Flow intermittency decreases nestedness and specialisation of diatom communities in Mediterranean rivers

Due to their ubiquity and highly efficient passive dispersal, benthic diatoms are an ideal group in which to explore the drivers of community spatial structure and the ultimate links between such patterns and species autoecology. However, studies addressing these issues have largely overlooked unicellular taxa to date. We examined the spatial order (i.e. nestedness) of a diatom meta-community of 122 lotic sites in the north-east Iberian Peninsula and the relationship between the contributions of individual species to nestedness and their niche breadths [measured using outlying mean index (OMI) analyses]. We observed a significantly nested pattern in the meta-community (T=14.05, NODF=22.57), although idiosyncratic species accounted for a substantial fraction (34%) of the global species pool. Hydrological stability was identified as the main driver of nestedness. Highly stable, moderately stable and intermittent sites differed in nestedness and in alpha diversity, with intermittent sites being the most species poor and the least nested. We also observed a significant association between nestedness and niche attributes, with idiosyncratic species exhibiting wider distributions and larger niche breadths compared with nested species. Overall, our results indicate that diatom communities inhabiting hydrologically stable rivers present a higher level of order in community spatial pattern and a higher proportion of specialist taxa than communities in intermittent streams. Because recent hydroclimatic models predict an increase in stream intermittency in the Mediterranean region, our findings may help anticipate future diatom biodiversity patterns resulting from global change. We suggest that the current need to protect a diverse network of sites (as opposed to a few species-rich sites) will be even greater in the near future because local communities are not only probably to become taxonomically and functionally poorer but also less predictable, making it more difficult to implement efficient programmes for biodiversity conservation at the regional scale.