Aims: The distribution and abundance of stream fishes are influenced by historical events as well as abiotic and biotic factors. Due to the high spatial heterogeneity and temporal differentiation of stream environmental characteristics, the composition and abundance of stream fish communities vary on spatial and temporal scales. On this basis, longitudinal patterns in the diversity and distribution of stream fish along upstream-downstream gradients have garnered the widespread attention of ecologists. Previous studies relevant to the longitudinal patterns in stream fish diversity along the upstream-downstream gradient mostly focused on taxonomic α diversity, while taxonomic β diversity and functional diversity have been given less attention. Therefore, this study aimed to (1) determine the effects of anthropogenic disturbances on species composition of fishes by comparing diversity across three streams, and determine the key species contributing to the between-stream variation in fish assemblages; (2) examine the influences of anthropogenic disturbances on functional diversity, and examine if differences in functional diversity relate to α and β diversity; (3) to assess whether the between-stream variations in functional diversity show dependence on the spatial positions along the river continuum or not, and further assess the effects of anthropogenic disturbances on the longitudinal patterns in the functional α and β diversities of stream fishes.
Methods: In this study, we focused on the functional diversity of fishes, which is an effective measure for assessing the response of communities to environmental changes and predicting changes in ecosystem functioning. Using data collected in three headwater streams of the Qingyi River, China, that experience different extents of anthropogenic disturbances, we examined the effects of anthropogenic disturbances on the longitudinal patterns in the functional α and β diversities of streams fishes. We considered multiple metrics of the α diversity, including the functional richness, evenness and divergence, which measure different aspects of functional space and abundance distribution. This work also analyzed β diversity, including its turnover and nestedness components, which can provide additional insights into the processes of community changing.
Results: We found that in the less disturbed stream, functional richness increased but divergence decreased downstream, and functional β diversity decreased downstream. On the other hand, in heavily disturbed streams functional richness was higher in the upstream-midstream but lower in the downstream, and functional divergence decreased in the upstream-midstream and β diversity increased in the downstream. Anthropogenic disturbances significantly altered species compositions and functional diversity of fishes in the heavily disturbed streams, and the mode and degree to which functional diversity varied related to both the metrics of functional diversity and the spatial position along river continuum. As a result, anthropogenic disturbances caused a shift in the longitudinal patterns of functional diversity, from linear variation in less disturbed streams to unimodal distribution in heavily disturbed streams. Similarly, variations in the functional β diversity of fishes mainly occurred in the lower segments, but not in the upper segments, and were derived by the turnover component. We found anthropogenic disturbances caused the endemic species to be replaced by the native-invasive species. Heavy land use and polluting emission could increase the environmental discontinuity, leading to community turnover and nestedness changing. Although functional β diversity was dominated by the nestedness component, the proportion of the turnover component increased significantly compared with less disturbed streams.
Conclusion: Our results highlight the need for a multifaceted approach, including spatial scale and diversity metrics, in assessing the diversity changes of stream fishes under anthropogenic pressure.