The ability of species to tolerate a warming climate has important implications for ecological functioning. Theory and empirical synthesis suggest species adapted to more thermally variable climates are more capable of acclimating to rising temperatures, and are therefore characterized by greater phenotypic plasticity, than species adapted to less thermally variable environments. But this pattern has not been extensively evaluated for populations within a species that may inhabit different parts of a thermal gradient. In addition, it remains unclear whether different populations with different thermal sensitivities will maintain the same functional ecological roles as thermal regimes shift. To address this question, we conducted a reciprocal transplant experiment using Melanoplus femurrubrum grasshopper populations from Connecticut and Vermont, USA. During summer, the Vermont site was 3°C cooler on average with 1.5-fold greater temperature variation than the Connecticut site. We measured thermal sensitivity (metabolic rate Q 10 ) of individuals from each population reared in home field and transplanted sites and the nature and strength of trophic interactions with grasses and goldenrod (Solidago). Both grasshopper populations exhibited plasticity, but Q 10 of both populations at Vermont was 1.5-fold broader than populations at the Connecticut site. All grasshoppers had similar survivorship but not similar effects on plants, exhibiting stronger effects on grasses in their home fields relative to their transplanted sites. Only Vermont grasshoppers transplanted to Connecticut significantly impacted Solidago. The study shows populations may physiologically acclimate quickly under new thermal conditions, suggesting stronger tolerance to change than often presumed. But, thermal acclimatization may not translate into the maintenance of a species' functional role. The work underscores the need to link analyses of physiological performance with ecological function to obtain a complete picture of climate change effects on communities.Adam E. Rosenblatt and Bryan T. Crowley have contributed equally to the work.