Native invasions, homogenization, and the mismeasure of integrity of fish assemblages

Human activities, particularly habitat destruction and species introduction s, are resulting in increased homogenization of once unique biogeographic r egions. In the southeastern United States, extensive endemism occurs among highland fish species that have specialized ecologies, are adapted to cool, clear, nutrient-poor conditions, and are sediment-intolerant. Highland str eams flow into lower elevation systems, which are often inhabited by more w idespread, generalist fish species adapted to warmer, more turbid, fine-sed iment-rich, and nutrient-rich conditions. Common land use practices, such a s deforestation, degrade stream habitats and reduce habitat diversity, whic h is often correlated with taxonomic and ecological diversity. Habitat homo genization can thus cause assemblage homogenization via loss of native spec ies and addition of nonindigenous species. However, midpoints in the homoge nization process may be characterized by constant or even increased species diversity because generalist, sediment-tolerant, "native" species invade f rom downstream areas. Perusal of a species list for a river system would no t reveal such invasions because lists seldom discriminate between upstream and downstream assemblages in a drainage. Traditional metrics often used in biological assessment, such as species richness (a diversity) and evenness , should not include invasive species, whether native or exotic. Greater at tention should be paid to the actual species present and their ecological r equirements, and to changes in overlap in species occurrence among regions. Aquatic ecosystem integrity can degrade despite apparent increases in spec ies diversity.