A major goal in ecology is to understand which factors control the distribution and abundance of species. Local factors, such as habitat size and heterogeneity, ecosystem productivity and biological interactions, seem to have a major impact on bacterial communities, but regional factors, such as the distance among ecosystems, may also contribute structuring these communities. However, a strong correlation between distance and change in environmental conditions is found in most of studies, which limits general conclusions. Here, we have decoupled the influence of distance and environment on bacterial communities sampling at soil tank-bromeliads. Tank bromeliads form a tank-shaped rosette with a central and several lateral wells that accumulate rainwater, forming a patchy micro-habitat for several groups of aquatic organisms. Furthermore, nearby bromeliads do not necessary show similar water conditions, eliminating possible co-variances between distance and environmental conditions. The main goal of this study was to evaluate the relative importance of dispersal limitation and environmental conditions in determining bacterial distributions. Forty-eight bromeliads of four different species were sampled. Bacterial community composition was characterized through Denaturating Gradient Gel Electrophoresis (DGGE). All distances among bromeliads were measured. We have also determinate in each bromeliad the following local characteristics: 1) number of tanks, 2) plant diameter, 3) water temperature during sampling, 4) maximum temperature in a sunny day, 5) amount of water during the sampling, 6) water capacity, 7) inorganic nutrient concentrations (ion ammonium, nitrate and phosphate), 8) dissolved organic carbon concentration, 9) chlorophyll-a concentration, 10) water color. The ratios between the concentrations of nitrogen and phosphorus (NP ratio) and the concentration of chlorophyll-a and water color were calculated. We determined the relative importance of environmental characteristics and distance in explaining differentiation in bacterial community composition by decomposing the total variation in the bacterial community matrix into environmental and spatial components using partial redundancy analysis (partial RDA). 

Results/Conclusions

Variation portioning showed a significant impact of local environmental factors on bacterial community composition (11% of total variance). On the other hand, spatial distance had no impact on bacterial community composition. The ratio between chlorophyll-a concentration and water color (5% of variance) and the maximum water temperature (4% of variance) were the main local factors regulating bacterial community composition. Our results indicate that dispersal was not limiting bacterial community composition even in these patchy ecosystems, and extreme conditions (high water temperatures) and changes between allochthonous and autochthonous substrate sources were the main local factors regulating bacterial community composition.