Understanding the impacts of trait-mediated effects on biodiversity and ecosystem function is of growing concern in our changing world - especially in rapidly degrading freshwater systems. In the highly spatially structured environments of fresh waters, hydrological connectivity and relative dispersal capacities are likely to critically influence biodiversity. Bryozoans provide excellent model systems to address this issue. They develop as sessile colonies that reproduce clonally via fragmentation and the production of seed-like overwintering stages (statoblasts) that vary in dispersal capacity. We characterized genetic diversity of: 1) Cristatella mucedo which is primarily found in still waters and produces hooked, floating statoblasts; 2) Fredericella sultana which occurs in running waters and produces smooth, adherent statoblasts. We then ask how hydrological connectivity within and between contrasting landscapes differentially impacts genetic diversity and gene flow in bryozoans that vary in dispersal capacity in lowland agricultural (Norfolk and Northern Ireland), upland (Cumbria), and urbanized (Greater Glasgow) regions. We used 10 and 11 microsatellite markers to genotype C. mucedo and F. sultana, respectively. 16-38 individuals from 11 to 15 populations were genotyped from sites categorized as hydrologically isolated, semi-isolated and connected.
Results/Conclusions
Analysis of over 1300 statoblasts of C. mucedo identified 269 unique clones across all regions. Measures of genetic diversity (numbers of alleles and clones) were significantly greater in C. mucedo populations from connected sites in all landscapes with the exception of those from Greater Glasgow where the inverse relationship pertained. These results provide evidence that hydrological connectivity generally enhances dispersal of floating statoblasts and hence promotes population genetic diversity except in urban environments perhaps through enhancement of stressor impacts. There were 9 cases of identical clones shared by populations in hydrologically connected sites and 1 case of common occurrence in isolated sites providing evidence of greater gene flow amongst populations in connected sites (p < 0.05). Accordingly, populations in hydrologically isolated sites were characterized by higher genetic differentiation (FST). Analyses to date of over 400 F. sultana colonies identified 121 unique clones in one region and similar levels of genetic diversity and differentiation between populations in connected and isolated sites, results in keeping with lower expected dispersal for F. sultana. There was 1 case of a clone occurring in separate catchments. Our study highlights how variation in both landscape and dispersal capacity may impact connectivity/biodiversity relationships in freshwater systems.