Larger crustacean zooplankton thrive at lower temperatures and under reduced predation in pond ecosystems

Body size is a fundamental trait influencing ecological functions, trophic dynamics, and organismal fitness across ecosystems. However, the environmental drivers shaping body size patterns in pond ecosystems across broad spatial scales remain poorly defined. In this study, we assessed the main determinants of body size in cladocerans and copepods, key crustacean zooplankton groups, across 168 ponds including regions in Europe, Asia, and South America. We assess how temperature and latitudinal variation, food availability, and predation influence mean and maximum crustacean body sizes in zooplankton taxa. Using linear mixed models, we identified annual temperature and predation pressure as the strongest predictors of body sizes. Body size increased with lower temperature for both taxa, supporting the temperature-size rule. Fish presence was associated with smaller body sizes for both taxa, while copepod size metrics were additionally influenced by pond morphometry, showing a negative relationship with depth and a positive association with area. These findings suggest temperature and predation pressure act as major selective forces shaping crustacean body size in pond ecosystems. Understanding these patterns is essential for predicting how crustacean communities and processes they mediate may respond to ongoing climate warming, particularly in small-shallow freshwater ecosystems highly sensitive to environmental change.