Flooding affects fluctuating asymmetry but not growth of a riparian orbweaving spider

Abstract Aquatic emerging insects play a critical role in riparian ecosystems by supporting terrestrial consumers, particularly generalist predators like spiders. Their emergence is highly sensitive to environmental changes such as river regulation and climate change. While previous studies have demonstrated that fluctuations in aquatic insect subsidies influence spider abundance and diversity, little is known about how these changes affect spider morphology and physiology. This study aimed to investigate how flooding affects both aquatic insect emergence and morphological responses of the web‐building spider Tetragnatha extensa (Linnaeus, 1785). We conducted an experiment using a riparian stream mesocosm facility, simulating four flooding events between May and September. Aquatic insects were sampled using emergence traps during all flooding events, while spiders were collected at the beginning and end of the experiment. To assess developmental stress, we measured spider head size and fluctuating asymmetry using landmarking and geometric morphometrics. Spiders in flooded mesocosms exhibited ~15% lower fluctuating asymmetry than those in controls, suggesting reduced developmental stress. This likely reflects the 45% higher emergence of chironomids, the main prey of T. extensa . These results indicate that spider developmental stability is responsive to prey availability and that geometric morphometrics is a sensitive tool for detecting such prey‐mediated ecological effects. Climate change, water use and river regulation, through altered flooding regimes, can disrupt cross‐ecosystem fluxes by affecting the emergence of aquatic insects. Reduced prey availability may, in turn, impact spider physiology and propagate into the riparian ecosystem through altered fitness, reproductive success and population dynamics of terrestrial consumers.