Land use change is one of the main drivers of freshwater ecosystems degradation worldwide. However, nowhere else are its consequences more far-reaching than in the Amazonian Basin, which harbors the greatest diversity of freshwater fish on the planet. We investigated the effects of deforestation, impoundments, and warming on stream fishes in this Amazonian “Arc of Deforestation” in the headwaters of the Xingu River. We conducted abiotic and fish assemblage surveys in lotic and impounded reaches in streams draining forested and converted watersheds. We also performed an experiment to test the hypothesis that high temperatures similar to those occurring in deforested streams could negatively affect body size in the most common fish species in the region. Declining body size has been suggested to be a universal response of organisms to rising temperatures, but no study to date had evaluated whether land use-driven warming could trigger a similar response.
Results/Conclusions
Deforestation and impoundments altered stream habitats and fish assemblages, and increasing water temperatures was an important factor influencing body size at the assemblage, population, and individual levels. Canopy cover was the best predictor of habitat conditions and fish assemblage structure, probably due to its influence on temperature, light, and inputs of allochthonous organic matter. Surprisingly, fish assemblages in deforested streams were as diverse and more abundant than in forested streams. However, they were also more homogeneous due to increases in abundance of opportunistic species in widened floodplains and impounded reaches. Impoundments favored lentic species adapted to slow velocities and locally eliminated species that were common in lotic reaches. Streams in deforested watersheds were up to 6 °C warmer than forested streams. Our laboratory experiment demonstrated that increased temperatures caused significant body size reductions in the most abundant fish species at our study site. Therefore, stream warming is a plausible causative factor for the 36% decline in average fish body size that we observed in deforested streams. A broad scale change in stream fish assemblages and body size may be occurring throughout the Amazonian Arc of Deforestation, with implications for biodiversity conservation and food supply for people around the Basin.