Scale(s) matter: Deconstructing an area of endemism for Middle American freshwater fishes

" Aim
The processes that generate biodiversity occur at finer scales than are often studied, particularly in freshwater systems of the northern Neotropics. We investigate whether fine‐scale biogeographic patterns are present within the larger Grijalva–Usumacinta Area of Endemism (AoE) —a region in Middle America where nearly 60% of freshwater fishes are endemic—and if present‐day river basins are single historical units.

Location
Northern Central America and southern Mexico.

Taxon
Freshwater fishes (e.g., Cichlidae, Poeciliidae, Ostariophysi)

Methods
We used fine‐scale distributional data for freshwater fishes in northern Middle America (based on museum records and fieldwork) and performed cluster analyses on a presence/absence data matrix of 117 species. We assessed statistical support of geographic clusters using global one‐way analysis of similarity and recovered endemic areas that were further supported by species indicator analyses. Endemic areas were diagnosed by the distribution of endemic species within these areas. For a broader scale we also performed phylogeographic analyses for widespread species representing the dominant families of the region.

Results
We found unrecognized biogeographic structure within the Grijalva/Usumacinta AoE. Different lineages possess varying degrees of geographic structuring and endemic species were not homogeneously distributed across the riverscape.

Main conclusions
We redefine the geographic boundaries of two northern Middle American areas of endemism. We identify five endemic areas nested within the larger Grijalva/Usumacinta AoE. We demonstrate that the upper reaches of the Usumacinta and Grijalva river basins are independent historical biogeographic units and can be differentiated from their lower reaches based on their assemblage and molecular diversity. The recognition of these endemic areas can help us define the geographic landscape and develop hypotheses about the processes that generated the ichthyofauna of northern Middle America "