Tamarisk trees are invading, crowding out native trees along rivers in the southwestern United States. But their removal could further imperil the endangered southwestern willow flycatcher, a sparrow-sized songbird.
For decades biologists have tried to remove tamarisk—which grows up to 16 feet tall—manually and with herbicides and other strategies. But they are still losing ground to the sprawling tree. Now, some scientists fear that one of those strategies—an imported, tamarisk-munching beetle—could become a scourge in its own right, wiping out tamarisk trees too quickly for other species in the ecosystem to adapt.
A Growing Problem?
The controversial tree is from Europe and Asia and was first planted in the United States in the early 1800s to decorate western yards and stabilize soil along rivers. Tamarisk, also called salt cedar because it exudes salt into the soil, spread like wildfire. It now dominates more than a million acres of mostly riverside habitat throughout the southwest, having crowded out native trees and keeping them at bay with the salty soils.
Since the 1940s, biologists have worked to control tamarisk trees wherever they’ve taken hold, with especially aggressive efforts in national parks. That’s because the National Park Service aims to protect native plants and animals from a whole host of threats, including invasive species such as tamarisk.
At the Grand Canyon, the trees are so well established along the main Colorado River that park managers have abandoned efforts there. Tamarisk eradication efforts in Grand Canyon National Park for the past decade have been targeted—successfully—at protecting pristine side canyons from the menacing tree’s creep. At last count, tree crews had removed 275,000 trees from 130 side canyons, out of more than 500, within the park.
“From the beginning we knew we were not getting rid of the source,” said Lori Makarick, Grand Canyon’s vegetation program manager.
But that could change with the salt cedar leaf beetle. The tamarisk-killing bug was introduced to areas far enough from the Grand Canyon that biologists thought they would never make it there. The beetle populations, however, kept spreading. And now tamarisk control could happen too quickly for biologists and tamarisk-loving wildlife to adapt.
The USDA’s Animal and Plant Health Inspection Service (APHIS) began importing salt cedar leaf beetles in the late 1990s from Kazakhstan and other regions where they feast naturally on tamarisk trees.
Starting in 2001, land managers in all the southwestern states—except Arizona and New Mexico—began releasing them into tamarisk thickets. Based on input from the U.S. Fish and Wildlife Service, no beetles were released within 200 miles of nesting locations for the southwestern willow flycatcher, a bird endangered since 1995 that now appears to rely on tamarisk trees, nesting in them even when native trees are still around.
In late 2009, Grand Canyon biologists got a surprise when they found just a few beetles about 12 miles downstream from Lees Ferry, Arizona.
Surveys last year revealed that those beetles—which probably followed the Colorado River down from release sites in Moab, Utah—are now eating tamarisk leaves between Lees Ferry, where Grand Canyon National Park begins, and Redwall Cavern, about 33 miles downstream. Another population of beetles apparently has migrated into the canyon from southwest Utah, and set up shop 100 or so miles downstream.
“I am a little freaked out about how fast they’re coming and moving,” Makarick said. “We didn’t even expect them to be here.”
The fear is that if the beetles kill tamarisk too fast, it will be a hit to wildlife species that have gotten used to them, especially the endangered flycatchers. Furthermore, without native trees to fill in the gaps left by dead tamarisk, other weedy invasives, like Russian knapweed, Russian thistle, pepperweed, and camelthorn, could take up occupancy. Many of these species are known for being water hogs, taking more out of the river than native species such as willow.
Makarick says she’s been trying to calm fellow biologists who are jittery about the arrival of the beetle.
She says her team is about a year out from being fully prepared to plant native trees in the wake of tamarisk, which could die out after just a few seasons of beetle feeding. But they’ve already started collecting and growing native saplings–which, mercifully, grow quickly.
Rising to the Challenge
Rusty Lloyd, program director for the Colorado-based Tamarisk Coalition, a nonprofit that aims to stem the tide of tamarisk invasion, said there are cases in Utah, Arizona, and Nevada where the beetles have defoliated, or de-leafed, tamarisk trees quickly enough to kill reptiles and insects by altering the microclimates they need to survive. The loss of tamarisk leaves has also fatally exposed flycatcher chicks to increased heat and predation.
But the beetles are inconsistent, and their patchy habits could give Grand Canyon biologists—and willow flycatchers—a break.
“In some tamarisk stands mortality has been very high—maybe 70 percent or more. In other stands, tamarisk mortality has been near zero,” said Kevin Hultine, a plant ecologist at Northern Arizona University in Flagstaff.
And in some places, the ecosystem is showing signs it can rise to the challenge of beetle invasion.
Defoliation in northern Nevada slowed down when local predators keyed into the presence of the beetles. Arthropods such as ladybird beetles, assassin bugs, stink bugs, preying mantis, spiders, and ants, tended to keep beetle numbers down, said Tom Dudley, a riparian ecologist at the University of California at Santa Barbara.
“Birds also fed readily on the beetles, and we saw increases in the diversity and abundance of birds in areas with beetles compared with nearby areas where they had not yet colonized,” he added.
Die-hard Beetle Fans
Jack Deloach, an entomologist recently retired from the USDA’s Agricultural Research Service, was one of the pioneers of the beetle introductions and once called the program “one of the most successful biological control projects ever in the U.S.” He stands by his assessment.
Deloach calls the southwestern willow flycatchers’ preference to nest in tamarisk trees a “fatal attraction:” the dense branches at the tree crown are appealing because they offer stability. But seen from below—as they are by some predators, like cowbirds—the nests are much more visible than they would be in a willow. Cowbird attacks on flycatchers are three times higher for flycatcher nests in tamarisk than willows, he said.
So far, southwestern willow flycatchers have only met the salt cedar leaf beetle in one watershed—along the Virgin River, which flows through southwestern Utah and Nevada before joining the Colorado River at Lake Mead.
There is anecdotal, but so far no scientific, evidence that flycatcher numbers suffered because of tamarisk defoliation, Dudley said. “On the other hand, where restoration has been done in the upper watershed near St. George, those flycatchers nesting in tamarisk did a very interesting behavioral switch last year to nesting in the willows.”
Once in the willows, one of the trees with which they co-evolved in the first place, the birds tripled the number of chicks they successfully fledged the year before, when they were living in tamarisk trees. Deloach takes it as a sign that southwestern willow flycatchers will end up better off in the wake of the tamarisk-chomping beetles.
“I think salt cedar is very destructive in the ecosystems,” he said. “And I think there’s good evidence that the flycatcher is not going to be at all harmed by this.”