Dynamics of species-rich predator-prey networks and seasonal alternations of keystone species

In nature, entangled webs of predator-prey interactions constitute the backbones of ecosystems. Uncovering the network architecture of such trophic interactions has been recognized as the essential step for exploring species with great impacts on ecosystem-level phenomena and functions. However, it has remained a major challenge to reveal how species-rich networks of predator-prey interactions are continually reshaped though time in the wild. We here show that dynamics of species-rich predator-prey interactions can be characterized by remarkable network structural changes and alternations of potential keystone species. Based on high-throughput detection of prey DNA from 1,556 spider individuals collected in a grassland ecosystem, we reconstructed dynamics of interaction networks involving, in total, 50 spider species and 974 prey species/strains through eight months. The networks were compartmentalized into modules (groups) of closely interacting predators and prey in some but not all months. As the modules differed in detritus/grazing food chain properties, complex fission-fusion dynamics of below-ground and above-ground energy channels was reconstructed across the seasons. The substantial shifts of network structure entailed alternations of spider species located at the core positions within the entangled webs of interactions. These results indicate that knowledge of dynamically shifting food webs is essential for understanding the temporally-varying roles of keystone species that interlink multiple energy channels.