Aim It is generally believed that communities of small organisms, or those with small propagules, are structured mainly by local niche-based processes, and less by dispersal limitation. Conversely, weaker environmental and stronger spatial structure, indicating dispersal limitation, are expected to occur more frequently in communities of large organisms. However, this hypothesis has rarely been tested by comparing spatial and environmental effects across groups of organisms of different size (or with different size of propagules) sampled at the same set of sites. Here, we test it in urban environments.
Location Thirty-two cities in 10 countries of Central Europe and Benelux.
Methods We compared effects of spatial location and climate on species composition of different groups of organisms sampled in corresponding types of urban habitats. The studied groups were: (1) subaerial cyanobacteria and algae, (2) vascular plants, (3) land snails; and subgroups of vascular plants with different life form and dispersal mode, namely: (4) herbs, (5) animal-dispersed trees and shrubs, and (6) wind-dispersed trees and shrubs. Data were analysed by variation partitioning based on redundancy analysis (RDA) with principal coordinates of neighbour matrices (PCNM). Eighteen PCNM eigenvectors (expressing spatial effects) and mean annual temperature, July–January temperature difference and annual precipitation sum (expressing environmental effects) were used as explanatory variables.
Results Pure effects of climate on species composition, indicating niche-based processes, were not significant for any group or subgroup of the studied organisms. In contrast, pure effects of space, indicating dispersal limitation, were significant for all groups and subgroups except herbs. Surprisingly, the community of cyanobacteria/algae possessed much stronger spatial structure independent of climate than communities of larger organisms, although cyanobacteria/algae had the lowest beta diversity among the studied cities.
Main conclusions We hypothesize that the community of subaerial cyanobacteria/algae is structured by natural processes which involve dispersal limitation, whereas communities of urban plants and snails are influenced by human-assisted dispersal of their propagules between cities, which results in weaker dispersal limitation. Our study indicates that dispersal vectors can be more important for community structure than size of organisms or of their propagules.