Patterns of endemism and species richness of fishes of the Western Gulf Slope
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Contemporary freshwater fish communities are influenced by selection mechanisms (i.e., species-area relationships; species-energy relationships) with the role of historical mechanisms (i.e., speciation and dispersal) poorly understood and largely understated. I assessed the influence of selection and historical mechanisms on species richness and endemism among fish communities of the Western Gulf Slope (WGS). Distributions within the WGS generally conformed to predictions generated from global freshwater fish distributions. Species richness was greatest (P < 0.01) in wetter regions, and endemism was greatest (P < 0.01) in drier regions of the WGS. To assess historical mechanisms, fish phylogenies were used to determine direction, origin, and timing of dispersion into the WGS. Based on these findings, multiple events of colonization into the WGS likely occurred from the late Miocene until the start of the Holocene. Contemporary lineages represent ancestral origins and routes from the north (28% of the 155 native fishes of the WGS; beginning during the Late Miocene), southwest (15%; Late Miocene/Early Pliocene), north-east (14%; Pleistocene), and east (44%; Pleistocene through Holocene). Recent colonizers (north-east and east) attributed to the greatest numbers (67 to 93%) of widespread taxa within the WGS, whereas older lineages (north and southwest) attributed to the greatest number (92%) of endemics. Furthermore, greatest species richness was in the eastern WGS, nearest the source (e.g., Mississippi River) of most recent colonizers during the last glacial maximum. Consequently, historical influences obscured contemporary influences of selection mechanisms. Nevertheless, a consistent pattern of colonization and extirpation has emerged since the Late Miocene, which likely will continue to influence species richness and endemism in the WGS and can be used to inform fish conservation policy.