The Impacts of the Tamarisk Beetle on Southwestern Willow Flycatcher Habitat on the Middle Rio Grande, NM
Kristen Dillon1
1 Bureau of Reclamation, Denver, CO
The Rio Grande in New Mexico currently supports one of the largest breeding populations of the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus; SWFL) in the United States. Approximately 300 SWFL breeding territories are documented on the Middle Rio Grande annually, and in recent years 75% of nests are constructed in salt cedar (Tamarix spp.). The tamarisk beetle (Diorhabda spp.) was first detected on the Rio Grande in 2012 and had expanded throughout the Middle Rio Grande river corridor by 2016. The Bureau of Reclamation began a photographic monitoring study of the impacts of beetle defoliation in occupied SWFL breeding habitat in 2015. The study employs hemispherical photography, landscape photography, and microclimate monitoring to document changes in SWFL habitat due to beetle defoliation. No defoliation was documented at the study sites in the first two years of the study. Severe defoliation was documented in 2017 and 2018, followed by two years without breeding season defoliation in 2019 and 2020. Despite the lack of direct beetle impact in 2019 and 2020, landscape and hemispherical photography found an overall decline in vegetation health and canopy cover in some sites as a persistent result of multiple previous years of defoliation. Although canopy cover remained within the apparent range of natural variation observed in the study, the decline was sufficient to result in increased temperature and aridity of SWFL breeding habitat.