Variation in processes structuring fish assemblages as inferred from metacommunity analyses: Differences between headwater and adventitious streams within a river network

Abstract

Community assembly is generally affected by the spatial position of local patches. Examining metacommunity processes and structures in small streams with different river network positions (e.g., headwater streams in the periphery of river networks vs adventitious streams in relatively central positions) may help us better understand how spatial position influences metacommunity structuring. In this study, based on data collected from sampling 24 headwater and 15 adventitious stream sites in the Xin’an Basin, China, we examined whether and how fish metacommunity processes and structures differed between the 2 stream types. We used distanced-based redundancy analysis coupled with forward selection to assess the relative roles of environmental and spatial factors in metacommunity processes, and we applied the elements of metacommunity structure analysis, extended by using different ordinations, to identify metacommunity structures. We found that the 2 types of streams were similar in environmental heterogeneity, but fish assemblages in adventitious streams were more dissimilar to one another than were assemblages in headwater streams and had more riverine immigrants. The amount of among-stream variation in species composition was low in both stream types. In headwater streams, assemblage–environment relationships were stronger than assemblage–space relationships, but we found the opposite pattern in adventitious streams. Species turnover was strong in both headwater and adventitious streams. However, assemblages in headwater streams showed both nested and quasi-nested structures, whereas the assemblages in adventitious streams were more consistent with Clementsian structure. Our results suggest that different metacommunity processes may occur in small streams with distinct river network positions and that extended elements of metacommunity structure analysis can help identify the underlying mechanisms that produce metacommunity structure. The effect of spatial position within river networks on stream fish metacommunity structuring appears to be weak, but our analyses do imply that river immigrants can influence species composition in adventitious streams to some extent.