Intraspecific variation of thermal tolerance in freshwater insects along elevational gradients: the case of a widespread diving beetle

Species distributed along wide elevational gradients are likely to experience local adaptation and exhibit high plasticity of thermal tolerance traits, as these gradients are characterised by steep environmental changes over short geographic distances (i.e., strong selection differentials). However, the prevalence of adaptive clinal intraspecific variation in thermal tolerance with elevation remains unclear, and this aspect has been poorly studied in freshwater insects. We explored variation in upper (heat coma temperature) and lower (supercooling point) thermal limits and acclimation capacity among Iberian populations of the widespread aquatic beetle Agabus bipustulatus (fam. Dytiscidae) across a 2,000-m elevational gradient, from lowland to alpine areas. As minimum, maximum and mean temperatures decline with elevation, we predicted that higher elevation populations will show lower heat tolerances and higher cold tolerances. We also explored whether acclimation capacity is positively related with climatic variability across different elevations. We found significant variation in upper and lower thermal limits among populations of A. bipustulatus, but no evidence of local adaptation to different thermal conditions along the altitudinal gradient, as relationships between thermal limits and elevation or climatic variables were in general not significant. Plasticity of upper and lower thermal limits was overall and consistently low in all populations. These results suggest conservatism of the thermal niche, which might be the result of gene flow counteracting the effects of divergent selection, or adaptations in other traits that buffer the exposure of populations to climate extremes. The limited adaptive potential and plasticity of thermal tolerance found here for A. bipustulatus imply that even generalist species, distributed along wide environmental gradients, may have little resilience to global warming.