Great Basin Naturalist


The thermal equilibrium hypothesis predicts that aquatic insect body size/fecundity and, consequently, population density and biomass will be maximized in geographic areas or along altitudinal gradients where the thermal regime is optimal with respect to growth and development. Seasonal growth analyses of three mayfly species,combined with detailed thermal descriptions were used to explore differences in body size and fecundity at three sites with similar elevations but different temperature regimes. Site 1 was located near the upper altitudinal distribution for each species, whereas sites 2 and 3 were located below a deep-release storage reservoir. The temperature pattern at site 1 had rapid seasonal changes, with a short summer and a long, freezing winter. Site 2 demonstrated gradual seasonal changes combined with winter warm and summer cool temperatures. Site 3 was intermediate with respect to seasonal change and winter harshness but had the highest maximum and mean annual temperatures. Mayfly development at site 1 was characterized by slow growth during the summer-autumn period, no growth during the winter, and a rapid increase during the spring-summer period, in contrast growth at site 2 was continuous throughout the year including the winter. Growth at site 3 was either continuous across sites or rapid during the spring-summer period, depending on the species. Based upon the interactions among temperature, body size, and metabolic costs, the thermal equilibrium hypothesis was successful at predicting body size and fecundity differences among sites. It was less successful at predicting variation in population density and biomass. Density-dependent and density-independent sources of mortality, including temperature, may interrupt the translation of higher fecundity into higher population density and biomass.