“Ecological Traps” and the Tamarisk Leaf Beetle

Unintended Consequences for the Southwestern Willow Flycatcher

Anyone familiar with southwestern rivers is also familiar with the salt cedar bush. Also known as tamarisk, this plant is able to rapidly replace native vegetation in riparian systems. It is now known that the presence of tamarisk is symptomatic of hydrologic changes to river ecosystems that have been brought on by changes in land use and heavily engineered river dam, diversion, and distribution systems. Native trees such as cottonwood will outcompete tamarisk when regular spring river flows of fresh water, resembling historic snowmelt patterns, are allowed to occur. But as very few rivers in the southwest still function with their natural hydrograph intact, tamarisk has flourished, now occupying more than 900,000 acres (1,406 square miles) by one estimate. Some recent efforts to eradicate tamarisk have resulted in unintended consequences for the Southwestern Willow Flycatcher.

All willow flycatcher subspecies spend time migrating in the United States from April to June and from July through September. Flycatchers, like most small, migratory, insect-eating birds require food-rich stopover areas in order to replenish energy reserves and continue their northward or southward migration; migration stopover areas are likely critically important for flycatcher productivity and survival. Also, the Southwestern Willow Flycatcher nests in riparian habitats containing mixtures of native vegetation and tamarisk, and those which are dominated by tamarisk, taking advantage of the presence of tamarisk, especially where tamarisk flourishes in areas where landscape stressors impact the occurrence of native vegetation.

The tamarisk leaf beetle, which causes defoliation and ultimately, death, of tamarisk was introduced across the southwest to control tamarisk, but is now creating challenges for flycatchers. For example, in 2009 along the Virgin River in St. George, Utah, beetle defoliation began with the onset of the flycatcher’s nesting season. Thirteen of the 15 (87%) flycatcher nests in this area of defoliation failed, a rate much higher than sites where beetles do not occur. This has been described by the USGS as a possible ecological trap, wherein the habitat initially appears green and inviting to nesting birds only to change vegetation characteristics in the middle of the nesting season.

With leaf beetles expanding their distribution, moving farther south and faster than ever anticipated, there will certainly be impacts from the leaf beetle on flycatcher recovery efforts. Unless and until river ecosystem functions are restored to allow for more natural flows and successful native plant establishment, any efforts to control tamarisk and support flycatcher recovery will be limited. In fact, the USFWS states that the ongoing presence and operation of dams remain the greatest threats to flycatchers and their habitat. Here again is another reason to protect the Gila River in New Mexico from a large-scale diversion project – tamarisk is largely absent along the Gila because naturally fluctuating river flows sustain the structure and vigor of its native riparian forest and provide healthy flycatcher habitat.

Audubon is actively fighting the delisting of the Southwestern Willow Flycatcher as a federally listed endangered species; more than 24,000 people recently joined National Audubon and state chapters, offices, and members in submitting comments to the federal government to protect this critical bird who is becoming a sentinel for river health across the west. For more information on the Southwestern Willow Flycatcher and current threats to the species’ recovery, including the recently recognized threat of the tamarisk leaf beetle, see the USFWS 5-year Review.

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