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Abstract While climate warming is widely predicted to reduce body size of ectotherms, evidence for this trend is mixed. Body size depends not only on temperature but also on other factors, such as food quality and intraspecific competition. Because temperature trends or other long‐term environmental factors may affect population size and food sources, attributing trends in average body size to temperature requires the separation of potentially confounding effects. We evaluated trends in the body size of the midgeTanytarsus gracilentusand potential drivers (water temperature, population size, and food quality) between 1977 and 2015 at Lake Mývatn, Iceland. Although temperatures increased at Mývatn over this period, there was only a slight (non‐significant) decrease in midge adult body size, contrary to theoretical expectations. Using a state‐space model including multiple predictors, body size was negatively associated with both water temperature and midge population abundance, and it was positively associated with¹³C enrichment of midges (an indicator of favorable food conditions). The magnitude of these effects were similar, such that simultaneous changes in temperature, abundance, and carbon stable isotopic signature could counteract each other in the long‐term body size trend. Our results illustrate how multiple factors, all of which could be influenced by global change, interact to affect average ectotherm body size. 

Abstract Population cycles can be caused by consumer–resource interactions. Confirming the role of consumer–resource interactions, however, can be challenging due to an absence of data for the resource candidate. For example, interactions between midge larvae and benthic algae likely govern the high‐amplitude population fluctuations ofTanytarsus gracilentusin Lake Mývatn, Iceland, but there are no records of benthic resources concurrent with adult midge population counts. Here, we investigate consumer population dynamics using the carbon stable isotope signatures of archivedT. gracilentusspecimens collected from 1977 to 2015, under the assumption that midge δ¹³C values reflect those of resources they consumed as larvae. We used the time series for population abundance and δ¹³C to estimate interactions between midges and resources while accounting for measurement error and possible preservation effects on isotope values. Results were consistent with consumer–resource interactions: high δ¹³C values preceded peaks in the midge population, and δ¹³C values tended to decline after midges reached high abundance. One interpretation of this dynamic coupling is that midge isotope signatures reflect temporal variation in benthic algal δ¹³C values, which we expected to mirror primary production. Following from this explanation, high benthic production (enriched δ¹³C values) would contribute to increased midge abundance, and high midge abundance would result in declining benthic production (depleted δ¹³C values). An additional and related explanation is that midges deplete benthic algal abundance once they reach peak densities, causing midges to increase their relative reliance on other resources including detritus and associated microorganisms. Such a shift in resource use would be consistent with the subsequent decline in midge δ¹³C values. Our study adds evidence that midge–resource interactions driveT. gracilentusfluctuations and demonstrates a novel application of stable isotope time‐series data to understand consumer population dynamics.