Does hydrology drive functional trait diversity differences in arid-land stream ecosystems?
The predicted increase in drought conditions and intermittency of stream flow in the deserts of the American southwest is a major concern for aquatic organisms and adjacent terrestrial habitats. Alterations from natural stream conditions can cause shifts in population and community structure, species extirpations, ecological function, and invasion by non-native species. Climate change and water abstraction is tipping perennial streams towards intermittent therefore, our investigation will give insight into what desert streams may look like in a drier future. We sampled invertebrate communities from 30 sites ranging in flow permanence across eight streams in the Huachuca Mountains in Southeastern Arizona from 2009 to 2011. We used five functional traits linked to feeding, drought tolerance, and body size to describe insect trait diversity. We quantified the patterns of species richness, species diversity, and functional trait diversity of desert stream invertebrate assemblages between intermittent and perennial stream reaches and used multivariate analyses to examine trait-environment relationships. We hypothesized perennial sites will have greater functional trait diversity than intermittent sites.
We found a strong positive relationship between functional trait diversity and species richness. The relationship shows invertebrate assemblages in intermittent sites had lower functional diversity than perennial sites and that the most species rich perennial sites show evidence of functional trait redundancy. Species richness was significantly different between flow types and ranged from 4 in the intermittent reaches to 68 in the perennial waters. A dendrogram of functional trait relationships among species resulted in two clusters, separated largely by functional feeding group and secondarily by dispersal ability. Group 1 consisted of 97 taxa that were predators and active dispersers. Group 2 consisted of 60 taxa that were in the scraper, shredder, and collector-gatherer functional feeding groups. Trait states associated with intermittent sites were small body size and multiple generations per year. We showed sites are composed of communities with distinct biological traits, suggesting variance in traits may indicate differences in ecosystem functioning and stability along a stream permanence gradient. Knowledge of the baseline functional and taxonomic diversity of stream invertebrates will improve our predictions of how stream communities will respond to changes in stream flow regimes.