Taxonomic richness of fish, arthropod, and macrophyte assemblages
We sampled fish, arthropod and macrophyte assemblages at each stream site. Those three assemblages were selected because they represent diverse biotic components of stream communities, and are associated with key ecosystem processes, such as standing biomass (Moi et al., 2021). All fish were identified to species; arthropods and macrophytes were identified to species or genus. We recorded 141 fish species, 321 arthropod taxa, and 43 macrophyte taxa across the two study areas. Importantly, taxonomic richness of the assemblages did not differ markedly between Amazonia and Uruguay stream sites (see, Figure 1d). However, to account for possible differences in population densities and sampling effort among sites, we estimated taxonomic richness as the Chao index with abundance-based data using the R package iNEXT (Hsieh et al., 2016). The Chao index is based on rarefaction and extrapolation of Hill numbers. It provides an unbiased estimate of asymptotic taxonomic richness, thus enabling comparisons among areas with different potential levels of assemblage richness.