Balancing environmental variability, habitat condition, and human pressures in shaping fish communities of Atlantic Forest streams

Understanding how environmental gradients and human pressures shape fish assemblages is crucial for conserving tropical streams. We examined how water physical and chemical variables, substrate characteristics, and local disturbances influence fish communities in three tributaries of the Guandu River (Atlantic Forest biome, southeastern Brazil). D'Ouro Stream showed stronger anthropogenic influences, whereas Santana and São Pedro showed greater habitat heterogeneity. Differences also occurred between upper and lower reaches, with higher disturbance levels downstream. Fish richness was highest in Santana and lowest in D'Ouro, varying among streams and reaches but consistent across seasons. The most impacted stretch (Lower D'Ouro) was dominated by tolerant taxa ( Poecilia vivipara, Oreochromis niloticus ), suggesting biotic homogenization driven by habitat degradation. In contrast, upper reaches with higher dissolved oxygen, rocky substrates, and greater heterogeneity supported higher diversity and specialist species ( Bryconamericus ornaticeps, Trichomycterus zonatus, Schizolecis guntheri ). Within-stream patterns suggested two disturbance regimes: a downstream gradient of increasing disturbance and tolerant taxa in Santana and São Pedro, and diffuse anthropogonic pressure in D'Ouro, associated with tolerant taxa across sampled reaches. Water physical and chemical variables accounted for the largest unique fraction of explained assemblage variation (26%), with an additional 19% shared with substrate and disturbance variables. These findings indicate that comparisons among streams alone may overlook important within-stream gradients and contrasting disturbance regimes. Conservation and restoration strategies should therefore prioritize maintaining water quality and habitat heterogeneity, protecting riparian corridors and upstream source areas, and tailoring interventions to reach-specific environmental conditions and disturbance regimes.