Data and code from: Disentangling the drivers of decadal body size decline in an insect population

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 midge Tanytarsus gracilentus and 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 with 13C 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.

Methods
Midges (Diptera: Chironomidae) were captured using window traps (Jónsson et al., 1986) at two locations around Lake Mývatn, Iceland. They were innumerated to species and separated into two cohorts (early summer and late summer), matching the voltinism pattern of the focal species (Tanytarsus gracilentus) which overwinter as larvae (Gardarsson et al., 2004; Lindegaard & Jónasson, 1979; Einarsson et al. 2002, 2004). Archived midges were used to measure carbon stable isotopes (McCormick et al. 2022) and wing lengths were measured from arculus to tip on 15-20 individuals per generation (where abundances were adequate). Because identifications of female midges is difficult and often impossible, the dataset includes only males. Air temperature data come from the Icelandic Meterological Office (https://www.vedur.is/). All processing to the data are included in scripts.