Restored streams recover food web properties but with different scaling relationships when compared with natural streams

Despite extensive studies revealing differences in the composition of aquatic assemblages between restored streams and natural or pre-restoration states, understanding the ecological consequences and trajectories of stream restoration remains challenging. Food webs are an important way of mapping biodiversity to ecosystem functioning by describing feeding linkages and energy transfer pathways. Describing food webs can provide ecological insights into stream restoration. This study analyzed an unprecedented large quantity of food web data (more than 1700 webs) based on long-term (2008-2018) biomonitoring data in South Korea using a feeding link extrapolation. By doing so, we aimed to describe general patterns for the reassembly of aquatic food webs in restored streams. Specifically, we analyzed 12 indices related to the food web structure and robustness of restored streams and compared them with those of natural streams. First, the species richness, link numbers, link density, and connectance of the restored streams were all lower than those of the natural streams, indicating smaller food webs with less complexity. Second, the scaling relationship analyses between the other food web indices and species richness and connectance showed different mechanisms for structuring food webs in restored streams compared with natural streams. In particular, greater generalist feeding by consumers was identified as a major mechanism that increased the connectance of restored streams, which may increase their robustness against external disturbances. The fractions of the top, intermediate, and basal nodes in the restored streams changed rapidly as species richness increased compared with those of natural streams. Food web connectance and robustness in the restored streams tended to increase over time, reaching a level similar to that of natural streams. This suggests that the long-term ecological recovery of the restored food webs is underway. Overall, our findings indicate that restored stream food webs have ecological features distinct from those of natural streams, suggesting high compositional flexibility, and that consumers with a broad diet are the major driving forces behind these differences. Our food web analyses provide a greater understanding of restored streams and help support sustainable stream management through restoration strategies. These results provide new insights into the ecological potential of stream restoration.