Mild impacts do not subsidize aquatic insect communities in an Atlantic Forest stream.

Odums perturbation theory hypothesizes that toxic pollutants cause damage to ecosystems early in the course of contamination. In contrast, organic pollutants enrich the ecosystem until it exceeds their carrying capacity, an effect known as the subsidy-stress gradient. Understanding this dynamic can improve the efficiency of river restoration programs and bring significant benefits to society by providing ecosystem services that were lost. However, the initial effects of the most common human-induced disturbances in Atlantic Forest streams are not well known, indicating the necessity to evaluate the subsidy-stress gradient in these vulnerable and diverse ecosystems. AimWe evaluated the composition and abundance of the community of aquatic insects from leaf litter of headwater streams in three conditions: a fully forested area (reference stream), a low-intensity urban settlement (urban stream), and a region with small farms dedicated to the cultivation of fruits and vegetables (agricultural stream). MethodsWe used alpha and beta diversity metrics and a specific biotic index to test the subsidy-stress gradient prediction. ResultsThe agricultural stream showed the most degraded ecological condition. The urban stream and the reference stream showed similarity in alpha diversity metrics. According to the biotic index, the streams showed a gradient of environmental quality, with the reference stream showing the best quality and the agricultural stream the worst quality. ConclusionsThe agricultural stream showed a decrease in the environmental quality consistent with the effect predicted by the subsidy-stress gradient due to toxic pollutants contribution. However, the low-intensity enrichment of organic matter from the urban settlement causes a disorder in the ecosystem that reduces its environmental quality, contrary to the predicted by the subsidy-stress gradient.