Exploring trophic interactions and cascades in the Baltic Sea using a complex end-to-end ecosystem model with extensive food web integration

Direct and indirect biological interactions and trophic cascades suppressing a trophic level in the food web can control entire marine ecosystems. It can, however, be misleading for our understanding and for ecosystem-based management to only investigate the influence of biological interactions and trophic cascades in terms of the biomasses on annual basis, and by comparison of separate species specific data time series in simple food chains. We applied an end-to-end marine ecosystem model including both intra-specific and inter-specific competition and density-dependent regulations to investigate certain trophic interaction dynamics of a disturbed ecosystem. Here all trophic levels of the full Baltic Sea food web were integrated in a holistic mechanistic approach. The model was able to capture the functional responses in interactions of the groups and levels and trophic cascades. Its main results emphasize the importance of integrating the whole food web and the implications of only considering the change in biomass on annual basis, instead of also including the individual condition and the abundance (number of individuals). This should not be neglected as density-dependent interactions will shape the state of the entire ecosystem. We conclude that, in order to better understand ecosystem interactions and trophic cascade state and control mechanisms, a holistic approach is needed integrating multiple processes and seasons and which includes the complexity of all the intra- and inter-specific interactions of the full food web.