Population genetic structure of 3 alpine stream insects: influences of gene flow, demographics, and habitat fragmentation

Estimating scales of dispersal for benthic macro invertebrates using neutral genetic markers requires consideration of genetic, demographic, and historical influences on population genetic structure. We used allozyme electrophoresis to investigate the population genetic structure of 3 species of alpine stream insects among major drainages of the Swiss Alps (Rhine, Inn, and Ticino rivers), among streams within each drainage, and within single streams. Within streams we examined reaches that were fragmented by lakes or resevoirs and unfragmented reaches. Rhithrogena loyolaea (Heptageniidae) exhibited little genetic differentiation (theta) within (theta = 0.01-0.03) and among (theta = 0.02-0.03) streams but significant differentiation among drainages (theta = 0.08), suggesting that dispersal occurs among stream fragments and among stream valleys. Allogamus auricollis (Limnephilidae) did not exhibit genetic differentiation at any scale, suggesting that dispersal occurs throughout the geographical range of the study. In contrast, Baetis alpinus (Baetidae) showed moderate to substantial differentiation both within (theta = 0.08-0.39) and among (theta = 0.06-0.09) streams. However, a distinct lack of genetic differentiation for B. alpinus among major drainages of the Alps (theta = 0.01) suggests that low theta values reflect historical rather than present-day levels of gene flow. We suggest that genetic population structure reflects a lack of equilibrium between gene flow and genetic drift, resulting from historical gene flow that continues to mask reduced dispersal and from recurring processes of recruitment that lead to random changes in genetic signatures. We conclude that demographic processes affect small-scale patterns and historical recesses affect large-scale patterns. The simultaneous study of multiple spatial scales helps determine the relative importance of each. A synthesis of our results and data from published studies indicated that 4 consistent patterns of genetic differentiation emerged when multiple spatial scales were investigated. These patterns are indicative of taxon-specific dispersal ability within and among streams and whether taxa are in gene flow-genetic drift equilibrium.