Predicting changes in bee assemblages following state transitions in North American drylands

Drylands worldwide are experiencing ecosystem state transitions: the expansion of some ecosystem types at the expense of others. Bees in drylands are particularly abundant and diverse, with potential for large compositional differences and seasonal turnover across ecotones. To better understand how future ecosystem state transitions may influence bees, we compared bee assemblages and their seasonality among three dryland ecosystem types of the southwestern U.S. (Plains grassland, Chihuahuan Desert grassland, and Chihuahuan Desert shrubland). Using passive funnel traps, we caught bees during two-week intervals from March through October during 2002 - 2014. The resulting dataset included 302 bee species and >70,500 individuals. Bee abundance, composition, and diversity differed among ecosystems, indicating the potential for future ecosystem state transitions to alter bee assemblage composition in drylands. We also found strong seasonal turnover in bee species, suggesting that bee phenological shifts may accompany ecosystem state transitions. Common rather than rare species drove the observed trends, and both specialist and generalist bee species were indicators of each ecosystem type or month; these species could be informative sentinels of community-wide responses to future shifts. Our work suggests that predicting the consequences of global change for bee assemblages will require accounting for both within-year and among-ecosystem variation.