Dispersal ability determines the genetic effects of habitat fragmentation in three species of aquatic insect

1. The dispersal ability of species and the geographic scale of habitat fragmentation both may influence the extent of gene flow between fragments, but their interactions have rarely been tested, particularly among co-occurring species. 2. Population genetic structures of three species of aquatic insect were compared in streams fragmented by reservoirs and in unfragmented streams in north-eastern Japan, using 52, 37, and 58 RAPD markers. The three species studied included a strong disperser Cincticostella elongatula (Ephemeroptera: Ephemerellidae), an intermediate disperser Stenopsyche marmorata (Trichoptera: Stenopsychidae), and a weak disperser Hydropsyche orientalis (Trichoptera: Hydropsychidae). 3. The patterns of genetic isolation by distance (IBD) supported a priori hypotheses of dispersal ability. The strong disperser (C. elongatula) exhibited significant IBD only at the largest spatial scale studied (among drainages, r = 0.50, P < 0.01). The intermediate disperser (S. marmorata) showed IBD both within (r = 0.22, P < 0.01) and among (r = 0.45, P < 0.01) drainages. The weak disperser (H. orientalis) did not exhibit significant IBD at any scale. 4. Pairwise genetic differentiation (theta) indicated that neither the weak disperser nor the strong disperser were genetically differentiated above and below reservoirs when compared with reference sites. This was in contrast to previous results for S. marmorata, for which subpopulations were genetically fragmented across larger (>4.1 km), but not smaller (<2.9 km) reservoirs. 5. We suggest that intermediate dispersers, i.e. those at equilibrium between migration and genetic drift within drainages, are more likely to be affected by fragmentation than either strong or weak dispersers. Intermediate dispersers could therefore be used as indicator species in studies aimed at detecting the effects of distance between habitat fragments (e. g. reservoir size) for conservation planning. Copyright (C) 2010 John Wiley & Sons, Ltd.