Communities and Habitats

Rivers are dynamic and diverse ecosystems composed of a variety of habitats including the main channel, side channel, flood plain and backwater lakes as discussed briefly in chapter 2. Each habitat typically contains a characteristic assemblage of turtles (herein considered synonymous with a chelonian community). Although riverine chelonians may appear anywhere in the river, most species specialize in one or more habitats where they occur in maximum numbers and biomass. Thus, the various habitats of the river may have similar species composition but the rank abundance for each species will differ. Basic divisions of typical rivers are lotic (flowing water) habitats and the lentic (still water) habitats. Beyond this, habitat divisions involve such physical features as gradient (headwaters versus lower reaches), substrate (sand, gravel, and mud), size (feeder streams versus the river proper), shoreline heterogeneity (wooded, marsh, swamp, etc.) and even temperature. One can expect differences in species composition associated with any of these habitat features. The composition of river turtle communities is affected by the biotic as well as the physical environment of the stream. The presence or absence of lower and higher plants, competing species, or predators can all affect community composition. This chapter is chiefly concerned with assemblages of species rather than the individual species comprising these groups. It will examine the composition of river turtle communities, their ecology, and their evolution. The concept that organisms occur in assemblages of species interacting to produce distinctive community characteristics was neglected for many years in the herpetological literature. Prior to the mid-1960s, most ecological research on reptiles was autecological (Scott, 1982). Nevertheless, a scattering of early papers described chelonian assemblages associated with particular habitats. Evermann and Clark’s biological survey of Lake Maxinkuckee in 1920 included an overview of the turtle community. In 1942, Fred Cagle delineated the species composition and relative abundance of turtles inhabiting six lentic habitats in southern Illinois. In 1950 he published a similar report with A. H. Chaney on ten lentic and two lotic habitats in Louisiana. Tinkle (1959) compared the species composition and relative abundance of species above and below the “fall line” in five United States rivers emptying into the Gulf of Mexico.