Fig 1.
Map of the Po River basin with the three river sections: The Upper section of the Po River (in blue), the Middle section of the Po River (in cyan) and the Lower Po River (in light blue).
The main tributaries of Po River (in grey) and the elevation of the basin are also shown (based on open source shapefiles downloaded from European Environment Agency, https://www.eea.europa.eu/en/analysis).
Table 1.
List of node codes and node names.
Presence is marked with an ‘x’. The Master list contains all nodes as they appeared in the original sources. The aggregation steps (T1-T5) show nodes belonging to the five taxonomical aggregation steps.
Fig 2.
Sankey plots showing the food webs for the (a) Upper, (b) Middle, and (c) Lower sections of the Po River, composed of functional groups resulting from the aggregation process.
In each color, the T2-T5 aggregation steps are indicated (T1: size and age aggregation, not relevant here; T2: Perciformes, Cypriniformes; T3: fish families; T4: invertebrates; T5: producers). The width of the flows (grey connectors) indicates the number of nodes (species or functional groups) that have been aggregated from the original networks. The self-loop in the Lower Po indicates cannibalism (Cypriniformes eating other Cypriniformes).
Table 2.
Global network metrics for the three river sections (Upper, Middle, Lower Po) and the combined Master food web, in each consecutive taxonomic aggregation step (T1-T5).
Six global metrics were computed: N (number of nodes), L (number of links), D (network density), CL (weighted overall clustering coefficient), d (average distance), SW (small-world index).
Fig 3.
The REGE similarity dendrograms for the (a) Upper, (b) Middle, and (c) Lower sections, based on the final food webs (aggregated T5).
Nodes are listed on the left, and the horizontal numbers indicate the similarity based on regular equivalence (100 = strict regular equivalence). Higher similarity (i.e., shorter distance in the dendrogram) between nodes i and j means that these nodes are structurally more similar (e.g., share similar direct and indirect neighbourhoods).
Fig 4.
Local network metrics (nDC = normalized degree centrality, nBC = normalized betweenness centrality) for the Po River and the Danube River master network’s shared nodes (species or functional groups).
(a) Normalized degree centrality (nDC) in both rivers, (b) normalized betweenness centrality (nBC) in both rivers, (c) nDC and nBC in the Po River, and (d) nDC and nBC in the Danube River. Correlation between these two indices is high (rpo = 0,76, rdanube = 0,93), reflecting also their mathematical relatedness. Points that are highly correlated indicate nodes that are important for both degree and betweenness centrality (e.g., C. auratus in the Po River, phytoplankton in the Danube), whereas others are more important for either nBC (e.g., Chironomidae in the Danube) or nDC (e.g., G. cernua in the Danube).
Table 3.
Local network metrics (nDC = normalized degree centrality, nBC = normalized betweenness centrality) for the shared nodes for the Po and Danube River food webs.