Victor Lemes Landeiro

Mantel test in population genetics

The comparison of genetic divergence or genetic distances, estimated by pairwise FST and related statistics, with geographical distances by Mantel test is one of the most popular approaches to evaluate spatial processes driving population structure. There have been, however, recent criticisms and discussions on the statistical performance of the Mantel test. Simultaneously, alternative frameworks for data analyses are being proposed. Here, we review the Mantel test and its variations, including Mantel correlograms and partial correlations and regressions. For illustrative purposes, we studied spatial genetic divergence among 25 populations of Dipteryx alata (“Baru”), a tree species endemic to the Cerrado, the Brazilian savannas, based on 8 microsatellite loci. We also applied alternative methods to analyze spatial patterns in this dataset, especially a multivariate generalization of Spatial Eigenfunction Analysis based on redundancy analysis. The different approaches resulted in similar estimates of the magnitude of spatial structure in the genetic data. Furthermore, the results were expected based on previous knowledge of the ecological and evolutionary processes underlying genetic variation in this species. Our review shows that a careful application and interpretation of Mantel tests, especially Mantel correlograms, can overcome some potential statistical problems and provide a simple and useful tool for multivariate analysis of spatial patterns of genetic divergence.

Metacommunity structuring in stream networks: roles of dispersal mode, distance type, and regional environmental context

Within a metacommunity, both environmental and spatial processes regulate variation in local community structure. The strength of these processes may vary depending on species traits (e.g., dispersal mode) or the characteristics of the regions studied (e.g., spatial extent, environmental heterogeneity). We studied the metacommunity structuring of three groups of stream macroinvertebrates differing in their overland dispersal mode (passive dispersers with aquatic adults; passive dispersers with terrestrial adults; active dispersers with terrestrial adults). We predicted that environmental structuring should be more important for active dispersers, because of their better ability to track environmental variability, and that spatial structuring should be more important for species with aquatic adults, because of stronger dispersal limitation. We sampled a total of 70 stream riffle sites in three drainage basins. Environmental heterogeneity was unrelated to spatial extent among our study regions, allowing us to examine the effects of these two factors on metacommunity structuring. We used partial redundancy analysis and Morans eigenvector maps based on overland and watercourse distances to study the relative importance of environmental control and spatial structuring. We found that, compared with environmental control, spatial structuring was generally negligible, and it did not vary according to our predictions. In general, active dispersers with terrestrial adults showed stronger environmental control than the two passively dispersing groups, suggesting that the species dispersing actively are better able to track environmental variability. There were no clear differences in the results based on watercourse and overland distances. The variability in metacommunity structuring among basins was not related to the differences in the environmental heterogeneity and spatial extent. Our study emphasized that (1) environmental control is prevailing in stream metacommunities, (2) dispersal mode may have an important effect on metacommunity structuring, and (3) some factors other than spatial extent or environmental heterogeneity contributed to the differences among the basins.

A comparative analysis reveals weak relationships between ecological factors and beta diversity of stream insect metacommunities at two spatial levels

The hypotheses that beta diversity should increase with decreasing latitude and increase with spatial extent of a region have rarely been tested based on a comparative analysis of multiple datasets, and no such study has focused on stream insects. We first assessed how well variability in beta diversity of stream insect metacommunities is predicted by insect group, latitude, spatial extent, altitudinal range, and dataset properties across multiple drainage basins throughout the world. Second, we assessed the relative roles of environmental and spatial factors in driving variation in assemblage composition within each drainage basin. Our analyses were based on a dataset of 95 stream insect metacommunities from 31 drainage basins distributed around the world. We used dissimilarity-based indices to quantify beta diversity for each metacommunity and, subsequently, regressed beta diversity on insect group, latitude, spatial extent, altitudinal range, and dataset properties (e.g., number of sites and percentage of presences). Within each metacommunity, we used a combination of spatial eigenfunction analyses and partial redundancy analysis to partition variation in assemblage structure into environmental, shared, spatial, and unexplained fractions. We found that dataset properties were more important predictors of beta diversity than ecological and geographical factors across multiple drainage basins. In the within-basin analyses, environmental and spatial variables were generally poor predictors of variation in assemblage composition. Our results revealed deviation from general biodiversity patterns because beta diversity did not show the expected decreasing trend with latitude. Our results also call for reconsideration of just how predictable stream assemblages are along ecological gradients, with implications for environmental assessment and conservation decisions. Our findings may also be applicable to other dynamic systems where predictability is low.

Variance partitioning of deconstructed periphyton communities: does the use of biological traits matter?

The use of species traits offers a promising approach to the understanding of the main processes underlying metacommunity patterns. We analyzed samples of periphytic algae in 30 environments of the Upper Paraná River floodplain in southeastern Brazil, to test the hypotheses that variation in species composition of algal groups with low dispersal abilities would be mainly explained by spatial variables; on the other hand, algal groups with higher dispersal abilities would be better explained by environmental variables. The variation in community structure was mainly correlated with environmental variables. This result is in line with a growing body of evidence indicating a predominant role of species-sorting processes. The more-refined prediction that the spatial variables would gradually become more important across a gradient of adherence or size was, however, not supported by our analyses. Also, the large unexplained variation suggested that these periphytic communities were assembled by idiosyncratic events, or that other variables that are often neglected in studies of aquatic metacommunities needed to be included.

Nutrient enrichment is related to two facets of beta diversity for stream invertebrates across the United States.

Beta diversity, the spatial or temporal variability of species composition, is a key concept in community ecology. However, our ability to predict the relative importance of the main drivers of beta diversity (e.g., environmental heterogeneity, dispersal limitation, and environmental productivity) remains limited. Using a comprehensive data set on stream invertebrate assemblages across the continental United States, we found a hump-shaped relationship between beta diversity and within-ecoregion nutrient concentrations. Within-ecoregion compositional dissimilarity matrices were mainly related to environmental distances in most of the 30 ecoregions analyzed, suggesting a stronger role for species-sorting than for spatial processes. The strength of these relationships varied considerably among ecoregions, but they were unrelated to within-ecoregion environmental heterogeneity or spatial extent. Instead, we detected a negative correlation between the strength of species sorting and nutrient concentrations. We suggest that eutrophication is a major mechanism disassembling invertebrate assemblages in streams at a continental scale.

Responses of aquatic invertebrate assemblages and leaf breakdown to macroconsumer exclusion in Amazonian "terra firme" streams

Many authors have reported a lack of insect shredders in tropical streams and some have suggested that macroconsumers, such as fish and shrimp, are potential substitutes for insect shredders. We experimentally excluded macroconsumers (fish and shrimp) from leaf packs to examine their role in determining the rate of leaf breakdown and their effects on the associated invertebrate community. The experiment was designed in blocks and replicated in two reaches of four streams. Temperature of all stream reaches studied was 24 °C (without variation), and water conductivity was low, varying from 8.8 to 10.8 μs/cm. Fish were never observed near the leaf packs and therefore the effects of the macroconsumer treatment were attributed to shrimps. We found a significant effect on leaf breakdown, with greater leaf breakdown (i.e., less mass remaining after 17 days) in the control (65 % leaf mass remaining) compared to the macroconsumer exclusion (70 % leaf mass remaining). However, the mechanism for this effect was not clear. Considerable variation in leaf decomposition occurred among blocked stream sites, suggesting that some factors differing among these sites, perhaps macroinvertebrate shredder abundance, was contributing to decomposition. Leaves were visually inspected at the conclusion of the experiment and there was no sign of shrimp directly feeding on the leaves. There was no difference in insect shredder abundance between treatments. There was, however, a significantly greater amount of non-mining chironomids in the absence of macroconsumers. This is probably due to the release from predation by shrimp.

Limited effects of dominant ants on assemblage species richness in three Amazon forests

1. Ants are highly interactive organisms and dominant species are considered to be able to control the species richness of other ants via competitive exclusion. However, depending on the scale studied, inter‐specific competition may or may not structure biological assemblages. To date, ant dominance–richness relationships have only been studied in small sample units, where a few dominant colonies could plausibly control most of the sample unit.

Freshwater diatoms as environmental indicators: evaluating the effects of eutrophication using species morphology and biological indices

Anthropogenic eutrophication is a major form of perturbation in freshwaters, and several approaches aim to recognise its effects on lake ecosystems. We compared the responses of diatom species morphology, diversity indices and diatom indices to total phosphorus, total nitrogen and distance from a point stressor causing eutrophication in a large lake. We specifically examined the degree to which extent nutrients and distance to the stressor affect variation in the values of various biological indices and diatom valve size. In addition, special attention was given to the adequate repetition of diatom valve width measurements in the context of environmental assessment. Our results showed that diatom valve width was a better indicator of nutrient concentrations than any of the diatom and diversity indices examined. However, the results varied between the two study transects, suggesting that the diatom-based variables not only respond to nutrients but also to other environmental factors (e.g. shoreline morphology). We also found that when using the method based on diatom morphology, one should measure more valves than has been originally suggested to provide a more reliable picture of response to eutrophication. We argue that diatom morphology could be considered as an additional environmental assessment tool, because it may complement the information provided by the traditional diatom indices. Diatom valve width may also be more sensitive to early phases of the eutrophication process and its effects on freshwater ecosystems than various diatom indices that were developed in regional contexts with wide ranges in nutrient levels.

Species richness and distribution of blackflies (Diptera: Simuliidae) in the Chapada Diamantina region, Bahia, Brazil

Taxonomically, blackflies (Simuliidae) are among the best-known aquatic insects in Brazil. However, information on their distribution is lacking for many regions. We sampled simuliids in 50 streams in the Chapada Diamantina region, State of Bahia, located in three distinct geographical areas, 20 at Lençóis, 18 at Mucugê, and 12 at Rio de Contas. We analyzed simuliid species richness and distribution in the Chapada Diamantina area as a whole, as well as in each of the three distinct geographical areas. We collected 20 species, three of which were not yet described and two were described after our sampling. The three areas are distinct in relation to environmental variables, mainly owing to differences in altitude and water pH. Four species were restricted to Rio de Contas, while four other species were restricted to Mucugê. Sixteen species were present in Mucugê and fifteen in Rio de Contas. Only twelve species were present in Lençóis, despite the higher sampling effort. Rio de Contas appears to be the area that harbors the highest species richness. Species richness was related to environmental factors only when evaluated on a small spatial scale (i.e. when each area was analyzed separately). Although we have not tested for causal relationships, our findings agree with other studies that showed that pH and altitude are important factors with which species richness and species distribution appear to be associated.

Interchange between flooding and drying, and spatial connectivity control the fish metacommunity structure in lakes of the Pantanal wetland

We evaluated if the homogenizing effect of flooding and the environmental differentiation during the dry period, as reflected in fish communities, are related to the type of connection between floodplain lakes and rivers. In order to do this, we sampled fish in the early dry and the early flood hydroperiods. We estimated the fish assemblage structure, as fish species richness, abundance, biomass, and beta diversity. We found that biomass varied among connectivity levels and hydroperiods, while richness varied among hydroperiods and density was influenced by an interaction between connectivity and hydroperiod. Species composition varied as a function of both connection type and hydroperiod, but the interaction between these factors was not observed. The fish assemblage in isolated lakes differed from those in temporarily and permanently connected lakes. Beta diversity increased from the early dry to the early flood period and was higher in temporarily connected lakes than in permanently connected lakes. These results indicated that the spatio-temporal variations in local fish communities of floodplain lakes can be attributed to differences in connectivity. Consequently, they strengthen the hypothesis that dispersal plays a fundamental role in shaping the pattern of fish species diversity in floodplain lakes.