Extensive and intensive cost/benefit thermodynamic indicators for the study of the development of trophic networks: application to the lagoon of Venice food web

Extensive and intensive cost/benefit thermodynamic indicators for the study of the development of trophic networks are presented. The characteristics of a suitable indicator are discussed and two state indicators are proposed, based on the assumption that costs and benefits of the ecosystem growth have to be in balance. The first one, the benefit/cost indicator (BC), is a function of well known state indicators, that fully satisfies the properties required although is difficult to compute and complex to understand. Nevertheless, from the researcher point of view, this indicator gives interesting insights into system behavior. The second, the supply demand balance (SDB), is an indicator based on two main assumptions, the first one being that an ecosystem can be represented by a network of compartments and flows, the second that the metabolic rates scale across species approximately as the (3/4) power of mass. The SDB indicator summarizes the distance of an ecosystem from an optimal state in a single number. Under these assumptions the SDB indicator can be regarded as a measure of ecosystem state. It is easy to compute and simple to understand. SDB looks like a good indicator both for scientific and practical uses to understand where the ecosystem is going. A tentative application of these indicators to the lagoon of Venice food web is presented.