Indirect effects and distributed control in ecosystems

Network environ analysis (NEA) was used to analyze a seven-compartment, steady-state model of nitrogen flow in the Neuse River Estuary, North Carolina, USA. Four perspectives for analyzing network properties emerge: (1) throughflow-specific oriented to output environs; (2) throughflow-specific oriented to input environs; (2) storage-specific oriented to output environs; (4) storage-specific oriented to input environs. Analysis of the model is based on a decomposition of total system throughflow, TST, into total environ throughflow, TET, for each of the seven compartments, and a further decomposition of total environ throughflow into compartmental boundary exchanges and environ flows generated by boundary inputs and outputs at individual compartments. The analysis provided a quantitative basis for the development of indirect effects between compartments and is the basis for analyzing the fate of nitrogen entering the system and the origin of nitrogen leaving the system. The decomposition of TST is also the basis for analyzing four network properties: pathway proliferation, ratio of indirect effects to direct effects, homogenization and amplification. Results indicate dominance of indirect effects within the model due to cycling. Moreover, insight about the interrelationship of all model compartments is developed quantitatively based on the NEA methodology.