Functional traits and habitat use: investigating community assembly in a montane community (Carabidae: Nebria)

The processes that influence community assembly, such as competition for
resources and environmental filtering, are often scale-dependent and vary
across ecotones. Trait-based ecology provides a useful framework for
testing which ecological processes most strongly influence local community
composition, especially across environmental gradients where species
diversity varies. Where environmental filtering dominates, species
distributions are expected to be defined by strong turnover along
environmental gradients, with more similar species occupying more similar
habitats. Where interspecific competition dominates, species are expected
to diverge in relative abundance and resource utilization at sites, so
species can co-occur. Here, we integrate measurements of functional
traits, microhabitat usage, isotopic composition (δ15N and δ13C), and
abundance to test the importance of environmental filtering and
resource/habitat partitioning in shaping a montane ground beetle species
assemblage (Carabidae: Nebriini: Nebria) in the isolated, volcanic peaks
of the northern Cascades Range, U.S.A. Across species of Nebria, body
size, pronotal shape, temperature preference, and isotopic enrichment
varied across habitats [gravel, rocks 10 cm – 50 cm in diameter), large
rocks (>50 cm in diameter), vegetation-covered rocks, and alpine
(snowfields and talus)], and habitat/microhabitat features were reliable
predictors of species presence. Resource consumption among mid-elevation
species on Mt. Rainier – the peak with the greatest species diversity – is
highly overlapping. Species turnover and nestedness varied significantly
across habitat gradients and peaks throughout this region and varied
nearly significantly across sites. Across habitat types and sites, more
similar species are more likely to coexist. These results suggest that
environmental filtering is the primary process structuring this species
assemblage, although we find detailed evidence for microhabitat niche
partitioning among species of Nebria at the site-scale.