Interplay between productivity and regional species pool determines community assembly in aquatic microcosms

The relative importance of deterministic and neutral processes in shaping assembly of communities remains controversial, partly due to inconsistencies between theoretical, empirical, and experimental studies. We investigate the interplay between local (productivity) and regional (size of species pool) assembly mechanisms in communities of phytoplankton and zooplankton in 72 experimental microcosms. Local environmental conditions were manipulated by varying the level of nutrients in the water (ambient, low, high). The size of regional species pool colonizing each microcosm was manipulated by mixing phytoplankton and zooplankton species from different numbers of source ponds (n=2, 4, 8 and 16). Our results show that local communities assembled differently depending on the numbers of sources available for colonization. Microcosms with larger species pools supported greater numbers of species. In contrast, the effects of productivity led to different results across trophic groups. Phytoplankton communities were, on average, more diverse on more productive treatments, while zooplankton communities were more diverse under less productive treatments. Phytoplankton and zooplankton communities responded to both sources of variation, although the size of species pool was a better predictor of communities' composition than the local effects of productivity. These results reinforce the view that community assembly is influenced by the interplay of both local and regional drivers but that the relative importance of these factors varies with trophic groups.