Biotic homogenization goes beyond the increase in taxonomic similarity among communities. It also involves the loss of biological differences in any organizational level (e.g., populations or communities) in terms of functional, taxonomic or genetic features. There are many ways to measure biotic homogenization, and the results depend on temporal and spatial scales, the biological group, and the richness of the communities. In freshwater ecosystems, the main investigated causes of biotic homogenization correspond to the introduction of non‐native species, damming, and changes in land use. However, other natural and anthropogenic causes also increase similarity among aquatic biota, such as climatic change, changes in productivity, and flood and drought events. The consequences of biotic homogenization in freshwater ecosystems are less explored than its causes, despite its severe implications, such as lesser resistant/resilient communities, loss of ecosystem functions, and higher vulnerability to diseases. Finally, biotic homogenization is a complex process that requires attention in conservation strategies, especially because forecasts suggest that freshwater biotas will continue to become more homogeneous in the future.

2020. Community size can affect the signals of ecological drift and niche selection on biodiversity. Ecology 101(6):

Aim: Biological diversity typically varies between climatically different regions, and regions closer to the equator often support higher numbers of taxa than those closer to the poles. However, these trends have been assessed for a few organism groups, and the existing studies have rarely been based on extensive identical surveys in different climatic regions. Location:We conducted standardized surveys of wadeable streams in a boreal (western Finland) and a subtropical (south-eastern Brazil) region, sampling insects identically from 100 streams in each region and measuring the same environmental variables in both regions.Taxon: Aquatic insects. Methods:Comparisons were made at the scales of local stream sites, drainage basins and entire regions. We standardized the spatial extent of the study areas by resampling regional richness based on subsets of sites with similar extents. We examined differences in genus richness and assemblage abundance patterns between the regions using graphical and statistical modelling approaches. Results:We found that while genus accumulation and rank-abundance curves were relatively similar at the regional scale between Finland and Brazil, regional genus richness was higher in the latter but regional abundance much higher in the former region. These regional patterns for richness and abundance were reproduced by basin and local genus richness that were higher in Brazil than in Finland, and assemblage abundance that was much higher in Finland than in Brazil. The magnitude of the difference in genus richness between Brazil and Finland tended to increase from local through basin to regional scales. Main conclusions:Our findings suggest that factors related to evolutionary diversification might explain differences in genus richness between these two

Biological communities are composed of a few common and many rare species. An understanding of the mechanisms that govern the distribution of these species is fundamental to knowledge regarding community ecology. Our hypothesis is that chironomid larvae follow a nested distribution in relation to hydrological periods in Neotropical floodplain lakes, whereby the flood period composition is a subset of the drought periods with a predominance of common species. We collected samples from 18 lakes in 2011 in a flood month and three drought months. The community followed a nested distribution where the spatial factors were more important for rare and common species during the flood and for the common species during all months. Thus, with the increasing connectivity and similarity of environments during the flood, neutral processes, as the dispersal, would govern the community. Conversely, environmental factors were more important for rare species in the drought, which suggest that these species are more specialists, largely influenced by niche-related processes. Thus, our study emphasizes the complexity of biological communities specifically concerning how environmental, spatial, and temporal factors influence community dynamics among species groups.

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Community structure of many systems changes across space in many different ways (e.g., gradual, random or clumpiness). Accessing patterns of species spatial variation in ecosystems characterized by strong environmental gradients, such as estuaries, is essential to provide information on how species respond to them and for identification of potential underlying mechanisms. We investigated how environmental filters (i.e., strong environmental gradients that can include or exclude species in local communities), spatial predictors (i.e., geographical distance between communities) and temporal variations (e.g., different sampling periods) influence benthic macroinfaunal metacommunity structure along salinity gradients in tropical estuaries. We expected environmental filters to explain the highest proportion of total variation due to strong salinity and sediment gradients, and the main structure indicating species displaying individualistic response that yield a continuum of gradually changing composition (i.e., Gleasonian structure). First we identified benthic community structures in three estuaries at Todos os Santos Bay in Bahia, Brazil. Then we used variation partitioning to quantify the influences of environmental, spatial and temporal predictors on the structures identified. More frequently, the benthic metacommunity fitted a quasi-nested pattern with total variation explained by the shared influence of environmental and spatial predictors, probably because of ecological gradients (i.e., salinity decreases from sea to river). Estuarine benthic assemblages were quasi-nested likely for two reasons: first, nested subsets are common in communities subjected to disturbances such as one of our estuarine systems; second, because most of the estuarine species were of marine origin, and consequently sites closer to the sea would be richer while those more distant from the sea would be poorer subsets. Understanding how community structure of many systems changes across space and how mechanisms, driven mostly by dispersal and environmental filters, determine species distribution patterns in local communities is a central question in community ecology 1-3. Testing how community assembly mechanisms determine species distribution has also become important in metacommunity ecology, an offshoot of community ecology, which has emerged to describe processes occurring at local and regional scales 1,4. A metacommunity can be defined as a set of local communities potentially, but not necessarily, linked by the dispersal of multiple, likely interacting, species 5,6. Assessing processes that affect metacommunity composition particularly in ecosystems characterized by strong environmental gradients is important to provide useful information on species responses to environmental changes across ecological gradients. To understand patterns of spatial variation in species composition, two different and complementary metacommunity approaches have been proposed 7 : one focusing on patterns 7,8 and another focusing on mechanisms 1,9. The patt...

Previous studies have found mixed results regarding the relationship between beta diversity and latitude. In addition, by influencing local environmental heterogeneity, land use may modify spatial taxonomic and functional variability among communities causing biotic differentiation or homogenization. We tested 1) whether taxonomic and functional beta diversities among streams within watersheds differ between subtropical and boreal regions and 2) whether land use is related to taxonomic and functional beta diversities in both regions. We sampled aquatic insects in 100 subtropical (Brazil) and 100 boreal (Finland) streams across a wide gradient of land use, including agriculture and exotic planted, secondary, and native forests. We calculated beta diversity at the watershed scale (among 5 streams in each watershed). We found higher taxonomic beta diversity among subtropical than among boreal streams, whereas functional beta diversity was similar between the 2 regions. Total land use was positively correlated with taxonomic and functional beta diversity among subtropical streams, while local environmental heterogeneity was positively correlated with beta diversity among boreal streams. We suggest that different types and intensities of land use may increase among-stream heterogeneity, promoting distinct insect assemblage compositions among streams. Our findings also suggest that beta diversity patterns and their underlying determinants are highly context dependent.

While land use intensification is a major driver of biodiversity change in streams, the nature of such changes, and at which scales they occur, have not been synthesized. To synthesize how land use change has altered multiple components of stream biodiversity across scales, we compiled data from 37 studies where comparative data were available for species' total and relative abundances from multiple locations including reference (less impacted) streams to those surrounded by different land use types (urban, forestry, and agriculture). We found that each type of land use reduced multiple components of within-stream biodiversity across scales, but that urbanization consistently had the strongest effects. However, we found that β-diversity among streams in modified landscapes did not differ from β-diversity observed among reference streams, suggesting little evidence for biotic homogenization. Nevertheless, assemblage composition did experience considerable species turnover between reference and modified streams. Our results emphasize that to understand how anthropogenic factors such as land use alter biodiversity, multiple components of biodiversity within and among sites must be simultaneously considered at multiple scales.