Indirect Effects of Biocontrol of an Invasive Riparian Plant

A biological control agent (tamarisk leaf beetle, Diorhabda spp.) is being used to defoliate and help control the spread of exotic saltcedar or tamarisk (Tamarix spp.) in riparian ecosystems in the western United States.  Although considered a useful tool, saltcedar biocontrol (like other biocontrol methods) has the potential to affect non-target species. Tamarisk leaf beetle populations are spreading in the Virgin River system of Arizona and Nevada and, because amphibians and reptiles respond to vegetation changes and forage in areas where the beetles are active, herpetofauna are model taxa for investigating potential impacts of biocontrol defoliation. Our study investigated the correlation of herpetofauna (toads, lizards, and snakes) abundance to vegetation cover and indices (NDVI and EVI) and timing of biocontrol defoliation.  We captured herpetofauna and ground-dwelling arthropods in trap arrays and measured vegetation using remotely sensed images and on-the-ground measurements at 16–21 sites 2 years before (2009–2010) and 2 years after (2011–2012) biocontrol defoliation. Following defoliation, riparian stands (including stands mixed with native and exotic trees and stands of monotypic exotic saltcedar) had significantly lower NDVI and EVI values and fewer re-captures of marked lizards. Total captures of herpetofauna were related to higher vegetation cover and sites with a lower proportion of saltcedar. Our results suggest that effects of biocontrol defoliation are likely to be site-specific and depend upon the proportion of native riparian trees established prior to biocontrol introduction and defoliation. The mechanisms by which habitat structure, microclimate, and ultimately vertebrate species are affected by exotic plant biocontrol riparian areas  are factors to be considered by natural-resource managers.