Allometry and Dissipation of Ecological Flow Networks

Abstract

An ecological flow network is a weighted directed graph in which nodes are species, edges are "who eats whom" relationships and weights are rates of energy or nutrients transfer between species. Allometric scaling is a ubiquitous feature for flow systems like river basins, vascular networks and food webs. By "ecological network analysis" method, we can reveal the hidden allometry directly on the original flow networks without cutting edges. On the other hand, dissipation law, which is another significant scaling relationship between the energy dissipation (respiration) and the throughflow of any species is also discovered on the collected flow networks. Interestingly, the exponents of allometric law ($η$) and the dissipation law ($γ$) have a strong connection for both empirical and simulated flow networks. The dissipation law exponent $γ$ rather than the topology of the network is the most important ingredient to the allometric exponent $η$. By reinterpreting $η$ as the inequality of species impacts (direct and indirect influences) to the whole network along all energy flow pathways but not the energy transportation efficiency, we found that as $γ$ increases, the relative energy loss of large nodes (with high throughflow) increases, $η$ decreases, and the inequality of the whole flow network as well as the relative importance of large species decreases. Therefore, flow structure and thermodynamic constraint are connected.