The relative importance of local, regional and historical processes to patterns of distribution and diversity of lower Mesoamerican (Panama and Costa Rica, LMA) freshwater fishes are examined at two different spatial scales. Biogeographic patterns of LMA freshwater fish communities at the spatial extent of LMA indicate that biological differences among species as well as regional and historical processes are quintessential to explaining the patters of turnover of community composition and species richness across the LMA Isthmus. The study of LMA freshwater fishes permits the inference that ecological interactions among species are not independent of a region's history, suggesting that regional and historical processes probably play an ongoing role in the maintenance of LMA freshwater fish communities. In addition, the study of the natural experiment of the Panama Canal provides a clear demonstration that dispersal, rather than local deterministic processes such as competition and predation, previously set the upper limit of the number of fish species coexisting in local communities. Comparisons of species inventories established prior to Canal construction with contemporary species lists demonstrates a large net increase in species richness of local communities, suggesting that the local communities of the Panama Canal watershed were not saturated.

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