Investigating the processes influencing community assembly in the montane ground beetle genus Nebria (Carabidae: Nebriini) in the northern Cascades Range, USA

Ecological theory predicts that species’ upper elevational distribution is limited by physiological constraints, while their lower elevational limit is limited by competitive interactions. The presence of microhabitats complicates this prediction. Functional traits are useful indicators of niche space and can be used to understand the processes which determine microhabitat usage and elevational distribution. The ground beetle genus Nebria (Carabidae: Nebriini) is speciose and abundant throughout the Cascades Range and occurs in a variety of riparian habitats – e.g., sandy rivers, streams with gravel and boulders, and snowfields (0 – 3000 m. a.s.l.). On Mt. Rainier, 12 species occupy sequentially overlapping, statistically distinguishable elevational ranges. The presence of many closely related species indicates strong environmental filtering could be the primary ecological processes influencing community assembly. This study found that the relationship between functional traits and habitat use is scale dependent. Morphological functional traits – body size and pronotum width ratio – are useful predictors of elevational range. The close association between substrate and species, and between substrate and elevation, supports this finding. Physiological functional traits – temperature preference, not desiccation tolerance – are useful predictors of microhabitat use. The importance of environmental filters appears to vary with scale. Future research should focus on understanding the relationships between functional traits and habitat use across continental scales.