Effects of anthropogenic disturbances on α and β diversity of fish assemblages and their longitudinal patterns in subtropical streams, China

Identifying the spatial patterns of alpha and beta diversity of biotics is an important yet little-understood area of basic and applied ecological research. Although the upstreamdownstream patterns of alpha diversity of stream fishes are numerously reported, beta diversity has received less attention. In this study, we surveyed fishes along the upstream- downstream gradients in three headwater streams of the Qingyi River, China, which were affected by different extents of human activities. We aimed to assess how anthropogenic disturbances affect alpha and beta diversity of stream fishes and their upstream-downstream patterns. We found that, compared with that in the Shuxi Stream disturbed less, endemic species decreased and cosmopolitan species increased in the Maxi and Puxi Stream disturbed heavily. The streams disturbed heavily showed lower alpha diversity and higher beta diversity than that in the stream disturbed less. This among-stream variations in fish diversity only occurred at the mid-downstream, not upstream segments. alpha diversity increased downstream and beta diversity decreased downstream from headwaters to mouth in the stream disturbed less, whereas this upstream-downstream pattern in fish diversity shifted in the streams disturbed heavily, in which both alpha and beta diversity showed the quadratic distributions with the hump-shape for alpha diversity and the U-shape for beta diversity respectively. Our results suggest that anthropogenic disturbances cause some cosmopolitan fishes replacing many endemic fishes in upland streams. This replacement processes result in alpha diversity decreasing and beta diversity increasing, and shift the spatial patterns in fish diversity along the upstreamdownstream gradient.