Traits of a bee assemblage are linked to aridity

Climate change in the American Southwest is altering the composition of species assemblages. However, the resulting patterns in mean trait values and functional diversity are poorly understood. Bee assemblages in Southwestern drylands are exceptionally diverse, with species varying greatly in their morphological traits. In this study we addressed two questions: Does aridity predict community-weighted mean trait values? Has bee assemblage functional diversity declined under increasing aridity? To answer these questions, we utilized 16 years of abundance data for 33 focal bee species within a hyperdiverse bee assemblage at the Sevilleta National Wildlife Refuge (NM, USA), combined with measurements of a suite of five morphological traits hypothesized to mediate climate change responses. We found that changes in aridity were associated with changes in community-weighted mean (CWM) trait values and functional richness. For the spring assemblage, three CWM trait values (body size, wing size, and lightness in color) increased during climate extremes in either direction; two hairiness traits also had their maxima during arid conditions, but showed more complex (cubic) relationships with aridity. During the summer monsoon season, CWM trait values generally increased with aridity for all five traits, but relationships varied from linear to cubic. The functional richness of the Sevilleta bee assemblage increased with aridity across both seasons, but other measures of functional diversity did not covary with aridity. We conclude that characteristics of bee assemblages, such as community average trait values and functional richness, may change predictably with the increased aridity forecast for this region in the future.