Spatial pattern of fish assemblage structure and environmental correlates in the Spring River Basin, with emphasis on the Neosho madtom (Noturus placidus).

Abstract

Collections of the threatened Neosho madtom (Noturns placidus) made in 1993-94 confirm the persistence of a disjunct population in the Spring River. We captured 87 Neosho madtoms at 19 sites, extending the known distribution of the species in the Spring River 1.5 km upstream and 26 km downstream, with one downstream site representing a newly discovered subunit of the species' distribution. Mean overall Neosho madtom densities in the Spring River were 0.9 -1.8 per 100 m2, substantially lower than those reported from other portions of the species' range. We also examined patterns of spatial heterogeneity in the Spring River basin fish assemblage along with environmental correlates to assess the relative importance of geographic distances and habitat differences among sites in explaining assemblage structure. Mantel tests and Mantel correlograms indicated that fish species composition and abundance were spatially autocorrelated and exhibited patch size of about 44 km at the basinwide scale. We used partial Mantel tests to remove the effects of spatial autocorrelation from habitat variables before modeling habitat factors influencing fish assemblage structure. Space-constrained cluster analysis and principal coordinates analysis revealed three primary groups of sites, reflecting relatively distinct fish faunas within the Ozark, Lowland, and mainstream regions of the basin. Within individual streams, longitudinal pattern was more apparent than it was at the basinwide scale, and spatial autocorrelation of species and environmental differences were of varying importance, consistent with the concept that stream systems act as mosaics of interacting patches. Spatial patterns of the fish assemblage and environmental correlates were consistent with a hypothesis of vicariance biogeography as the primary organizing factor, but a linkage between mainstream and tributary assemblages, along with spatial autocorrelation in species composition, suggested biotic contagious processes are important in maintaining assemblage structure, particularly at the interface between the mainstream Spring River and its tributaries.