Optimized joint envelope score ecological niche models of four species of tamarisk beetles (Diorhabda spp.) introduced to North America

Joint Envelope Score ecological niche models are developed to project potential distributions of four tamarisk beetles (Diorhabda spp.; Coleoptera: Chrysomelidae) introduced for biological control of tamarisk (Tamarix spp.)  in western North America. Feature selection was used to optimize subsets of 50 climate variables at 1 km resolution for modeling each species individually. Distribution data of all species were used to inform the models of each individual species. Individual Envelope Score models were overlapped to produce a joint model with ties indicating areas of potential overlap of the four species. Subtropical tamarisk beetles, D. sublineata, are projected to have the greatest potential to spread westwards from Texas and New Mexico and dominate most of the Chihuahuan, Sonoran, and Mojave deserts, eventually reaching southern California. Models project that northern tamarisk beetles, D. carinulata, already occupy most of the potential southern portion of their North American range in suitable habitats of the Colorado Plateau Shrublands and Great Basin Shrub Steppe. They may spread mostly to the east into the Western Short Grasslands from Texas to Kansas. Larger tamarisk beetles, D. carinata, are projected to continue spreading north and west from grasslands of Texas, Oklahoma, and Kansas to eventually reach California and Nevada (excluding most of the Sonoran Desert). Mediterranean tamarisk beetles, D. elongata, are projected to remain mostly confined to currently occupied areas of the California, west Texas, and eastern New Mexico. Subtropical tamarisk beetles have the greatest potential to spread into critical habitat of the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus) where it nests in dense tamarisk in central New Mexico and central Arizona.