Effects of loss of lateral hydrological connectivity on fish functional diversity

Abstract

Loss of lateral hydrological connectivity (LHC) is a major cause of biodiversity decline in river floodplains, yet little is known about its effects on aquatic functional diversity in these ecosystems. We quantified functional alpha and beta diversity of fish assemblages in Yangtze River floodplain lakes and explored their responses to loss of LHC with generalized linear mixed models. Functional richness was much lower in lakes that were not connected to the Yangtze River (i.e., disconnected lakes), where functional evenness and divergence were higher. LHC was the most important factor shaping fish diversity patterns in this region. Predicted reductions in functional richness and taxonomic richness due to LHC loss were higher for functional richness (0.47-0.82) than taxonomic richness (0.32) for all species assemblages except nonmigratory species. The distribution of functional strategies of migratory and nonmigratory fishes was highly uneven throughout the floodplain. Taxonomic beta diversity was much higher than functional beta diversity. The former was due mainly to spatial turnover (73.6-83.8%), which suggested that dissimilarity of diversity among fish assemblages was largely induced by species replacement. The latter was induced by the nestedness-resultant component of overall beta diversity (70.7-86.0%), which indicated a high degree of function loss without replacement. Both taxonomic and functional beta diversity were higher in disconnected lakes, where they were significantly correlated with fishing activity and water quality, than in river-connected lakes. We showed for the first time the effects of loss of LHC on fish functional diversity in large river floodplains. We found a serious decline of fish functional richness in the Yangtze floodplain, and functional diversity remained highly vulnerable to loss of LHC even though this is a species-rich ecosystem.