Tamarisk biocontrol alters bird community composition in the absence of cottonwood and willow vegetation

Abstract

Abstract Invasive plants threaten biodiversity worldwide, but control of non-native species may affect native species in complex ways. Non-native tamarisk (Tamarix spp.) is widespread in western U.S. riparian environments, and tamarisk beetles (Diorhabda spp.), a tamarisk-specific herbivore, were subsequently introduced as biocontrol. The primary effect of biocontrol is defoliation and branch dieback, with repeated defoliation killing the plant. We investigated the initial stages of site recovery after biocontrol and how tamarisk decline affected birds, their arthropod prey, and microclimate. We compared avian community diversity and composition, arthropod abundance, humidity, and temperature at sites along the Virgin River in Nevada and Arizona, USA characterized as either mixed native vegetation, tamarisk-dominated, or tamarisk-dominated where the majority of tamarisk died from biocontrol. We compared avian communities sampled after biocontrol to counts at the same locations before biocontrol. Prior to biocontrol, community compositions of all but one site grouped together using unbiased clustering algorithms. Following biocontrol, tamarisk-dominated sites grouped separately, and mixed sites grouped with the pre-biocontrol cluster. Comparison of pre- and post-biocontrol communities showed 7 common species declined by ≥30% in dead tamarisk sites, while one species did so at mixed sites and 3 at tamarisk-dominated sites. Individual census points in dead tamarisk had significantly lower Simpson diversity than the same points censused before biocontrol, unless native vegetation was present, suggesting tamarisk death was the cause of dominant species abundance changes. Tamarisk-dominated sites were hotter and drier than sites with native vegetation and supported fewer non-tamarisk-obligate arthropods, consistent with the hypothesis that bird reductions were driven by changes in microclimate and prey abundance. How long these effects last will depend upon the rate of native vegetation recovery after biocontrol, therefore we recommend monitoring sites to determine the trajectory of vegetative recovery and considering the need and feasibility of active restoration in those sites with slow or no native regeneration.