Molluscan metacommunity dynamics in the Colorado River estuary, Mexico before upstream water diversion

The magnitude and effect of rapid environmental change during the “Anthropocene” is often underestimated due to a paucity of pre-impact data with which to contextualize the change, potentially compromising the efficacy of conservation and restoration efforts. Geohistorical records can fill this gap. This study utilizes accumulations of molluscan shells from the past 100–300 years at three sites along a past salinity gradient in the Colorado River estuary to determine which of four major paradigms of metacommunity theory (environmental filtering, mass effects, patch dynamics, and neutrality) best explained community assembly. The effectiveness of ongoing conservation efforts in this anthropogenically altered estuary may depend on the dominant paradigm controlling community assembly. Differentiating between paradigms using model fits of species abundance distributions and their subsequent deconstruction by commonness and environmental preference showed that community assembly in the past molluscan community was controlled by a combination of environmental filtering and mass effects. It was highly probable (p>0.999) to find a species with estuarine preference in the common component of the low-salinity northern site as compared with the two more southern marine sites, suggesting an important role for environmental filtering. At each site, species with mismatched environmental preferences and strong dispersal capacities were present in the common component, providing evidence to support mass effects. Given the importance of species’ environmental preferences in environmental filtering and that of dispersal capacity under mass effects, these results suggest that the most effective conservation strategies for the molluscan metacommunity in the Colorado River estuary would be to restore the brackish conditions preferred by estuarine species and maintain populations of those species so as to ensure their capacity to disperse.