Currently, the expansion of the agricultural frontier in the Amazon represents a profound change in the world’s vegetation cover. This expansion causes decrease of forest cover, affecting the environmental integrity of streams, as well as the richness of associated communities. We aimed to evaluate the effects of loss of forest cover on the physical integrity of streams, and on the taxonomic and functional diversity of fish assemblages in a Cerrado-Amazon transitional zone. We sampled nine streams in the Alto Xingu river basin. The streams possess microbasins covered by different percentages of transitional Cerrado-Amazon forest and croplands. We did not detect the effect of loss of forest cover on the physical integrity of streams, and neither on fish taxonomic diversity. However, we found a negative effect on the functional diversity. The physical integrity of streams was not associated with taxonomic and functional diversity of fish assemblages. The conversion of forest areas to croplands, on the riparian zone, affects negatively the functional richness of fish assemblages, acting as an environmental filter, leading to the disappearance of species that could have performed important ecosystem functions. Furthermore, the establishment of restoring and conservation strategies of areas affected by deforestation must be a priority on all streams drainage systems, especially on the riparian zone scale, to minimize impacts on species. To understand which factors can better structure fish assemblages in streams, it is necessary to do research, that can give basis to formulate strategies for the preservation of those organisms.
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