Tell me where you flow and I will tell you who you are: basin context shapes the insect fauna of small tributaries

The traditional view of river basins as linear corridors has led to numerous studies examining the effects of stream size on aquatic communities. However, similar‐sized streams may harbor distinct faunal assemblages depending on their spatial context within the basin. Headwater tributaries (HTs) that flow into small streams, in the periphery of basins, are spatially isolated and influenced by species pools characteristic of small streams. In contrast, mainstem tributaries (MTs), situated in more central areas, flow into large streams and are expected to support species typical of both small streams and adjacent main channels. We tested whether HTs exhibit species‐poorer communities of aquatic insects that are nested subsets of those in MTs. We also investigated whether beta diversity among HTs is higher than among MTs. We sampled aquatic insects in paired tributaries across nine small basins. The tributaries were first and second order streams that flowed either into small (first or second order) or large streams (third to fifth order). Taxa composition differed between the two stream types, with the fauna of HTs consisting of species‐poorer nested subsets of that present in their paired MTs. Beta diversity among HTs was higher than that among MTs. The low taxa richness observed in HTs is consistent with theoretical and empirical studies suggesting higher spatial isolation compared to MTs. Additionally, the richer and mixed faunas in MTs suggest that taxa typically found in main channels may also disperse upstream into adjacent small tributaries and homogenize their faunas. This finding is consistent with observations of upstream movements of adult insects, particularly gravid females. While this downstream‐to‐upstream influence has been documented for fishes, our study represents, to our knowledge, the first to report this effect for aquatic insects.