CONICET | Buscador de Institutos y Recursos Humanos

To study the relationships between Uruguay and neighbouring geographicalareas based on distributions of Ephemeroptera species (mayflies: an ancient order of aquatic insects). We wanted to evaluate whether Uruguay more closely represents (1) the southern limit of the tropical (Paranense and Amazonian) fauna or (2) the northern limit of the temperate (Pampean-Bonaerense) fauna.We compiled more than 5000 collection records of mayfly speciesthroughout South America and evaluated these using current taxonomy andgeographical validity. We used the Network Analysis Method (NAM) on thesedata to identify units of co-occurrence (UCs: mutually exclusive groups of codistributed species, with each group connected through strong links of sympatry and disconnected from the others). We focused solely on those UCs that included Uruguay in their spatial ranges and used these to infer the vicinity relationships.We recovered four UCs consisting of many species that link Uruguaywith tropical areas of Brazil and NE Argentina. These groups followed a geographically nested pattern. The results contradict the previously held view that the Uruguayan fauna holds strong affinities to that of temperate grasslands that lie to the south in central oriental Argentina (i.e. the currently accepted concept of Pampas). A comparison of the genera known from Uruguay and Buenos Aires Province further reinforces the distinction between Uruguay and temperate areas to the south.The hypothesis that Uruguay represents the southern limit of tropical affinities is strongly supported by mayfly distributions, indicating that a reappraisal of the Pampas as a cohesive biogeographical province is needed. We suggest that Uruguay and Buenos Aires should belong to different provinces, the former aligned with tropical provinces and the latter aligned with more temperate areas.