Heterogeneity affects the functional composition and diversity of montane benthic macroinvertebrate communities

Background/Question/Methods

Traditionally, benthic macroinvertebrate communities have been characterized by taxonomic groupings and diversity indices. Recently, a trait-based approach has emerged that allows comparisons of macroinvertebrates by functional traits rather than their taxonomic affiliation. This approach has the potential to identify traits necessary for maintaining stream ecosystem function. Here we explore how benthic macroinvertebrate diversity and functional trait composition vary with streambed heterogeneity.  Heterogeneity was assessed in terms of near-bed velocity, stream depth, and substrate size. We collected Surber samples from Copper Creek, a montane stream flowing through the Rocky Mountain Biological Laboratory in Gothic, Colorado. For each sampled area, nine measurements of near-bed velocity and depth were taken, and substrate size was measured. Standard deviations (SD) of near-bed velocity, depth, and substrate size were used to quantify heterogeneity, such that higher SD values indicated greater heterogeneity. All macroinvertebrates collected were identified to species and body length was measured. Other functional traits were assigned using the classification scheme in Poff et al. 2006.

Results/Conclusions

For all observed families, body length decreased as near-bed velocity increased (R² = .91, P = .003). The most abundant families were found in areas with higher near-bed velocities, and abundant families had a smaller average body length than rarer taxa (5.25 and 7.41 mm, respectively; R² = .79, P < .001). Ordination plots revealed a separation of genera based on their functional traits, as well as by average near-bed velocity and substrate size. The greatest factors contributing to diversity (i.e. effective richness, e^H) were near-bed velocity and substrate size. As heterogeneity in near-bed velocity increased, e^H decreased, while greater variation in substrate size yielded an increase in e^H.  Results from this study suggest that both functional traits and taxonomic diversity are strongly influenced by habitat heterogeneity. Moreover, families present at fast near-bed velocities showed greater relative abundance, thereby decreasing evenness and e^H in regions of fast near-bed velocity. Together, these data suggest that near-bed velocity and its heterogeneity across natural streambeds are elemental in structuring the fine-scale diversity of macroinvertebrate communities.