River drying lowers the diversity and alters the composition of an assemblage of desert riparian arthropods

Many studies have shown negative effects of river drying on in-stream animals. However, the influence of river drying on riparian animals remains poorly studied. We examined ground-dwelling riparian arthropod assemblages along a drying section of the semi-arid San Pedro River in southeastern Arizona, U.S.A. We found strong differences in assemblage composition, taxon diversity and the abundance of key taxa between dry and flowing sites, with higher diversity and abundance of most taxa at flowing sites. Changes in assemblage composition, taxon diversity and abundance of representative taxa were associated with a combined measure of water availability that included distance to water and type of water. Other environmental variables showed a weaker association with changes in these arthropod assemblages. Thus, we found evidence that desert riparian arthropods are sensitive to river drying and to reduction in water resources. Increases in drying along this river may reduce the diversity and the abundance of many groups of ground-dwelling arthropods, leading to marked shifts in community composition.