The distance to large rivers affects fish diversity patterns in highly dynamic streams of Central Amazonia.

Diversity patterns are driven by different combinations of local effects, dispersal limitation, and historical factors. In tropical systems, the occurrence of flood pulses of large rivers generates marked changes in landscape connectivity by creating temporary corridors between habitat patches, which amplifies the role of regional factors in determining the local assemblages of species. In this study, we investigated the effects of the stream position in the fluvial continuum and its proximity to large rivers on the taxonomic and functional patterns of fish assemblages along an extensive region of the Central Amazonia. We sampled 31 headwater stream stretches in nine different catchments along the interfluvial area of the Purus and Madeira rivers, between December 2014 and March 2015.

A total of 97 fish species from seven orders and 19 families were captured.

The results show that, at large spatial scales, hydrological distance to large rivers is more important than geographic distance in determining diversity patterns of stream-fish assemblages, due to a decrease in taxonomic and functional richness towards headwaters. Proximity to floodplains seems to be one of the main factors favoring the constitution of assemblages with higher species richness, since flood pulses allow the seasonal opening of dispersal corridors
and increase the heterogeneity of resources and habitats.

We also found high values of species turnover not associated to any predictor variables suggesting the effect of stochastic mechanisms. The lack of a single pattern indicates that several processes may act concomitantly to determine the structuring of the stream fish assemblages at broad scales, and being influenced by the degree of drainage isolation, local environmental conditions and by the adaptive response of species.