Freshwater ecology templates were developed in temperate streams, but whether they also apply to tropical streams that harbour a higher biological diversity than their temperate counterparts remains uncertain. This is particularly true for tropical fish assemblages inhabiting small streams that have been less studied than larger, higher-order lowland streams. Here, we disentangled the strength of spatial (longitudinal and environmental) drivers, and scale-specific (drainage basin, reach and local scale) determinants of species richness and composition of freshwater fish assemblages inhabiting small streams in French Guiana. We found that species richness increased from upstream to downstream but also with increasing local habitat structural diversity independently of stream position in the upstream–downstream gradient. This pattern was shared by the two most speciose fish orders (Characiformes and Siluriformes), demonstrating that species addition rather than species replacement shaped species richness in these assemblages. Species composition of fish assemblages was determined equally by their spatial structure within drainage and by the environment, and assemblages differed both with distance and along an upstream–downstream gradient. The environmental effect on species assemblages indicated by the fact that almost all environmental descriptors had slight but nonetheless significant effects on assemblage composition, probably reflecting species-specific responses to the local environment. In contrast, despite a strong micro-endemism between drainages for some taxa, assemblages were only slightly affected by river drainage identity, since widespread species were a common constituent of assemblages in all rivers. We identified five species assemblages characterising different local habitat features from torrential areas to lowland muddy areas. We also distinguished fish assemblages from confluence areas with larger rivers, which differed from the other five assemblages. The fish zonation patterns we report can constitute a benchmark for future studies measuring the impact of anthropogenic disturbances on Neotropical forest streams.

MITARAKA