We analyzed the metacommunity structure of volant and nonvolant small-mammal assemblages along the San Pedro-Mezquital River to provide insights on its role as a biological corridor connecting the eastern and western versants of the Sierra Madre Occidental. Because the river bed descends from 1,800 m to sea level, and from arid to tropical environments, we expected high species turnover and significant relationships of species composition with the vegetation and elevation. Different metacommunity structures were identified for volant (Gleasonian) and nonvolant (quasi-Gleasonian) mammals; i.e., species along the gradient replace each other, but replacement is not by groups but rather species respond idiosyncratically to local environment. We found no significant relationship between species composition and vegetation or elevation, likely because mammals that would otherwise be confined to one vegetation type can occur here in more than one, probably tracking specific resources along the elevation–vegetation gradient. Results support the hypothesis that it is not species richness in itself that accounts for the high biodiversity in Mexico but rather the high environmental heterogeneity that in turn produces high species turnover rates at relatively low scales.