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BIOLOGICAL REPORT 90(5) 11 Ecology of Southeastern Stream Fishes: Geographic Macro- and Micro-habitat Considerations Stephen T. Ross by Department of Biological Sciences University of Southern Mississippi Hattiesburg, Mississippi 39406 An understanding of natural variation in fish as- semblage composition is important in developing and implementing stream management plans. Over the last decade, fish ecologists have worked to under- stand the patterns of organization and variation in communities and then to explore the mechanisms responsible for these patterns. Studies of stream fish assemblages necessarily emphasize current condi- tions and interactions, simply because we are faced with finite lifespans, careers, and budgets. While human and professional mortality are facts that we must deal with, our mortality-and the resulting short-term view of ecological communities-has led to problems of interpretation. I will identify three problems confronting fish community ecologists and suggest approaches to dealing with them. The first problem is that ecologists often approach communities with the idea that biotic processes that structured such systems are still ongoing and are unaltered in the particular habitat under study-the fallacy of community stasis. It is important to have a temporal perspective on the subjects that we study as fish ecologists-namely species, assemblages, river systems, populations, and microhabitats. In terms of species, the age of most of the fish fauna in the southeastern United States probably dates from the late Miocene or early Pliocene. For in- stance, ancestral suckers, minnows, and darters are thought to have dispersed over a North American land route (Beringia) during the early Tertiary (Gilbert 1976), when such a connection between Europe and America still existed. Somewhat later, the modern darter (tribe Etheo- stomatini) arose from an unknown percid ancestor. Vicariant events, such as isolation in many small streams and the disruption of highland areas, led to the evolution of numerous species of Etheostoma and Percina (Collette and Banarescue 1977). The min- nows date from about the same age, with diversi- fication of the largest genus, Notropis, probably ~ occurring in the Mississippi Basin in the early to mid- dle Pliocene (Gilbert 1964, for Luxilus). Fossil data indicate that centrarchids may date from the Oligocene (Gilbert 1976), and Lepomis occurred in the Miocene (Swift and Wing 1968). Thus, the three largest families of freshwater fishes have diverged since the Miocene or Pliocene (Gilbert 1964; Collette and Banarescue 1977), and therefore we are most concerned with recent, Pleistocene, and Pliocene geology in attempting to understand fish distribu- tions and the history of interactions. In dealing with the diverse fishes of the North American highlands, which include the area of greatest diversity of North American freshwater fishes, Mayden (1987a, 1987b) concluded that these faunas are older-probably much older-than the Pleistocene. While actual dates of origin of individual species are uncertain and will probably remain so, it is evident that the organisms we are studying have existed as species for 2-5 million years. The message for ecologists studying stream fishes is clearly that these species are carrying con- siderable "historical baggage." Also, it is most realistic to expect that adaptations to reduce com- petition or predation should be generalized rather than specific to particular species or microhabitats. Many studies of fish resource partitioning attempt to interpret interspecific differences in resource use as due solely to local spatio-temporal interactions, ignoring the evolutionary history of species. The history of the southeastern fish assemblages tells us that each species in an assemblage represents a long, spatially integrated, and potentially independent evolutionary history, and at any given point a species may not even be well adapted to its environment (Jaksic 1981). Whether or not assemblages vary significantly over time has been a major issue in the attempts to understand factors responsible for maintaining com- munity structure (e.g., Grossman et al. 1982; Ross