Apparent Competition Between Two Submerged Macrophytes via Fish: Evidence From a Mesocosm Study

ABSTRACT The coexistence of submerged macrophytes is crucial for maintaining ecosystem health and biodiversity in shallow lake ecosystems. The modern coexistence theory (MCT) has been extensively used to explain species coexistence in grasslands and microbial communities by considering average fitness differences and niche differences at local scales. However, this theory has not yet been applied to investigate the coexistence of submerged macrophytes. In freshwater ecosystems, fish play a vital role in regulating the structure of the submerged macrophytes community. Fish could affect inter/intra specific competition of submerged macrophytes by indirectly changing the water environment or directly acting as herbivores, which might be described as apparent competition. Here, we employed a short‐term mesocosm experiment to explore how omnivorous fish ( Rhodeus ocellatus ) and herbivorous fish ( Megalobrama amblycephala ) affect the co‐existence of rosette‐forming macrophytes ( Vallisneria denseserrulata ) and canopy‐forming macrophytes ( Myriophyllum spicatum ), based on the MCT. We hypothesised that both herbivorous fish and omnivorous fish could influence the co‐existence dynamics of these macrophytes through apparent competition, that is two macrophytes interact indirectly through their shared grazer. In the absence of fish or with only omnivorous fish present, V . denseserrulata and M. spicatum couldn't stably coexist, with V . denseserrulata exhibiting stronger growth and clonal reproductive with more ramets, winning the interspecific competition according to the prediction of MCT. However, the herbivorous fish, with preferential grazing, altered their fitness and niche differences between the macrophytes, promoting their coexistence. Our results reveal a case study of apparent competition between submerged macrophytes via fish presence, which provides insightful solutions for maintaining high species diversity of submerged macrophytes in restored shallow lakes.