Factors Facilitating the Coexistence of Hydropsychid Caddis Larvae (Trichoptera) in the Same River System

Abstract

SUMMARY (1) Six species of Hydropsychidae were found to coexist in the River Usk and its tributaries (South Wales). There is a marked downstream sequence of species. Some of them have disjunct distributions, others have zones of overlap. On the basis of extensive temperature records from three river systems it appears that this succession is related to sequential differences in temperature. Experimental observations on respiration rates of three species suggest that they are adapted to their own characteristic temperature regimes. (2) Hydropsyche pellucidula and Hydropsyche siltalai coexist in most of the main river. They exhibit differences in their water velocity preferences, life cycles, microhabitat preferences and diet. Hydropsyche siltalai and Hydropsyche instabilis coexist in some sites. No ecological differences between them were revealed. (3) Both sequential distribution patterns and ecological distinctions between species in the same river section could effectively partition the supply of net-spinning sites or food between the species. It is suggested that net-spinning sites are in short supply and that differences in their use are necessary for stable coexistence.