ABSTRACT
Trophic interactions are important determinants of the structure and functioning of ecosystems. As the metabolism and consumption rates of ectotherms increase sharply with rising temperature, there are currently major concerns that global warming will increase the strength of trophic interactions, destabilizing food webs, and altering ecosystem structure and function. We used geothermally warmed streams that span a ~10 ºC temperature gradient to investigate the interplay between the thermal response of respiration, local adaptation, and the interaction strength between the keystone gastropod grazer, the wandering snail Radix balthica, and a common algal resource. Populations from a warm stream (~28ºC) had higher maximal metabolic rates and optimal temperatures across all measurement temperatures than those from a colder stream (~17ºC), suggesting local adaptation of metabolic rates. A reciprocal transplant experiment demonstrated that the interaction strength between the grazer and its resource were highest for both populations when transplanted into the warm stream. In line with the thermal response curves for respiration, interaction strengths of the warm-adapted grazers were higher than their cold-adapted counterparts in both the warm and the cold stream. These findings suggest that warming can increase the strength of algal-grazer interactions both through the thermodynamic effects of higher temperatures on physiological rates and through correlated increases in per capita metabolism and consumption as organisms adapt to warmer temperatures.