High-intensity fish disturbance reduces ecosystem multifunctionality by diminishing planktonic bacterial diversity and network complexity

Global environmental change threatens biodiversity and ecosystem functioning, yet the role of species interactions in buffering these impacts remains understudied. In freshwater ecosystems, small omnivorous fish can disrupt ecosystems through predation and nutrient excretion, which could provide models for human pressures on ecosystem structure and functioning. However, the effects of disturbance intensity, as represented by fish densities, on microbial community structure and ecosystem multifunctionality (EMF) are not well understood. Here, we experimentally explored the impact of disturbance intensity using two small omnivorous fish species (Misgurnus anguillicaudatus and Pseudorasbora parva) on the species diversity, co-occurrence network complexity of planktonic bacteria, and multifunctionality of freshwater ecosystems. We found that high-intensity disturbances by the two fish species significantly reduced species diversity, network complexity and EMF, especially for the treatments involving P. parva. However, low-intensity disturbances led to a significant increase in EMF, supporting the intermediate disturbance hypothesis. In addition, the results of linear regression analysis showed that species diversity and network complexity of planktonic bacteria had significantly positive correlations with the EMF. Structural equation modeling (SEM) further confirmed that high-intensity fish disturbances indirectly reduced EMF by decreasing the diversity and network complexity of planktonic bacteria, whereas low-intensity disturbances primarily had direct positive effects on EMF. These findings underscore that protecting planktonic bacteria community structure is critical for sustaining freshwater ecosystems under anthropogenic pressures. We propose managing small omnivorous fish disturbance regimes to preserve microbial diversity and network complexity, offering a novel framework for interaction-oriented conservation in a rapidly changing world.