Competition or Habitat: Co-Occurrence of Endemic and Cosmopolitan Black Bass Species in a Changing Landscape

The black basses of the southeastern United States are an excellent study group for investigating the relative roles of habitat and biotic interactions for determining species co-occurrence. This group is comprised of generalist species such as Largemouth Bass Micropterus nigricans, as well as specialist species such as those within the M. coosae complex. Bartram's Bass M. sp. cf. coosae is a provisional species of particular concern endemic to the upper Savannah River basin of Georgia, South Carolina and North Carolina, USA. Bartram's Bass and Largemouth Bass may occur in the same rivers, but do not necessarily co-occur in the same micro- or mesohabitats. Recent surveys in numerous Savannah River tributaries where Bartram's Bass was previously known to have existed have either detected only Largemouth Bass or no black basses at all. The apparent loss of endemic Bartram's Bass and frequent replacement with cosmopolitan Largemouth Bass is concerning from a conservation perspective because it could indicate either (a) ongoing biotic homogenisation via habitat shifts, (b) habitat-mediated asymmetric interspecific interactions resulting incompetitive exclusion, or (c) some combination of these processes. In this study, we assessed differences in landscape-level factors and proximity to reservoirs on the occurrence and co-occurrence of these species using a multispecies occupancy model developed for two or more potentially interacting species. This approach can be used to untangle the relative roles that biotic interactions or habitat covariates have on species distributions. The most compelling model evidence suggested that Bartram's Bass and Largemouth Bass were associated with land cover effects on instream habitat and provided less evidence that co-occurrence was driven by interspecific interactions. Human land use change and impoundment of free-flowing rivers have created new opportunities for Largemouth Bass to access upstream lotic habitats and are associated with the loss of lotic habitats and restricted the distribution of Bartram's Bass.