Global warming is now an undeniable reality: the IPCC has forecast an average 3-5°C increase in global surface temperatures by 2100, with even stronger increases predicted for Arctic areas. Our current understanding of ecological responses to warming is still largely observational, whereas we need to move rapidly towards more predictive mechanistic frameworks based on first principles. It has been hypothesized that Thermal Performance Curves (TPCs) underlie community and ecosystem-level responses to climate change, although empirical evidence is lacking. Further, it has been suggested that such functional, trait-based approaches to understanding changes in species assemblages are more powerful than phenomenological taxon-based approaches (e.g. species richness) because the former provide a mechanistic understanding of relationships based on first principles that can be applied universally across habitats and regions. We worked in a catchment of geothermally-heated streams in Iceland as a model system for the ecological effects of climate warming. We quantified the metabolic TPCs of all abundant aquatic macroinvertebrates from six streams, ranging from 5°C to 18°C.
For species from the same community (stream), we found significant interspecific variability in physiological thermal responses, indicating that some species are more sensitive to warming than others. In addition, we found significant variability in thermal responses between populations of the same species, living in different streams (ie, thermal habitats). This interspecific variability could lead to physiological mismatches between interacting species as temperatures rise, leading to changes in biodiversity and food web structure in a warmer world.