Tightening the loose equilibrium concept: New seasonal insights with benthic insect data from National Ecological Observatory Network streams

The loose equilibrium (LE) hypothesis predicts that biotic assemblages will exhibit modest change from year-to-year but will not follow directed trajectories away from a stable baseline over longer periods of time. Using 8 years (2016–2023) of data from 21 National Ecological Observatory Network streams, we test the LE hypothesis for benthic insect assemblages. We also expand the traditional LE method, which focuses on a single sampling season, by using multi-season data to determine whether LE is influenced by seasonal change. Results indicate that most benthic insect assemblages may be in a state of LE. Seventeen of the 21 assemblages exhibited random, non-directional change among sampling events, consistent with the LE hypothesis. However, directional movement away from a baseline condition was detected for two insect assemblages, while directional movement from the baseline, followed by a return toward the baseline was detected for the remaining two assemblages. Supplementary taxonomic, feeding behavior, and environmental data suggest that the four instances of directional change may be responses to recent disturbances or altered physicochemical habitat. Distinct seasonal clusters (spring, summer, fall) of insects are also detected for 13 of the 21 assemblages. Among the 13 clustered assemblages, 11 have at least one significantly small cluster that reflects minimal interannual change within the respective season. We conclude by explaining how strong seasonality may support LE. Specifically, we hypothesize that seasonally distinct, yet predictable environments promote inter-seasonal consistency in assemblage structure, thereby constraining opportunities for insect assemblages to shift far from the baseline condition. This work adds to the broader field of equilibrium theory in ecology and may help anticipate future changes in benthic assemblage structure.