Species-specific foraging behaviors define the functional roles of sympatric stingrays

Animals that disrupt sediments through burrowing or foraging contribute to ecosystem function through bioturbation and ecosystem engineering processes linked to their excavation behavior. Empirical evidence linking behavior with function is rare; yet this information is critical for assessing species-specific functional roles. Using two stingray species, Himantura australis and Pastinachus ater, as model ecosystem engineers, we empirically investigated how foraging behavior influenced the functional roles of sympatric species. Drone observations of stingray foraging revealed a strong link between behavior and function. Excavation feeding created the largest feeding pits and accounted for 58-67% of sediment turnover despite occurring in only 22-31% of feeding events. Although both H. australis and P. ater were capable of excavation feeding, less disruptive foraging behaviors were often favored over excavation. Differences in space use and behavior revealed that functional roles of sympatric stingrays are different yet complementary, which may enhance ecosystem productivity. High feeding rates combined with frequent use of disruptive feeding behaviors resulted in higher bioturbation rates for H. australis than for P. ater. On the other hand, P. ater made fewer feeding pits but foraged over a broader sandflat area than H. australis. Consequently, H. australis have intense, localized bioturbation and ecosystem engineering impacts, while P. ater is likely to promote nutrient dispersal over a larger area. Overall, results suggest that functional roles are dependent on complex interactions between feeding behavior and space use, meaning an extensive understanding of engineering activities is required before similar functional roles can be assumed for even morphologically similar species.