Responses of Southwestern Willow Flycatchers to Tamarisk Defoliation
Mary Anne McLeod1*
1SWCA Environmental Consultants, Flagstaff Arizona, USA; mmcleod@swca.com
Tamarisk leaf beetles (Diorhabda spp.), released in the western U.S. as biocontrol for tamarisk (Tamarix spp.), began defoliating breeding areas of the federally endangered southwestern willow flycatcher (Empidonax traillii extimus) in 2008 along the Virgin River around St. George, Utah. Beetles then expanded their range downstream, affecting multiple flycatcher breeding sites along the Virgin and Muddy Rivers in subsequent years and arriving at additional breeding sites along the Lower Colorado and Bill Williams Rivers in 2017. Flycatchers consistently had poor reproductive success when they attempted to nest in stands with a significant component of defoliated tamarisk. Breeding sites varied in the prevalence of tamarisk, the timing and duration of defoliation, and the degree of subsequent tamarisk mortality, and the trajectories of the local flycatcher populations varied in accordance with the amount and quality of the remaining habitat. Tamarisk beetles are continuing to expand farther into the flycatcher’s breeding range and are expected to arrive at several large flycatcher sites, some of which consist primarily of monotypic tamarisk, in the next few years. Active restoration of native riparian woodlands in watersheds where flycatchers currently nest primarily in tamarisk is urgently needed to provide flycatchers with alternate nesting sites. Although flycatchers are selective in choosing breeding sites, showing an affinity for areas with dense vegetation in proximity to surface water, sites do not need to be large to support breeding flycatchers. Flycatchers have persisted for several years in small (0.5 ha) patches of native vegetation within large expanses of defoliated and dead tamarisk. Careful selection of restoration sites and implementation of restoration in advance of or immediately following the arrival of beetles may provide the best chance for flycatcher populations to persist post-beetle in areas where breeding sites are dominated by tamarisk.