Grand Canyon’s colorful cliffs. The last of Brighty’s kind were removed in the late 1970s: feral burros had invaded the desert habitat and inflicted serious damage on the landscape. Like rabbits in Australia or kudzu vines in the Southeast, many invasive species are exotics that multiply out of control, overrunning their adopted habitats, pushing out native species, and disrupting native ecosystems.
The story of the desert’s “wild” burros, native to North Africa, illustrates some of the issues complicating invasive-species management in our parks, including public sentiment, explosive numbers, and a lack of funds.
Itinerant prospectors first brought Brighty and his kind to the Southwest’s deserts as pack animals during the late 1800s. Once feral, the diminutive donkeys with the ridiculously long ears adapted all too well to their new home, establishing themselves throughout the Southwest.
“Burros can reproduce [and increase their numbers] 25 percent per year, doubling their population pretty quickly,” says Linda Manning, wildlife biologist at Death Valley National Park in southeastern California. Combine that fecundity and a life expectancy of nearly 40 years with their mobility and tendency to congregate around scarce desert springs, and that spells bad news for native species. “Burros will quickly foul a water source with their feces and urine,” Manning says. “Bighorn sheep won’t come drink when the burros are there.
“Burros are opportunistic grazers,” she adds. “They browse on whatever is available.” Each adult burro consumes around 6,000 pounds of forage annually, which deprives other species like the federally threatened desert tortoise—North America’s largest and longest-lived land turtle. These ambling herbivores emerge from underground burrows each spring to feast on the desert’s brief green-up, a flush of annual plants that burros also relish.
Ecology professor Scott Abella of the University of Nevada-Las Vegas believes grazing burros alter whole ecosystems. For example, by overgrazing native plants, burros could contribute to the spread of red brome, an invasive annual grass from the Mediterranean that burns easily, charring desert habitats not adapted to fire like the “forests” of Joshua trees in Joshua Tree National Park.
Although burros are equina non grata in Death Valley and Mojave National Preserve, removing them is not simple. In 1971, Congress passed the Wild Free-Roaming Horse and Burro Act, giving the feral equines permanent tenancy on Western public lands. Although the act exempted national parks and wildlife refuges, it clearly demonstrated the popularity of feral horses and burros and the power of their fans.
At Death Valley, Manning estimates that 400 to 500 burros roam the park’s 3.4 million acres of desert basin and spiny mountain range, a landscape slightly smaller than the state of Connecticut but nearly all designated wilderness. A 33-mile-long fence built on the park’s northeastern border in the 1980s, at a cost of around $400,000, helps prevent burros from migrating to the area, but the wily feral donkeys still enter the western part of the park from Bureau of Land Management (BLM) and Department of Defense lands.
Rounding up burros in such remote country and trucking them out isn’t cheap. Helicopters alone can run $1,000 an hour, Manning says, plus the cost of veterinary care, neutering, feeding, and adoption through BLM. “We have a goal of zero burros, but there isn’t much funding,” says Manning. “If we could get $75,000 a year for five years, we could go a long way toward removing burros from the park.”
A Horse of a Different Color
Unfortunately, feral burros are only one of hundreds of exotic invasive species saddling the national parks.
Another kind of feral equine, popularized by yet another childhood book, roams Assateague Island National Seashore, established to preserve a 37-mile-long crescent of sandy barrier island off the shores of Virginia and Maryland. Like Death Valley’s burros, Assateague’s feral horses came from the Old World, and their descendants continue to damage island habitats, though there’s no mention of that fact in Marguerite Henry’s best-selling children’s book, Misty of Chincoteague.
Horses’ presence on the island dates to the late 1600s, when mainlanders herded stock out to graze on the publicly owned fertile marshes, which allowed them to evade livestock head taxes. The National Park Service’s involvement started when the agency acquired 28 horses from the Ocean City Jaycees shortly after the National Seashore was established in 1965. (Oddly, Congress directed the Park Service to acquire the horses during testimony that led to the parks’ establishment.) The population of horses had nearly quadrupled by 1994, when managers began annually administering a contraceptive vaccine by dart gun. The animals’ numbers peaked at 175 in 2001 but then dropped to the current figure of about 140. The National Seashore now spends about $55,000 per year to manage the herd.
“The horses affect the entire system from top to bottom,” says Carl Zimmerman, chief of resource management at Assateague. “Most of the species here have not evolved with large grazing herbivores. For instance, we’ve got a federally listed plant species—the sea beach amaranth—that the horses just love to munch.” After an absence of three decades, a few specimens of this beach-loving annual sprouted in 1999. Park staff promptly harvested seeds and spent several years reestablishing a viable population, only to have the horses eat it. Today, the Park Service surrounds the sea beach amaranth with protective wire caging, and as a result, it’s thriving.
Horse grazing has also altered the species composition and structure of the island’s salt marshes. A salt marsh heavily grazed by horses, says Zimmerman, “comes up to your ankles, where it should reach your knees,” leaving secretive marsh species like seaside sparrows and clapper rails without cover. Salt marsh cordgrass, the horses’ preferred food source, has also declined, resulting in less forage for both horses and wildlife and a system that’s more susceptible to erosion. And horses eat the native dune-stabilizing grasses, preventing new dunes from forming—something that even park staff didn’t anticipate.
After studying the balance between the optimum herd size necessary to preserve genetic diversity and to manage the horses’ ecological effects, the National Seashore aims to reduce the herd to 80 to 100 horses.
The feral horses’ enduring popularity makes the subject of population management a delicate topic, so Assateague is gathering public comment about a proposal to reduce the herd’s size, in the hope of “bringing the public along for the ride,” in Zimmerman’s words. “We care what people think and want to act accordingly.”
When Good Plants Go Bad
Many exotic plants are common in gardens and farmland, and some of these have the ability to quickly invade an area and proliferate, which leaves Park Service managers with the dual headache of explaining how a good plant went bad while combating the invasion. Grand Teton National Park in Wyoming is launching a multiyear effort to remove one such species—smooth brome, an exotic grass still widely planted for hay—from some 9,000 acres of former farmland at the south edge of the park. This area is crucial spring and fall range for the Jackson Hole elk herd, plus a thousand or so bison, says park ecologist Kelly McCloskey.
“Smooth brome is palatable for only a short while in spring, but then it gets pretty tall and tough,” McCloskey says, and the elk and bison won’t eat it. “It’s invading the adjacent big sagebrush rangelands,” reducing the rich diversity of native plant species that other wildlife depend on, from greater sage grouse to sagebrush voles. The soil under these old fields has fewer of the microbes that help native plants sprout and grow, she says, and that makes restoration more difficult.
Park biologist John Moeny wrote his graduate thesis on control of the invasive grass that sprouts from dense mats of underground stems. “We use fire to knock back the stems, forcing the grass to put out a new flush of growth,” he says. “Then we follow with herbicide.” After the smooth brome has been killed, the area will be restored, acre by acre, using native seeds that volunteer crews have collected by hand from nearby sagebrush habitat.
Another formerly cultivated plant, baby’s breath—the cloud of tiny white blossoms commonly used in bridal bouquets—infests the world’s largest system of freshwater sand dunes along Lake Michigan’s eastern shore, including Sleeping Bear Dunes National Lakeshore. The plant with the innocent-sounding name has invaded about 1,325 acres, 25 percent of the open dune habitat, according to Tom Ulrich, the park’s assistant superintendent.
“Baby’s breath tends to stabilize the dunes,” says Ulrich, “preventing dynamic movement of sand.” Native species including the threatened Pitcher’s thistle, a federally listed plant, depend on sand movement to create new habitat. Ulrich adds that baby’s breath also crowds out native dune plants because of its height and tendency to grow “cheek-by-jowl.” The constant winds that shape and reshape the dunes also send dried baby’s breath stems tumbling over the sand, dispersing the invader’s seeds for miles along the shoreline.
And the invasive plant is quite difficult to eradicate: “A single plant can have roots as big around as your wrist that go down twelve feet,” Ulrich says. Crews work plant by plant, digging into the sand to cut the thick roots and spraying herbicides.
The national lakeshore is multiplying its efforts by cooperating with The Nature Conservancy, which received funding from Meijer, a Michigan retailer, to eradicate baby’s breath along 160 miles of lakeshore. John Legge, the Conservancy’s West Michigan conservation director, says that after sharing crews for several years, in 2007, the lakeshore asked the Conservancy for a grant of $50,000. That sum will be matched by funds from the Park Service’s new Centennial Initiative, allowing Sleeping Bear Dunes to accelerate baby’s-breath-removal efforts.
“It’s been a great partnership,” says Ulrich. “Together we accomplished more than we expected. The Park Service and Nature Conservancy crews treated about 100 acres last year, and we’re hoping for 500 this year.”
Baby’s breath is not Sleeping Bear Dunes’ only invasive species; it is simply the one that most needs control now, before it gets even further out of hand. Like all park managers, Ulrich can reel off a list of worrisome exotics, ranging from aggressive mute swans to garlic mustard, an ornamental garden plant poised to invade the hardwood forest.
In time, the Park Service may overcome some of the more serious challenges facing Assateague, Mojave, and Sleeping Bear Dunes, but with 391 park units that cover more than 80 million acres, invasive species will continue to take root throughout the system. That doesn’t mean they’ll overrun the landscapes that the Park Service was established to protect. It simply means that invasive-species management—with its costs, constraints, and the creative partnerships it engenders—is here to stay, too.
The Cost of Invasive Species
The U.S. spends more than $120 million a year to control and mitigate more than 800 invasive species, from the Mediterranean fruit fly to the Burmese python. Invasive species affect more than 300 Park Service units.
How You Can Help
- Learn to identify the invasives in your area. For help, visit www.epa.gov/greenacres.
- Plant only native species in your yard.
- Avoid releasing exotic animals into the wild.
- If you fish, clean your shoes, waders, boat, and trailer after each outing, to avoid spreading invasive species from one ecosystem to the next.
- Join a volunteer invasive patrol—control is often labor-intensive, but very rewarding.