Characteristics of Lotic Ecosystems and Consequences for Future Research Directions

Abstract

Stream ecologists often have more problems than others in defining the boundaries of their operational unit “ecosystem” because running waters are essentially open systems: they are closely linked to terrestrial ecosystems through their long shoreline; and the water in ail streams of the earth has, statistically, a turnover rate of about 2 weeks (Czaya 1981). Recent books (Barnes and Minshall 1983; Fontaine and Bartell 1983; Resh and Rosenberg 1984) have discussed various means by which stream communities respond to this openness. This chapter concentrates on two major aspects of this response on which possibilities and limitations of running water ecosystem analyses largely depend: (1) that stream ecosystems are viewed as old, co-evolved and well- adapted entities (Vannote et al. 1980), as systems which can adapt their biological organization very fast to changing environmental conditions (Townsend and Hildrew 1984), or even as systems which are often more physically than biologically controlled (Reice 1985); and (2) intermediate levels of environmental fluctuations or disturbances produce high species diversity (intermediate disturbance hypothesis), the maximum of which is expected in mid-reaches of watersheds (Vannote et al. 1980; Stanford and Ward 1983; Minshall et al. 1985).