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Ecological drift can override the effects of deterministic niche selection on small populations and drive the assembly of some ecological communities. We tested this hypothesis with a unique data set sampled identically in 200 streams in two regions (tropical Brazil and boreal Finland) that differ in macroinvertebrate community size by fivefold. Null models allowed us to estimate the magnitude to which β-diversity deviates from the expectation under a random assembly process while taking differences in richness and relative abundance into account, i.e., β-deviation. We found that both abundance- and incidence-based β-diversity was negatively related to community size only in Brazil. Also, β-diversity of small tropical communities was closer to stochastic expectations compared with b-diversity of large communities. We suggest that ecological drift may drive variation in some small communities by changing the expected outcome of niche selection, increasing the chances of species with low abundance and narrow distribution to occur in some communities. Habitat destruction, overexploitation, pollution, and reductions in connectivity have been reducing the size of biological communities. These environmental pressures might make smaller communities more vulnerable to novel conditions and render community dynamics more unpredictable. Incorporation of community size into ecological models should provide conceptual and applied insights into a better understanding of the processes driving biodiversity.

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.

Local communities and individual species jointly contribute to the overall beta diversity in metacommunities. However, it is mostly unknown whether the local contribution (LCBD) and the species contribution (SCBD) to beta diversity can be predicted by local and regional environmental characteristics and by species traits and taxonomic relatedness, respectively. We investigated the LCBD and SCBD of stream benthic diatoms and insects along a gradient of land use intensification, ranging from streams in pristine forests to agricultural catchments in southeast subtropical Brazil. We expected that the LCBD would be negatively related to forest cover and positively related to the most unique streams in terms of environmental characteristics and land use (hereafter environmental and land use uniqueness, respectively). We also expected that species with a high SCBD would occur at sites with reduced forest cover. We found that the LCBD of diatoms and insects was negatively related to forest cover. The LCBD of insects was also positively related to environmental and land use uniqueness. As forest cover was negatively related to uniqueness in land use, biologically unique streams were those that deviated from the typical regional land cover. We also found that diatom traits, insect traits, and taxonomic relatedness partly explained SCBD. Furthermore, the SCBD of diatoms was positively correlated with forest cover, but the inverse was found for insects. We showed that deforestation creates novel and unique communities in subtropical streams and that species that contribute the most to beta diversity can occur at opposite ends of a land use gradient.

Human activities have resulted in higher levels of turbidity in aquatic systems due to increased sediment runoff from agriculture, urbanization, and mining activities into nearby water bodies. Previous research has suggested that as turbidity increases, fish may increase their mobility to counteract the reduction in low visual acuity and maintain prey consumption similar to those in clear water. However, other studies have reported negative and non-significant relationships between turbidity and fish mobility. Here, we conducted a systematic review and meta-analysis to investigate the effect of turbidity on fish mobility. We analysed potential sources of variation in the turbidity–fish mobility relationships (moderators), such as body size, eye size and the trophic position of fish, as well as turbidity type, range and ecosystem types, while also accounting for phylogenetic relationships among fish species. Our results revealed that the overall effect size of turbidity on fish mobility was not significantly different from zero, indicating that turbidity does not consistently affect fish mobility. Instead, some fish species may increase their mobility, while others decrease or remain unchanged. Furthermore, the effects of turbidity on fish mobility varied greatly among studies. However, none of the moderators tested explained such variability in the turbidity–fish mobility relationships. These findings demonstrate that the effects of turbidity on fish movements are complex and that some fish species and populations may be particularly vulnerable to increasing turbidity levels in water bodies worldwide caused by human activities.

The effects of introduced species on native species have been widely studied, however, invader-invader interactions are still poorly explored. Two non-native invasive species, the bivalve Limnoperna fortunei and the submersed macrophyte Hydrilla verticillata, have infested many aquatic ecosystems worldwide, and they are rapidly becoming nuisances in several water bodies in Brazil. We tested the hypothesis that H. verticillata facilitates the establishment of L. fortunei more than do native macrophytes. We surveyed mussels attached to two native macrophytes and to this invasive macrophyte in three Brazilian reservoirs and compared the length, density and biomass of the attached mussels relative to the host macrophyte species. All of the values of these attributes in L. fortunei were higher in the mussels attached to H. verticillata than in those attached to the native macrophytes. These results supported our initial hypothesis because the settlement of L. fortunei appeared to be facilitated by the invasive macrophyte, allowing higher abundances of the mussels as well as the more effective establishment of the mussel population. Various mechanisms could interact to facilitate the superior performance of L. fortunei in the invasive plant. H. verticillata is more flexible and withstands more mussels without breaking. Furthermore, this plant has different attached algae, which may be more beneficial for mussels. Thus, as a response to the facilitation suggested by our work, the mussel has greater opportunities to succeed in habitats colonised by the non-native H. verticillata. Although we have not evaluated the effects of this facilitation on aquatic communities or the ecosystem, our results could represent the first step of an invasional meltdown.

Deforestation is a major driver of biodiversity loss in the Tropical region, but the role of upstream refugia and dispersal ability on the community response to this disturbance is unknown. We assessed the relevance of undisturbed upstream patches (“refugia”) on the responses of benthic communities to forest cover loss. We selected four Andean rivers with a well-protected forest in their upstream section and different degree of forest cover loss downstream and evaluated the dissimilarity patterns of three benthic communities (diatoms, Chironomidae, and the assemblage of Ephemeroptera, Plecoptera and Trichoptera—‘EPT’) along their water courses. We evaluated the dissimilarity by using Euclidean (environment), Sørensen (incidence data) and Morisita–Horn (abundance data) pairwise distances. We found that diatom beta-diversity, as organisms with passive but higher dispersal ability, significantly tracked the environmental changes caused by forest loss. However, insect communities, whose a priori are active dispersers and can track for suitable conditions, were weakly affected by deforestation. These results provide evidences that the existence of well-preserved upstream reaches along patched corridors may allow non-tolerant species to remain extant throughout dispersal-driven feedbacks. This being the case in the Andean streams, effects of deforestation on benthic communities were small, and not uniform. Our results reinforce the strategy of preserving upstream sections in order to achieve successful restoration or rehabilitation goals.

Exotic species have invaded freshwater ecosystems, causing biodiversity loss of associated communities. We investigated the influence of the invasive macrophyte Hydrilla verticillata on taxonomic and functional richness, and on taxonomic and functional beta diversity of associated Chironomidae community, comparing this macrophyte with the structurally similar native macrophyte, Egeria najas. We conducted a field experiment where the native and invasive plants were kept side-by-side and colonized by invertebrates. We used accumulation curves to test if richness differed between macrophytes. We used multivariate dispersion to investigate whether taxonomic and functional composition differed between both macrophytes and to test if beta diversity was higher among individuals of the native than among individuals of the invasive macrophytes. We measured beta diversity values as an average distance of the compositions to the centroid in a multidimensional space (taxonomic and functional) given by a Principal Coordinates Analysis. We found similar taxonomic richness and similar taxonomic and functional Chironomidae composition between macrophytes. However, functional richness as well as taxonomic and functional beta diversity of Chironomidae were higher among the native than among the invasive macrophytes. Thus, although H. verticillata did not cause changes neither in Chironomidae taxonomic richness nor on taxonomic and functional composition, its spread may drive the simplification of species and functional traits of associated Chironomidae communities.

Ecological drift can override the effects of deterministic niche selection on small populations and drive the assembly of small communities. We tested the hypothesis that smaller local communities are more dissimilar among each other because of ecological drift than larger communities, which are mainly structured by niche selection. We used a unique, comprehensive dataset on insect communities sampled identically in a total of 200 streams in climatically different regions (Brazil and Finland) that differ in community size by fivefold. Null models allowed us to estimate the magnitude to which beta diversity deviates from the expectation under a random assembly process while taking differences in species richness and relative abundance into account, i.e., beta deviation. Beta diversity of small tropical communities was consistently higher but closer to null expectations than β-diversity of large communities. However, although β-deviation and community size were strongly related in both regions, the direction of the relationship varied according to dissimilarity metrics. While incidence-based β-diversity was lower than expected (communities were less dissimilar than null expectations) and negatively related to community size in Brazil, abundance-based β-diversity was higher than expected (communities were more dissimilar than null expectations) and positively related to community size in both regions. We suggest that ecological drift drives variation in small communities by increasing the chances of species with low abundance and narrow distribution to occur within the metacommunity. Also, while weak niche selection and high dispersal rates likely reduced variation in community structure among large tropical streams, niche selection was likely sufficient to cause non-random variations in the relative abundances of genera among large communities in both regions. Habitat destruction, overexploitation, pollution, and reductions in connectivity have been reducing the size of biological communities. These environmental pressures will make smaller communities more vulnerable to novel conditions and render community dynamics more unpredictable, as random demographic processes should prevail under these conditions. Incorporation of community size into ecological models should provide conceptual, empirical and applied insights into a better understanding of the processes driving changes in biodiversity. Footnotes * This new version includes many of the suggestions made by two reviewers and a recommender of PCI Ecology in a second round of revisions. * https://doi.org/10.5281/zenodo.2620550

Dams affect aquatic biota in running water by altering the environmental dynamics. One of the communities affected are benthic invertebrates, that perform important functions such as nutrient cycling and energy transfer in reservoirs. We investigated the influence of the following factors: spatial, abiotic variables, reservoir characteristics and land use on the richness, biomass and composition of benthic invertebrates in 29 reservoirs in southern Brazil. Sediment samples and abiotic variables were collected in the littoral and profundal zones of reservoirs during the dry (July) and rainy (November) seasons in 2001. We used principal coordinates of neighbour matrices to obtain the spatial factors. We analysed the unique and overall effects of the four factors. We found that the factors analysed significantly influenced the composition and biomass of the invertebrates (22%). The percentage of variation explained by the unique effects of the different factors showed similar values, but the spatial factors showed the highest value (4%). This indicates that closer reservoirs have higher similarities in terms of composition and biomass of benthic invertebrates. On the other hand, the abiotic variables had the highest explained value when evaluating the overall effects (10%). Therefore, for invertebrate richness, only the reservoir characteristics were significant for the overall and unique effects (49%). More specifically, we observed lower taxon richness in older reservoirs, probably because the age of such artificial aquatic ecosystems influences other variables, which structure the benthic communities. In conclusion, the structure of benthic invertebrate communities in reservoirs of southern Brazil are mainly driven by spatial and reservoir characteristics.