Impact of birds on intertidal food webs assessed with ecological network analysis

The Wadden Sea is one of the most important stop-over sites for breeding and migrating birds. About 10–12 million birds per year use the area for foraging and consume about 25%–45% of the standing stock of macrozoobenthos. However, little is known about the influence of birds on the entire ecosystem. We conducted ecological network analysis (ENA) on an important bird breeding and resting site, the Norderaue tidal basin in the northern German Wadden Sea, to determine the influence of birds on the entire food web. The model was based on the yearly average biomass of system compartments empirically sampled every season at the study site. The analysis revealed a complex and well-connected food web (high flow diversity and effective link-density) dominated by short pathways but with a high throughput of energy. The system's energy had a high degree of freedom for its development. However, there is a strong dependency on external imports of phytoplankton due to the dominance of suspension feeders suggested by a low recycling and the detritivory to herbivory ratio. A large variety of bird species uses the area for foraging and induces a negative impact on their prey items revealed by the mixed trophic impact analysis. The food sources and also the competitors of the birds' prey items were positively influenced by the birds via indirect pathways in the network. Furthermore, there is a negative impact among the bird compartments probably due to competition between the bird species. A sensitivity analysis, conducted by changing the bird biomass in the model, revealed only small changes in the activity, complexity and connectivity of the system. The linkage to other system components could amplify these changes in the system functioning. It is therefore recommended to include birds in coastal food web studies which has rarely been done before. The use of holistic approaches such as ENA would facilitate decisions on management measures by providing a simplified representation of the entire ecosystem including all direct and indirect connections between the system components.