The Influence on Birds of Tamarisk Beetle Defoliation along the Dolores River in Southwestern Colorado
Charles van Riper III1*, Abigail J. Darrah2, Harold F. Greeney3, and Andrew J. Boyce4
1U. S. Geological Survey and School of Natural Resources and the Environment, University of Arizona, 520 N Park Ave, Tucson, AZ 85719, USA
2Audubon Mississippi, 5009 Main Street, Moss Point, MS 39563, USA
3Yanayacu Biological Station & Center for Creative Studies, km 5, Via Las Caucheras, Cosanga, Napo, Ecuador
4School of Natural Resources and the Environment, University of Arizona, 520 N Park Ave, Tucson, AZ 85719, USA
The tamarisk leaf beetle (Diorhabda carinulata), introduced as a biological control agent for the invasive plant Tamarix ramosissima, has spread throughout the western USA. With D. carinulata now very abundant, scientists and restoration managers have questioned what influence this introduced arthropod might have upon the avian component of riparian ecosystems. From 2009 through 2012 we studied the consequences of tamarisk leaf beetles on avian diets and bird density along the Dolores River in southwestern Colorado, USA. We documented D. carinulata abundance, plant species where the beetles occurred, and the degree to which they were consumed by birds compared to other arthropods. We hypothesized that if D. carinulata is an important new avian food source, birds should consume beetles at least in proportion to their abundance. Despite D. carinulata being the most abundant arthropod in the environment, these invasive beetles were infrequently consumed by birds and seem not to provide a significant component of avian diets. We also assessed the effects of tamarisk defoliation by monitoring defoliation rates, changes in vegetation composition, and changes in density of six obligate riparian breeding bird species at two sites along the Dolores River in Colorado. We also conducted bird point counts from 2010 to 2014 and modeled bird density as a function of native vegetation density and extent of defoliation using hierarchical distance sampling. We found that maximum annual tamarisk defoliation by beetles decreased throughout the study period, peaking at 32–37% in 2009–2010 and dropping to 0.5–15% from 2011–2014. Density of all bird species declined concomitantly throughout most of the study, with Blue Grosbeak and Yellow Warbler densities negatively related to defoliation, while Lazuli Bunting exhibited a positive relationship with defoliation. These findings suggest that tamarisk defoliation may have short-term negative impacts on some riparian bird species, but potentially positive effects on others.