Freshwater Biology (1998) 39, 117–127 Dispersal and recruitment of Tasiagma ciliata (Trichoptera: Tasimiidae) in rainforest streams, south-eastern Australia JANE M. HUGHES, STUART E. BUNN, DAVID A. HURWOOD AND CATH CLEARY Centre for Catchment and In-Stream Research and Cooperative Research Centre for Tropical Rainforest Ecology and Management, Australian School of Environmental Studies, Griffith University, Nathan, Queensland 4111, Australia SUMMARY 1. This study examined genetic variation within and among populations of the caddis fly Tasiagma ciliata (Tasimiidae: Trichoptera) from rainforest streams in south-east Queensland, Australia. 2. Very low levels of genetic differentiation at large spatial scales, between subcatchments and between catchments, indicated that dispersal by the winged adults is widespread. However, significant genetic differentiation at the smallest spatial scale examined, within reaches in a single stream, suggested limited movement by larvae within streams. 3. A patchy distribution of deviations from Hardy–Weinberg equilibrium and differences in patterns among allozyme loci suggested that populations in particular reaches were the result of only a few matings. 4. These results are surprising, given the large numbers of larvae present within a single reach. We suggest that stochastic effects of recruitment may underlie much of the spatial and temporal variation in population numbers in these rainforest streams. Introduction Populations of stream and river invertebrates are generally considered to be highly connected (Wallace, 1990; Mackay, 1992). In part, this is evident in the extensive geographical distributions of some lotic species, suggesting that viable mechanisms for dis- persal exist or, at least, have existed in the past. However, the unidirectional flow of water and the hierarchical structure of the stream channel network undoubtedly limit the degree of in-stream movement (Meffe & Vrijenhoek, 1988). Furthermore, the physical nature of some streams provides many natural barriers to aquatic dispersal (Hughes et al., 1995, 1996). Although adult flight (of insects), downstream drift and, to a lesser extent, swimming or crawling represent the principal mechanisms of dispersal available to stream and river invertebrates (Williams & Hynes, 1976; Minshall & Petersen, 1985; Mackay, 1992), debate continues over the importance of each of these mechanisms to the maintenance of stream © 1998 Blackwell Science Ltd 117 populations (Wilzbach & Cummins, 1989; Hershey et al., 1993; Williams & Williams, 1993; Anholt, 1995). Although many studies of dispersal in stream invertebrates have reported considerable movement of adults and larvae past a fixed point (Hynes, 1970; Lee & Fielder, 1979; Mu ¨ller, 1982; Benson & Pearson, 1987; Ryan & Choy, 1990; Williams & Williams, 1993), few direct measures of the actual distances moved by individuals in the long term have been made (Erman, 1986; Hershey et al., 1993). An alternative approach to direct measurement of individual movement is to consider the consequences of dispersal on the genetic structure of populations. If dispersal is high, little genetic differentiation among populations would be expected (Slatkin, 1985). In contrast, if dispersal is restricted by barriers and/or low because of poor dispersal abilities, differentiation among populations will occur due to natural selection and/or random