Ectothermic species have body temperatures that reflect their environment to varying degrees. Environmental temperature drives all cellular and physiological functions, including metabolism, development, growth, migration, and reproduction. Extreme temperatures are occurring more frequently with climate change, and understanding the thermal tolerance and adaptive traits of species is critical.We hypothesized that 1) geographic location of stream ecosystems, such as elevation and latitude, influence the habitable water temperature of lotic (stream) invertebrates because the thermal habitat of species directly influences their life cycle and consequently fitness and 2) species functional traits (e.g., voltinism and feeding behavior) are influenced by habitable temperature. Here, we tested these hypotheses across diverse taxa and geographic regions using a dataset for stream invertebrates traits across North America. We showed that maximum water temperature in habitats and thermal breadth were significantly lower and narrower across streams ranging in elevation, from 0 to 3000 m, suggesting that invertebrate taxa across various elevations are less tolerant of warmer water temperature. Also, we identified thermal sensitivity differences among species traits, especially functional feeding group traits, as these are related to habitat selection in stream ecosystems. Our synthesis suggests that elevation and species traits can help predict thermal breadth and thermal tolerance for different species under a changing climate.

The biomass ratio of herbivores to producers reflects the structure of a community. Four primary factors have been proposed to affect this ratio, including production rate, defense traits, and nutrient contents of producers as well as predation by carnivores. However, the relative importance of these factors across natural communities is elusive, in part because of the lack of a framework for quantitatively comparing their effect sizes. Here, we develop a framework based on Lotka-Volterra equations for examining the relative importance among these factors in determining the biomass ratio. We further utilize it to analyze plankton communities in experimental ponds with different carnivore (fish) abundance and light input. We found that all four factors contributed significantly to the biomass ratio, but carnivore abundance had the largest effect size, followed by the stoichiometric nutrient content. The present framework is useful for quantifying relative roles of these factors shaping terrestrial and aquatic communities.