Life histories and production of three Rocky Mountain aquatic insects along an elevation-driven temperature gradient

Although temperature is known to influence individual traits such as growth, body size, and fecundity, few studies have examined how these relationships influence population-level secondary production in natural settings. We quantified life history traits and production of three dominant aquatic insects (Ephemeroptera: Ephemerella infrequens; Drunella doddsii; Trichoptera: Hydropsyche cockerelli) along an elevation-driven thermal gradient in the northern Rockies. We predicted that production would be highest at sites where temperature regimes lead to the largest terminal body size, individual fecundity, and reproductive potential (i.e., eggs per female x abundance of mature nymphs or larvae). In general, we found that temperature had idiosyncratic effects on life history traits of the study taxa, with no consistent effect of temperature on production. Although growth rates were generally highest during the warm months, growth did not consistently covary with temperature among sites along the elevation gradient. Terminal body size also differed among sites and was inconsistently related to mean temperature and reproductive potential. One of three taxa, D. doddsii, showed patterns entirely consistent with predictions, including smallest body size, reproductive potential, and secondary production at the warmest, low-elevation sites. Our findings suggest that connections among temperature regime, life history characteristics, and secondary production may not be straightforward, and are likely influenced by characteristics unmeasured in our study, including factors that influence survivorship throughout the larval phase, as well as adult mating or oviposition success. Such additional information will enrich our understanding of thermal effects on aquatic insects, and may contribute to predicting how these taxa may respond to ongoing changes in climate.