Organismal traits determine how species perform and contribute to ecosystem functioning. Functional diversity quantifies the value and range of the traits that influence species performance and contribution to ecosystem functioning. Functional diversity (rather than simply taxonomic diversity), provides a mechanistic framework to understand community productivity and resilience to perturbations or invasion. Aquatic insect functional diversity using multidimensional metrics has been examined in the past decade, but not in the context of strong environmental gradients. We applied a multifaceted framework to answer the question of does functional diversity of aquatic insects change along strong environmental gradients? We quantified three primary components of functional diversity of stream insects along environmental gradients of canopy coverage, water temperature and elevation across the Rocky Mountain Streams of Colorado. We calculated Functional Richness (FRic), Functional Evenness (FEve) and Functional Divergence (FDiv) indices based on a matrix of 21 traits per taxon collected, to test for significant differences in aquatic insect functional diversity in 24 small streams in three adjacent catchments, spanning an elevational range of ca. 2000–3500 m.
Results/Conclusions
Our results showed that only FRic differs significantly with elevation, and that the pattern of change remains constant across catchments. A principal component analysis showed that traits related to voltinism (number of generations per year), development, ecological habit, and adult dispersal ability, showed the highest correlation to changes in the environment; particularly to water temperature changes over 1°C in areas with open canopy (less than 60% coverage). These findings support the understanding that small tributary streams collectively exhibit high beta diversity; and that taxonomic (as well as functional) richness of aquatic insects declines with elevation in headwater mountain streams. Habitat heterogeneity and changes in water temperature along the elevational gradient, coupled with increased pressure on dispersal traits between isolated communities, contributes to sustaining large biotic heterogeneity among small montane streams. Observed results regarding changes in taxonomic and functional diversity along canopy coverage gradients have important implications for understanding the impact of riparian cover loss on aquatic insect communities and stream ecosystem function.
Our findings provide independent information concerning position and relative abundances of species in a multidimensional functional space, and allow for inferences on patterns of aquatic insect functional beta diversity along environmental gradients in mountain streams.