Trophic amplitude as an indicator of aquatic food web complexity

Real food webs are characterized by complex set of interactions that are considered fundamental to keep the system stable as well as to provide resilience and resistance to perturbations. The complexity of food webs is intrinsically connected with the degree omnivory of a system, therefore omnivory provides a basis for defining existing and new indicators of food web complexity. Here I first compare Omnivory Index, i.e. the variance of the trophic position of each food web element, with the Trophic Amplitude that accounts for the spectrum of trophic levels of the preys instead of their average value. I then define measures of food web complexity: the System Omnivory Index (SOI), the System Trophic Amplitude (STA) and the Average Trophic Amplitude (ATA) that are compared using both static and dynamic food webs. Results for a large set of static food webs highlight the independency of these indicators from the number of nodes used to describe the food web structure. Calculation of these indicators of complexity on the basis of results of a calibrated dynamic model showed that Average Trophic Amplitude is easy to calculate, responsive and a sensitive measure of the trophodynamic changes occurred in the system. Findings of this paper are thus suggesting the potential usefulness of the Average Trophic Amplitude in the context of Marine Strategy Framework Directive for evaluating good environmental status of marine food webs.