Native Invasions, Homogenization, and the Mismeasure of Integrity of Fish Assemblages

Abstract

Human activities, particularly habitat destruction and species introductions, are resulting in increased homogenization of once unique biogeographic regions. 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 streams flow into lower elevation systems, which are often inhabited by more widespread, generalist fish species adapted to warmer, more turbid, fine-sediment-rich, and nutrient-rich conditions. Common land use practices, such as deforestation, degrade stream habitats and reduce habitat diversity, which is often correlated with taxonomic and ecological diversity. Habitat homogenization can thus cause assemblage homogenization via loss of native species and addition of nonindigenous species. However, midpoints in the homogenization process may be characterized by constant or even increased species diversity because generalist, sediment-tolerant, “native” species invade from downstream areas. Perusal of a species list for a river system would not 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 (α diversity) and evenness, should not include invasive species, whether native or exotic. Greater attention should be paid to the actual species present and their ecological requirements, and to changes in overlap in species occurrence among regions. Aquatic ecosystem integrity can degrade despite apparent increases in species diversity.