LAUSR

This study investigated the effects of environmental variation on fish diversity patterns in two coastal streams in northwestern Ecuador. Specifically, we examined the role of topography, instream conditions (pH, conductivity), and human influence (land-use modifications) on fish species richness, diversity and evenness as well as body size diversity and size evenness. Study region comprised 19 sites sampled bimonthly during the dry and wet seasons in 2016–2017. The results showed that the contribution of each measure of fish diversity responded differently to the local drivers. For size-based metrics, fish assemblages displayed greater size diversity (more diversity of body sizes) at lower pH and conductivity levels. Taxonomic metrics showed a negative relationship between the number of species and the downstream–upstream gradient. In both measures of diversity, human influence negatively affected size evenness and Shannon diversity, with less equitable abundance distribution and fewer species in more human-altered locations. These results improve our understanding of how assembly processes operate in shaping local fish assemblages in Neotropical coastal streams.