The birds and the bee(tle)s

The end of a controversial tamarisk biocontrol program may be good news for habitat

  • Tamarisk trees along the Colorado River downstream of Moab, Utah, turn brown as tamarisk beetles gain a foothold. The native willows, bright green in the distant background, have been increasing here as the tamarisk continues to decline.

    Tom Dudley UCSB
  • Tamarisk beetle

    USDA Agricultural Research Service
  • Introduced tamarisk leaf beetles are now well-established on the Colorado Plateau, and spreading on their own. In June, the feds put the kibosh on a region-wide effort to use the beetles to kill invasive tamarisk, because the insects have spread into the Virgin River watershed (in the vicinity of St. George) where they could threaten nesting endangered southwestern willow flycatchers.

    the Tamarisk Coalition

Page 2

The beetles strip the trees where flycatchers nest at exactly the wrong time of year, creating an ecological trap for the birds, according to biologists. "The southwestern willow flycatchers like it where it's dense and moist," explains Greg Beatty, a biologist with the U.S. Fish and Wildlife Service in Arizona. When the small brown birds arrive from their winter homes in Central America to build nests early in the summer, the tamarisk is attractive because it's still green. Then the beetles emerge, start munching and upend everything. Different humidity levels, more sunlight, and new predators stress the birds -- and in 2009, 13 out of 15 Virgin River nests in the affected area failed once the beetles arrived. (The average nest failure rate is more like 40-50 percent.)

This year, the beetles advanced another 20 miles down the Virgin. By next year, they could be at Lake Mead. Researchers still don't know how deep into the flycatcher's range the beetles will spread, nor exactly how they will affect the birds in the long term. Even the statistics for the current overlap are preliminary, though this year's results are encouraging: The birds learned from last year's nest failures and chose to nest in willows instead. In the long run, there's a chance the beetles could even help improve the bird's habitat, if they clear the way for a more diverse community of native plants. But that's a big if.

Because it takes many years for biocontrolled ecosystems to reach a new equilibrium, it's too early to know for sure what will happen once beetles transform a site. "Generally, whatever is on the edges is going to move in as tamarisk mortality occurs," says Tim Carlson, research and policy director of the nonprofit Tamarisk Coalition. Along the main stem of the Colorado, native willows are growing more vigorously now that the beetles have hit the tamarisk. But on the lower Humbolt River in Nevada, the invasive Russian knapweed that was already in the understory is thriving as the tamarisk dies out.

Beatty worries that, especially in the Southwest, the willows won't return. And it's not because tamarisk is inherently aggressive: Newer studies suggest that dams, water diversion and overgrazing have created conditions that favor the plant. "Killing tamarisk thinking your ecosystem problems are going to go away is kind of like wiping your nose thinking it's going to take care of the cold," he says. In fact, Beatty says, tamarisk may not be the evil invader everyone thought.

It turns out that tamarisk uses more water than native plants only under certain conditions. Removing it rarely results in more water for people or wildlife. Tamarisk doesn't necessarily make soils saltier. And in some cases, wildlife -- including the flycatcher -- does fine in tamarisk and would likely do worse if the plants were removed.
Much of this information isn't brand-new, but it seems to have reached a new threshold of attention: APHIS cited "changes in perception of saltcedar (water usage, wildlife habitat, etc.)" as part of the new information that led it to re-evaluate the beetle program. APHIS and Beatty both reference a 2009 paper tracing the historical "monstering" of the plant. If blanket hostility to the tree is hyperbolic and outdated, it may matter less that many land managers are no longer free to cultivate the leaf beetles to fight it.

Still, Beatty concedes that removing tamarisk makes sense in some cases, especially where it's ecologically, economically and practically feasible for natives to rebound. As Dudley points out, tamarisk burns fiercely and generally supports less biodiversity than native plant communities. And the plant narrows stream channels by holding sediment, thus speeding water and increasing flooding. (Removing it all can cause trouble, however, as the sediment dumps back into the system.) Carlson of the Tamarisk Coalition sees the issue in terms of time-scales: "Generally speaking, long-term, there's good consensus that native species are better than a monoculture of non-natives."

Ultimately, though, the scuffle over the beetle may lead to more comprehensive efforts to restore flycatcher habitat. In early September, the Tamarisk Coalition assembled scientists and agency representatives to talk about how to prepare the birds for the beetles' arrival. Fish and Wildlife's default practice of simply leaving tamarisk in flycatcher habitat no longer makes sense. Instead, the idea is to be proactive. On the Virgin River and other sites, researchers and restoration professionals could create patches of willows near flycatcher nest sites, so the birds won't have to go far if tamarisk declines. And in the long term, the vision includes grazing reduction, leaving more water in streams where feasible, and protecting patches of native plants to serve as seed sources. These tactics will help not just the flycatchers, but the ecosystem as a whole. But if they don't work, don't get funded, or don't happen fast enough, the flycatcher could still be in trouble.

Even so, Dudley now believes that "for the first time in years there's actually some kind of a progress towards doing positive work," his optimistic tone a marked change from his earlier frustration. He's eager to get at the root causes of habitat problems. "In a weird way, the presence of the beetles really brought a smoldering issue to a head."

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