Citation: L opez-Delgado, E. O., K. O. Winemiller, and F. A. Villa-Navarro. 2020. Local environmental factors influence beta-diversity patterns of tropical fish assemblages more than spatial factors. Ecology 101(2):Abstract. A major goal in ecology is to understand mechanisms that influence patterns of biodiversity and community assembly at various spatial and temporal scales. Understanding how community composition is created and maintained also is critical for natural resource management and biological conservation. In this study, we investigated environmental and spatial factors influencing beta diversity of local fish assemblages along the longitudinal gradient of a nearly pristine Neotropical river in the Colombian Llanos. Standardized surveys were conducted during the low-water season at 34 sites within the Bita River Basin. Physical, chemical, and landscape parameters were recorded at each site, and asymmetric eigenvector maps were used as spatial variables. To examine the relative influence of dispersal and environmental variables on beta diversity and its components, distance-based redundancy analysis (db-RDA) and variation partitioning analysis were conducted. We proposed that spatial scale of analysis and position within the river network would constrain patterns of beta diversity in different ways. However, results indicated that in this system, high beta diversity was consistent among species assemblages no matter the scale of analysis or position within the river network. Species replacement (turnover) dominated beta diversity, an indication of the importance of species sorting. These findings suggested that conservation of fish diversity in tropical rivers requires maintenance of both habitat heterogeneity (spatial variation in habitat conditions) and connectivity at the scale of entire river basins.

The Amazon rainforest has experienced rapid land-use changes over the last few decades, including extensive deforestation that can affect riparian habitats and streams. The aim of this study was to assess responses of stream fish assemblages to deforestation and land cover change in the eastern Amazon. We expected that percentage of forest in the catchment is correlated with local habitat complexity, which in turn determines fish assemblage composition and structure. We sampled 71 streams in areas with different land uses and tested for relationships between stream fish assemblages and local habitat and landscape variables while controlling for the effect of inter site distance. Fish assemblage composition and structure were correlated with forest coverage, but local habitat variables explained more of the variation in both assemblage composition and structure than landscape variables.Inter site distance contributed to variance explained by local habitat and landscape variables, and the percentage of variance explained by the unique contribution of local habitat was approximately equivalent to the shared variance explained by all three factors in the model. In these streams of the eastern Amazon, fish assemblages were most strongly influenced by features of instream and riparian habitats, yet indirect effects of deforestation on fish assemblage composition and structure were observed even though intact riparian zones were present at most sites. Long-term monitoring of the hydrographic basin, instream habitat and aquatic fauna is needed to test for potential legacy effects and time lags, as well as assess species responses to continuing deforestation and land-use changes in the Amazon. Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for PROCAD/ CAPES funding (project no 88881.068425/2014-01), the graduate scholarship for TOB and senior internship scholarship for LFAM to conduct research at Texas A&M University (process 88881.119097/2016-1). The manuscript was improved during the review process by addressing the thoughtful comments from Philip Kaufmann and an anonymous reviewer.

Understanding processes driving patterns of species distribution and diversity is one of the main objectives of community ecology. There has been a growing recognition that local environmental conditions are not the only factor structuring ecological communities, and that large‐scale spatial variation and dispersal also have major influences. The aim of our study was to evaluate spatial variation in fish assemblage structure along the longitudinal fluvial gradient of the Bita River, a nearly pristine tributary of the Orinoco River in the Llanos region of Colombia. Standardised surveys conducted at 34 sites throughout the basin during the low water period in January and March 2016 yielded 25,928 fish specimens representing 201 species. Twenty‐seven environmental variables were recorded at each site, and asymmetric eigenvector maps were used to model spatial variables. To understand spatial variation in local fish assemblages and their relationships with the environmental and spatial variables, two approaches were used. First, we applied the elements of metacommunity structure framework, followed by a variation decomposition analysis that allowed the metacommunity to be classified according to six alternative patterns of species distribution and four alternative metacommunity paradigms. We hypothesised that at a basin scale a major fraction of variation in structure is explained by a pure environmental effect and metacommunity patterns should reveal a Clementsian distribution. At a more regional scale (localities within a river section), assemblages in upstream and downstream regions may reflect different metacommunity processes. Because headwater streams are isolated within the river network, they should receive fewer migrants and may have local assemblages strongly influenced by local environmental conditions and species sorting with one of three possible distributional patterns (Clementsian, Gleasonian, or evenly spaced) would be observed. Conversely, downstream sites closer to the river mouth should be influenced by high dispersal, resulting in a greater importance of spatial factors and the mass effect, with metacommunity patterns nested along the longitudinal gradient. Our results suggest that the fish metacommunity in the Bita River exhibits a Clementsian distribution, implying that species respond to the environmental and fluvial gradient as groups along the longitudinal gradient. These replacements were associated with environmental heterogeneity, especially regarding habitat features. Similarly, the variation partitioning analysis showed that the pure environmental component was higher than the pure spatial component, which is consistent with species sorting. In this paper, we demonstrated that variation partitioning and metacommunity structure analyses provided complementary findings to infer processes structuring fish assemblages in the Bita River. Both approaches identified species sorting as the principal structuring processes in this system. Therefore, strategies to preserve fish diversity...