Separation of the trophic niches in a community of riparian generalist predators

One striking feature of arthropod communities is their high diversity. Even generalist predators with a strong potential diet overlap can occur at a high local species richness, raising the question of why they do not competitively exclude each other. However, the diet of generalist predators may vary with hunting mode, microhabitat use and habitat specialization. We studied the diet of seven syntopic spider genera in the riparian zones of twelve floodplain mesocosms using gut content metabarcoding. Spider communities in riparian ecosystems are characterized by a mix of habitat specialist and generalist species, and by their reliance on prey of both aquatic and terrestrial origin. We found a surprisingly high differentiation of spider trophic niches, with different combinations of prey taxa dominating the diet of each spider genus. As expected, diet variation could partly be explained by higher dominance of herbivore prey in the diets of plant-dwelling versus ground-dwelling spiders, and by a higher dominance of aquatic prey in the diets of riparian specialists versus habitat generalists. Surprisingly, winged insects were not only the dominant prey source of web-building spiders, but also of free-hunting spiders. Our study revealed that microhabitat use and habitat specialization can lead to strong differentiation of trophic niches even in generalist predators, which may facilitate species coexistence. Gut content metabarcoding proved highly efficient in documenting species-specific differences in the prey choice within a community of generalist predators. Additionally, we found a significant difference in the number of detected prey taxa between direct sampling methods (vacuum and hand sampling) and indirect sampling methods (pitfall traps). Our study highlights that although many spiders are considered generalist predators, they successfully use different niches to coexist within the same habitats.