Milena Gonçalves-Silva , Elvira D'Bastiani, Thibault Datry, Carla Ferreira Rezende

Intermittent rivers and ephemeral streams are dynamic ecosystems characterized by periodic interruptions in flow, leading to temporary or complete absence of water. These dynamics create a mosaic of isolated pools, reducing habitat volume and connectivity, and impacting resource availability and predation levels. This study addresses three key questions: (i) Do species composition and environmental variables change during the hydrological phases in intermittent streams? (ii) Do hydrological phases in intermittent streams affect predator-prey network structures? (iii) Do stochastic factors during the dry phase affect predator-prey network structures? We analyzed predator-prey interactions using fish food items along 1 km of the intermittent river Tabocas during flowing (23 sections) and dry (22 isolated pools) phases, identifying 18 predator species and 11 ecological categories of food items. Our results indicate that fish species composition and environmental variables shift between flowing and dry phases. Despite similar richness, significant differences in fish species composition underscore the importance of temporal fluctuations in community dynamics. Hydrological fluctuations notably affect predator-prey network structures. During the flowing phase, interaction networks exhibited a nested pattern. In contrast, the dry phase networks within isolated pools displayed diverse patterns, including nested, modular, highly specialized, and random structures. Stochastic factors during the dry phase contribute to the formation of isolated pools, leading to predator-prey networks with random structures. The reduction in predator-prey links and increased specialization within pools are key factors contributing to the emergence of random networks, making fish species more vulnerable. Our findings emphasize the importance of identifying priority aquatic refuges to sustain freshwater biodiversity in intermittent streams. Considering hydrological variability and stochastic factors is crucial in devising conservation strategies to mitigate the impact of climate change on the biota of tropical semi-arid ecosystems.