Evaluating ecological mechanisms and optimization strategy of rice–fish co–culture system by ecosystem approach

The rice–fish co–culture ecosystem has a long history and has been designated as a “globally important agricultural heritage system”. Recognizing the ecological legacy of this system from an ecosystem perspective may help develop novel sustainable agriculture and aquaculture. This study investigated the structure and function of two different ecosystems of rice and common carp (Cyprinus carpio) using the Ecopath model, which is an ecosystem approach. Two treatments were designed, including the control ecosystem (without common carp, RM) and the treatment ecosystem (with common carp, RF). Seventeen and eighteen functional groups were incorporated into the models of these two ecosystems. The results showed that 1) the introduction of carp increased the energy transfer pathways, and the number of pathways of the RF system was 78.69% more than that of the RM system; 2) the feeding of carp on detritus, benthos, and hydrophytes enhanced the energy transfer efficiency of the RF system at all trophic levels, and the energy transfer efficiency of the RF system was 67.86% higher than that of the RM system; and 3) the indices of flow fluxes, matter cycling, and network information showed that the RF system was more complex, stable, and mature. To improve the energy utilization efficiency on the basis of the long–term energy balance, the maximum stocking density (34.850 g·m−2) was determined by regulating the biomass of common carp. Overall, co–culture with common carp could increase the number of energy transfer pathways, improve the energy utilization efficiency, and enhance the stability and maturity of the rice field ecosystem, representing sustainable agriculture practices. This may be the ecological mechanism by which rice–fish co–culture survives. It also demonstrates that management strategy evaluation utilizing ecosystem models is a powerful policy–screening tool that can inform the resource management decision process.