Metacommunities from bacteria to birds: stronger environmental effects in Mediterranean than in tropical ponds

The metacommunity concept provides a theoretical framework that aims at explaining organism distributions by a combination of environmental filtering, dispersal and drift. With the development of statistical tools to quantify and partially isolate the role of each of these processes, empirical metacommunity studies have multiplied worldwide. However, few works attempt a multi-taxon approach and even fewer compare two distant biogeographical regions using the same methodology. Under this framework, we tested the expectation that temperate (Mediterranean) pond metacommunities would be more influenced by environmental and spatial processes than tropical ones, because of stronger environmental gradients and higher isolation of waterbodies. We surveyed 30 tropical and 32 Mediterranean temporary ponds and obtained data on 49 environmental variables (including limnological, hydrogeomorphological, biotic, climatic, and landscape variables). We characterized the biological communities of Bacteria and Archaea (from the water column and the sediment), phytoplankton, zooplankton, benthic invertebrates, amphibians and birds, and estimated the relative role of space and environment on metacommunity organization for each group and region, by means of variation partitioning using Generalized Additive Models (GAMs). The residual (unexplained) variation was larger in tropical pond metacommunities, suggesting a higher role for stochastic processes and/or effects of unmeasured processes such as biotic interactions in the tropics. Environmental filtering was important in both tropical and Mediterranean ponds, but markedly stronger in the latter, probably related to higher environmental heterogeneity. The variability between taxonomic groups in spatial and environmental contributions was very wide, and only some similarities between geographic settings were observed for the environmental effects of passively dispersing organisms, with rotifers and diatoms being among the most and least affected by the abiotic environment, respectively, in both areas. Overall, these results provide support, in a wide variety of aquatic organisms, for the classical view of stronger abiotic niche constraints in temperate areas compared to the tropics.