The Absence of Mitigating Effects of Higher-Order Interactions on Competition in Interspecific Competitive Communities

Higher-order interactions (HOIs) involve interactions among three or more species within a community. Although researchers have long recognized the ability of HOIs to reduce direct competition by altering the strength of species interactions, their specific effects on competition outcomes and community structure remain inadequately understood. This study aims to explore the mitigating role of HOIs in interspecific competition within highly competitive plant communities. We focused on a naturally assembled submerged macrophyte community, which is characterized by highly competitive intensity. We collected propagule banks from Poyang Lake to reconstruct natural habitats under controlled conditions, applying three levels of nitrogen and phosphorus enrichment and four water level fluctuation regimes. The propagules were allowed to germinate naturally, leading to the formation of experimental communities. Given the multi-species nature of the community, we applied MacArthur's consumer–resource model to quantify Niche Differences (ND) and Average Fitness Differences (AFD), which are key determinants of species coexistence. We then assessed the potential presence of higher-order interactions (HOIs) through correlation-based qualitative inference. Our results revealed a striking 87.19% decline in community diversity, as measured by the Shannon–Wiener index. Furthermore, under all control treatments, the proportion of communities exhibiting species coexistence dropped dramatically from 33.3% to only 0.3%, suggesting that HOIs may not be sufficient to mitigate competitive intensity in communities characterized by inherently high competition. Our study challenges the role of HOIs in mitigating interspecific competition in highly competitive communities. This study also found that even non-stressful environmental changes, in conjunction with HOIs, could alter community structure. This underscores a limitation in current coexistence theory and predictive models. These findings underscore the need to incorporate complex, compound, and subtle environmental drivers into future models of community dynamics. Future research should focus on understanding how these multifactorial interactions affect stability and coexistence in real-world ecological systems.