Relationships between fish assemblage structure and stream order in South Carolina coastal plain streams

Forty-seven sample sites were electrofished in 22 streams on the South Carolina coastal plain. Average species numbers adjusted to a constant stream surface area were 12.7, 17.5, 21.4, and 22.0 in first- through fourth-order streams, respectively. Species addition and replacement led to large changes in species composition among stream orders. Relatively small fishes, most of which were generalized insectivores, numerically dominated headwater (first- and second-order) streams. Relatively large fishes, many of which were piscivores or benthic insectivores, were most common in fourth-order streams. Headwater species richness was higher and longitudinal species replacement was greater than often observed in other geographic regions of the United States. A comparative assessment of long-term temperature and precipitation records suggested that high species richness at headwater sites was related to mild climate and lack of steep elevation gradients. The presence of numerous small headwater species created the potential for multiple species replacements as downstream increases in habitat volume permitted the establishment of larger fish with predatory and competitive advantages. Because they support many species uncommon in larger streams, headwater streams in the southeastern coastal plain contribute importantly to biodiversity.