Evidence of stochasticity driving anuran metacommunity structure in the Pantanal wetlands

1. The association of species distribution and abundance patterns with environmental and spatial variables is often used to identify signatures of niche-based and spatial processes in metacommunity structures. If there is no association, however, then stochastic processes may be interpreted as a possible factor driving community assembly. 2. We sampled 20 ponds and more than 5800 adult individual anurans in one of the world's largest Neotropical wetland systems, where an annual and unimodal flood pulse is the most important force operating in the landscape. We aimed to determine the relative importance of environmental abiotic factors (pond physical, chemical and morphometric variables) and geographic distance in structuring the metacommunity of aquatic-breeding anurans during the end of the dry season. 3. We predicted that species composition would be strongly associated with environmental variables (niche-based processes) and species richness would be positively related to habitat heterogeneity. Moreover, we expected that massive dispersal, promoted by the preceding flood season, would result in low evidence of spatial processes in the metacommunity structure of anurans. 4. Contrary to our prediction, we found minimal association between anuran composition and environmental gradients. Nonetheless, as expected, we found a positive relationship between species richness and environmental heterogeneity, and no spatial signal. 5. Our results suggest that the annual floods increase the probability that species disperse massively and colonise ponds in an unpredictable way. This may be interpreted as a case where stochastic processes are probably the main force structuring the anuran metacommunity in seasonally flooded systems.