Why does freshwater fish species richness differ between Pacific and Atlantic drainages of the Americas?

Abstract Aim To identify the contributions of geomorphological and hydrological/climatic factors to differences in total and endemic freshwater fish species richness between Atlantic and Pacific drainages. Location Glaciated and unglaciated catchments draining to the Atlantic and Pacific Oceans in North ( NA ) and South America ( SA ). Taxon Fish. Methods A database of native freshwater fish species richnesses in drainage basins was compiled from published information, and the contributions of temperature, rainfall, catchment area, river length, channel gradient, discharge and geographic location in accounting for variation in total and endemic species richness were assessed using boosted regression trees. Results Atlantic drainages have larger catchments, with longer rivers, lower channel gradients, greater discharges and lower flow variability than Pacific ones of the same size. Unglaciated Atlantic drainages in both NA and SA have more species than Pacific drainages, but glaciated Pacific NA had significantly more species than the other Pacific areas. Geographic location had the greatest influence on total richness, but catchment area, river length and temperature were more important for endemic species. Pacific drainages had proportionally more endemics than Atlantic drainages. Main conclusions Tectonically driven topographic change, by altering relief, affects catchment size, slopes and connectivity, with the latter also affected by climatic changes in ice cover and aridity. These geomorphic and long‐term climatic changes, by influencing speciation, extinction and dispersal, govern freshwater fish species richness in Atlantic and Pacific catchments.