The Role of Filter Feeders in Flowing Waters

Net-spinning trichopteran larvae are used as examples of filter-feeding stream insects to show that various species feed upon a range of particle sizes. Evolution of individual species has resulted in cropping various particle sizes of drifting stream seston. Mechanisms of how this is achieved are discussed. The evolutionary diversity of filter feeders has important consequences for stream ecosystems that transcend the individual species involved. Organic drift in flowing waters is exploited in many ways by various invertebrates (HYNES, 1970). This fact is certainly obvious to those who have pursued the study of fauna in flowing waters. We think the importance of filter-feeding in flowing waters has never been fully stated by aquatic biologists and in this paper we present some ideas on the significance of the great diversity of filter feeders encountered in lotic habitats, using primarily net-spinning Trichoptera as examples. In the past 20 years net-spinning Trichoptera have been the subject of a number of studies. Details of nets and/or retreats have been shown by SATTLER (1955,1958,1963 a, 1963 b, 1968), KAISER (1965), BRICKENSTEIN (1955), GIBBS (1968), SCHUHMACHER (1970), WILLIAMS & .HYNES. (1973), WALLACE (1975 a, 1975 b), WALLACE & MALAS (1975), and WALLACE & SHERBERGEH (1974, 1975). The preference of certain species for various stream sizes has been documented (e.g. DECAMPS, 1968; ULFSTRAND, 1968; EDINGTON, 1968; MECOM, 1972; and GORDON & WALLACE, 1975). FREMLING (1960), EDINGTON (1965, 1968), PHILIPSON (1969), KAISER (1965), ULFSTRAND (1968) WILLIAMS & HYNES (1973), and WALLACE (1975 a, 1975 b) have discussed the importance of various environmental factors such as current velocity on net-spinners. * ULFSTHAND (1967) and EDINGTON (1968) stressed the importance of microdistributions of filter-feeding trichopterans as associated with food preferences. EDINGTON (1968) also found various netspinners were associated with specific areas within streams and suggested nets were constructed for rather specific current velocities and food preferences. WILThe Role of Filter Feeders in Flowing Waters 507 LIAMS & HYNES (1973) found no distinct differences in two hydropsychid caddisflies in a Canadian stream but they did find that a Philopotamidae, Chimarra aterrima HAGEN, occupied a different niche in terms of food and feeding. WALLACE (1975 b) suggested size as well as type of particles consumed were important criteria in niche segregation by feeding in three species of hydropsychids. He suggested that several mechanisms by which this might be achieved would also result in more efficient use of drifting seston food resources. In this investigation we present evidence from two diverse stream types, a small, rapid headwater river and a large coastal plain river, which shows that each locality has filter-feeding trichopteran species which consume diverse types and sizes of stream seston. The purpose of this article is to focus on the importance of this evolutionary diversity of filter-feeders and their role in stream ecosystems.