Identifying Biogeographic Modules in La Plata Basin and Patos-Merin Drainage System

The network approach recently emerged as a novel perspective in biogeography. Biogeographic networks could be defined on the base of species occurrences among locations, where species and locations are nodes and occurrences among them the links. In this context, modularity analysis offers a robust tool to identify groups of species and habitats more mutually linked than with the rest of the network. These biogeographic modules could be related to physical barrier or other source of isolation, but are not constrained to them. Based on the species and habitat connections four topological roles are defined: peripherals, connectors, module hubs and network hubs. In a bipartite killifish-basin network from the La Plata basin and Patos-Merin drainage system of South America, we identify a significant modular organization (Q=0.55; p=0.037), which is composed by five biogeographic modules. Most species are restricted to one or two basins and so classified as ultra-peripherals, that is, all their links are within their modules, while a few species connect modules to form the large network. The results herein found emphasize the relevance of modularity to identify areas of endemism, and also to capture intermediate levels of geographic aggregation not evident from other analysis.