Environmental filtering and low trait redundancy characterise fish assemblages in Lake Mamo, Orinoco River floodplain, Venezuela

Abstract

The study of assembly rules of fish communities in Neotropical floodplain lakes represents a major interest in community ecology due to their high species diversity and environmental variation, both spatial and temporal. In this study, assembly rules of freshwater fishes in Lake Mamo were explored. The main goals of this research were to analyse the seasonal variations in taxonomic and functional diversities of fishes in sandbanks and patches of aquatic macrophytes, in order to identify whether the fish assemblages are organised according to environmental filtering or limiting similarity, and to explore the relationships between taxonomic and functional diversity to determine if fish assemblages are either functionally redundant or complementary. In both mesohabitats, fishes were collected using seine nets during four hydrological seasons corresponding to low waters, rising waters, high waters and falling waters. Taxonomic and functional structure of fish assemblages in each habitat and across hydrological seasons was examined through multidimensional scaling (MDS), PERMANOVAs and principal co-ordinate analyses. Seasonal variations in taxonomic diversity were determined through Fligner tests of Shannon diversity indices, and for functional diversity, of functional richness (FRich), functional dispersion (FDisp), functional evenness (FEve) and functional divergence (FDiv). Null model analyses were used to determine if the functional indices diverged from randomness across hydrological seasons. Finally, relationships between functional indices and Shannon diversity were explored using linear, power, asymptotic and logistic models. There were significant differences in taxonomic and functional composition between mesohabitats and more markedly across hydrological seasons. All fish assemblages were significantly underdispersed at all instances, while functional evenness was higher than expected in both mesohabitats during low waters, signaling both a strong environmental filtering and niche packing. There were significant linear associations between the Shannon diversity and functional dispersion and between the Shannon diversity and functional evenness, suggesting that functional redundancy in these assemblages is low. In Lake Mamo, fish assemblages are filtered first by seasonality and then by habitat type. As more species are added to the assemblages, niches are more packed in the functional space, but the number of traits increases at a constant rate. This means that if a lake becomes impaired as a consequence of potential oil spills, the loss of fish species could represent also the loss of traits and functions in the ecosystem.