Ecological uniqueness of fish communities from streams in modified landscapes of Eastern Amazonia

Ecological uniqueness is one aspect of beta (β) diversity that shows the relative contribution of sites (local contribution to beta diversity – LCBD) and taxa (species contribution to beta diversity – SCBD) in the formation of a unique environment in terms of species composition, and may be directly related to habitat quality. Our objective was to evaluate the uniqueness of fish communities from streams in modified areas of the Amazon, and to investigate the main environmental predictors at local and landscape scale. We sampled 58 streams in the Capim river basin (Pará, Brazil) located within areas of preserved forest, reduced-impact logging, conventional logging and pasture. We found greater β diversity and higher LCBD in the pasture areas, making this land use the largest contributor to β diversity in the study area. At local scale, this high contribution was primarily influenced by environmental heterogeneity, thalweg depth, percentage of land use and cover (all positively), and volume of large woody debris in the riverbed (negatively). This indicates that the β diversity of fish is highly affected by streams with greater thalweg depth, possibly due to the reduced amount of large woody structures entering streams within pasture areas. These streams also showed greater environmental heterogeneity due to the large variation in disturbance levels of this area, which renders their sites suitable for potential occupation by different species, making them high contributors (high LCBD) and also leading them to present high β diversity. On the other hand, areas with a higher percentage of forest at landscape scale (including preserved forest, reduced-impact logging and conventional logging) were the main contributors, while pasture areas had a higher percentage of exposed soil. We did not find any association between SCBD values and the habit of the species, as the taxa that contributed most to β diversity can be classified as having reduced niches (specialists) as well as broader niches (generalists). We conclude that modified areas may contribute substantially to β diversity because they have a distinct species combination, however different patterns can be observed at local and landscape scales.