Landscape-Scale Environmental and Floristic Variation in Costa Rican Old-Growth Rain Forest Remnants

Studies of tropical rain forest beta-diversity debate environmental determinism versus dispersal limitation as principal mechanisms underlying floristic variation. We examined the relationship between soil characteristics, terrain, climate variation, and rain forest composition across a 3000 km² area in northeastern Costa Rica. Canopy tree and arboreal palm species abundance and soils were measured from 127 0.25-ha plots across Caribbean lowlands and foothills. Plot elevation, slope, temperature, and precipitation variation were taken from digital grids. Ordination of forest data yielded three floristic groups with strong affinities to foothills and differing lowland environments. Variation in floristics, soil texture, and climate conditions showed parallel patterns of significantly positive spatial autocorrelation up to 13 km and significantly negative correlation beyond 40 km. Partial Mantel tests resulted in a significant correlation between floristic distance and terrain, climate and soil textural variables controlling the effect of geographical distance. Separate comparisons for palm species showed significant correlation with Mg and Ca concentrations among other soil factors. Arboreal palm species demonstrated a stronger relationship with soil factors than did canopy trees. Correlation between floristic data and geographical distance, related to seed dispersal or unmeasured variables, was not significant after controlling for soil characteristics and elevation. Canopy trees and palms showed differing relationships to soil and other environmental factors, but lend greater support for a niche-assembly hypothesis than to a major role for dispersal limitation in determining species turnover for this landscape.     

Primate communities are structured more by dispersal limitation than by niches

1.?A major goal in community ecology is to identify mechanisms that govern the assembly and maintenance of ecological communities. Current models of metacommunity dynamics differ chiefly in the relative emphasis placed on dispersal limitation and niche differentiation as causal mechanisms structuring ecological communities. Herein we investigate the relative roles of these two mechanisms in structuring primate communities in Africa, South America, Madagascar and Borneo. 2.?We hypothesized that if dispersal limitation is important in structuring communities, then community similarity should depend on geographical proximity even after controlling for ecological similarity. Conversely, if communities are assembled primarily through niche processes, then community similarity should be determined by ecological similarity regardless of geographical proximity. 3.?We performed Mantel and partial Mantel tests to investigate correlations among primate community similarity, ecological distance and geographical distance. Results showed significant and strongly negative relationships between diurnal primate community similarity and both ecological similarity and geographical distance in Madagascar, but significant and stronger negative relationships between community similarity and geographical distance in African, South American and Bornean metacommunities. 4.?We conclude that dispersal limitation is an important determinant of primate community structure and may play a stronger role in shaping the structure of some terrestrial vertebrate communities than niche differentiation. These patterns are consistent with neutral theory. We recommend tests of functional equivalence to determine the extent to which neutral theory may explain primate community composition.     

Dispersal modes affect tropical forest assembly across trophic levels

We examined assemblages of trees and two major groups of vertebrate seed dispersers, birds and primates, in Ugandan protected areas to evaluate the roles of dispersal limitation and species sorting in community assembly. We conducted partial Mantel tests to investigate relationships between community similarity, environmental distance and geographic distance. Results showed that environmental factors, specifically temperature and rainfall, significantly and more strongly structured tree assemblages than geographic distance. Analysis of tree dispersal modes revealed wind‐dispersed tree guilds were significantly dispersal limited but trees dispersed by animals were not. For assemblages of vertebrate seed dispersers, dispersal limitation significantly and more strongly structured assemblages of primates than species sorting whereas environmental factors significantly and more strongly structured assemblages of birds than dispersal limitation. We therefore examined whether trees dispersed by primates were more dispersal limited than trees dispersed by birds. We found consistent trends that primate fruit trees were more dispersal limited than bird fruit trees using three definitions of dispersal syndromes based on fruit color. Our results suggest that the dispersal abilities of primary consumers may affect the distribution of primary producers at large spatial scales.     

黄土高原腹地人工林下草本层群落构建机制

群落构建机制是生态学研究的核心论题。生态位理论和中性理论是阐明群落构建的两种主要理论, 但这两种理论分别强调的环境过滤和扩散限制对群落构建的影响尚存争议。该研究以黄土高原人工林下草本层群落为研究对象, 将群落物种组成数据与地理空间、气候、地形以及生物因子相结合, 运用随机森林模型(RF)和基于距离矩阵的多元回归方法(MRM), 探究了各个因子的重要性, 并通过邻体主坐标矩阵(PCNM)和基于距离的冗余分析(db-RDA)分别筛选显著的空间结构和环境因子, 最后结合筛选出的PCNM特征值和环境变量进行变差分解。研究结果显示: 1)林下草本层群落组成相似性随地理距离和生境差异的增加而减小。2)地理距离与生境差异共同解释了群落组成相似性变化的47.8%。其中, 空间因子和环境因子的独立解释率分别为14.1%和9.8%, 两者的联合解释率为23.9%。3)地理距离和年降水量是引起研究区内物种组成变化的关键因子, 且地理距离的重要性大于年降水量。在黄土高原腹地, 扩散限制与环境过滤共同主导了人工林下草本层群落的构建过程。     

Environmental filtering affects soil fungal community composition more than dispersal limitation at regional scales

The relative importance of dispersal limitation versus environmental filtering for community assembly has received much attention for macroorganisms. These processes have only recently been examined in microbial communities. Instead, microbial dispersal has mostly been measured as community composition change over space (i.e., distance decay). Here we directly examined fungal composition in airborne wind currents and soil fungal communities across a 40 000 km² regional landscape to determine if dispersal limitation or abiotic factors were structuring soil fungal communities. Over this landscape, neither airborne nor soil fungal communities exhibited compositional differences due to geographic distance. Airborne fungal communities shifted temporally while soil fungal communities were correlated with abiotic parameters. These patterns suggest that environmental filtering may have the largest influence on fungal regional community assembly in soils, especially for aerially dispersed fungal taxa. Furthermore, we found evidence that dispersal of fungal spores differs between fungal taxa and can be both a stochastic and deterministic process. The spatial range of soil fungal taxa was correlated with their average regional abundance across all sites, which may imply stochastic dispersal mechanisms. Nevertheless, spore volume was also negatively correlated with spatial range for some species. Smaller volume spores may be adapted to long-range dispersal, or establishment, suggesting that deterministic fungal traits may also influence fungal distributions. Fungal life-history traits may influence their distributions as well. Hypogeous fungal taxa exhibited high local abundance, but small spatial ranges, while epigeous fungal taxa had lower local abundance, but larger spatial ranges. This study is the first, to our knowledge, to directly sample air dispersal and soil fungal communities simultaneously across a regional landscape. We provide some of the first evidence that soil fungal communities are mostly assembled through environmental filtering and experience little dispersal limitation.     

Geographical and environmental controls of palm beta diversity in paleo-riverine terrace forests in Amazonian Peru

The palm (Arecaceae) community on low paleo-riverine terraces (terrace forest) in the north-western Amazon, is described, and we assessed the importance of environmental differences and geographic distance as drivers of its local (25² grain and 0–500 extent) and regional scale (500² grain and 0.3–143 km extent) beta diversity using ordination, multiple regressions on distance matrices and Indicator Species Analysis. A total of 15,869 individuals and 37 species of palm were sampled in 10 terrace forest transects, while 3758 individuals and 21 species were sampled in two adjacent floodplain forest transects for comparison. The terrace and floodplain forest were clearly different in their diversity and floristic composition. The relative importance of geographical distance and environmental difference as controls of terrace forest beta diversity was scale dependent, with environmental differences, notably in soil moisture, dominating at local scales and geographical distance dominating at regional scales. In fact, none of the environmental factors had a significant influence on regional-scale beta diversity. The geographical distance decay in floristic similarity was markedly steeper at local scale ( −0.25 km ⁻¹) than at regional scale ( −0.003 km⁻¹). Such a nonlinear decay is expected if simple dispersal limitation controls beta diversity. However, the absent flattening of the distance decay at the largest distances and the sub-Andean affinities of the westernmost palm communities suggest that large-scale biogeographical processes also contribute to the regional-scale beta diversity. Hereby our results indicate that not only local environment, but also dispersal limitation and biogeographical history can be important controls of the diversity and composition of local plant communities.     

地理距离及环境差异对云南元江干热河谷植物群落beta多样性的影响

beta多样性反映了群落间物种组成的差异, 是生物多样性研究的热点之一。本研究通过对云南元江干热河谷41个植物群落样方进行调查, 用Jaccard相异系数表征物种beta多样性, 用样方之间的最近谱系距离(mean nearest taxon distance, MNTD)及平均谱系距离(mean pairwise distance, MPD)表征谱系beta多样性, 采用基于距离矩阵的多元回归和方差分解方法, 探讨了该区域干热河谷典型植物群落的物种beta多样性和谱系beta多样性与样方间环境差异(主要是气候)及地理距离之间的关系。结果表明: (1)群落间的地理距离和年平均温度差异对干热河谷植物群落的物种beta多样性和谱系beta多样性有显著影响; (2)地理距离对物种beta多样性和MNTD的影响最大; 地理距离和年平均温度差异对MPD的影响均较大; (3)样方间年平均温度与年平均降水量的差异和地理距离能够解释群落间beta多样性及谱系beta多样性11-13%的变异。以上结果表明, 生态位分化和扩散限制对该地区植物群落的beta多样性均有显著影响, 其中扩散限制的影响可能更大。此外, 人类活动等其他因素也很可能对元江干热河谷的群落组成具有非常重要的影响。     

Hydrological connectivity structures concordant plant and animal assemblages according to niche rather than dispersal processes

1. Understanding the processes that structure community assembly across landscapes is fundamental to ecology and for predicting and managing the consequences of anthropogenically induced changes to ecosystems. 2. We assessed the community similarity of fish, macroinvertebrate and vegetation communities against geographic distances ranging from 4 to 480 km (i.e. distance–decay relationships) to determine the balance between local environmental factors and regional dispersal processes, and thus whether species‐sorting (niche processes) or dispersal limitation (neutral processes) was more important in structuring these assemblages in Australia’s wet‐dry tropics. We investigated whether the balance between niche and dispersal processes depended on the degree of hydrological connectivity, predicting that dispersal processes would be more important at connected sites, and also whether there was spatial concordance among these three assemblage types. 3. There was significant but weak spatial concordance among the study communities, suggesting limited potential for surrogacy among them. Distance–decay in community similarity was not observed for any study assemblage at perennial sites, suggesting dispersal was not limiting and assemblages were structured more strongly by local niche processes at these connected sites. At intermittent sites, weak distance–decay relationships for each assemblage type were confounded by significant relationships with environmental dissimilarity, suggesting that dispersal limitation contributed, albeit weakly, to niche processes in structuring our three study assemblages at disconnected sites. 4. Two environmental factors, flow regime and channel width, explained significant proportions of variation in all three assemblages, potentially contributing to the observed spatial concordance between them and representing local environmental gradients along which these communities re‐assemble after the wet season, according to niche rather than dispersal processes.     

Soil‐based habitat partitioning in understorey palms in lower montane tropical forests

Aim Dispersal assembly and niche assembly are two competing theories proposed to explain the maintenance of species diversity in tropical forests. Dispersal theory emphasizes the role of chance colonization events and distance‐limited seed dispersal in explaining species abundance and distribution, whereas niche theory emphasizes differences among species in requirements for potentially limiting resources. Species distribution patterns in tropical forests often correlate with geology and topography, but tests of the relative importance of dispersal and niche partitioning have been hampered by an inadequate characterization of resource availability. The aim of this study was to explore how soil chemical and physical properties, climate, and geographic distance affect understorey palm communities in lower montane forests. Location Fortuna Forest Reserve, Chiriqui Province, and Palo Seco Forest Reserve, Bocas del Toro Province, in western Panama. Methods Understorey palms and soil nutrient concentrations were surveyed within 10 sites on different soil types across a 13‐km transect. Variation in palm community composition was examined in relation to spatial and environmental variables. Results The 25 understorey palm species recorded in the study were non‐randomly distributed among forests differing in soil nutrient availability. In support of dispersal theory, floristic similarity decreased predictably with increasing geographic distance. However, environmental and soil variables were also correlated with geographic distance. Floristic similarity was also highly associated with a subset of environmental variables. Variation in palm community similarity was most strongly correlated with inorganic nitrogen availability and cation concentration. A subset of soil variables had a stronger relationship with floristic similarity when geographic distance was controlled for than did geographic distance when differences in soils were controlled for. Main conclusions Both dispersal and niche processes affect palm species distribution patterns. Although spatially limited dispersal may influence species distribution patterns, soil‐based habitat associations, particularly with respect to soil nitrogen, cation availability and aluminium concentrations, remain important factors influencing palm community composition at the mesoscale level in this tropical montane forest.     

Autogenic-allogenic status affects interpond community similarity and species area relationship of macroparasites in the bluegill sunfish, Lepomis macrochirus, from a series of freshwater ponds in the Piedmont area of North Carolina

The effects of autogenic-allogenic status on the species-area relationship and the relationship between geographic distance and intercommunity dissimilarity were investigated in macroparasite communities of the bluegill sunfish Lepomis macrochirus. Rank correlation analyses were used to examine the relationship between pond surface area and species richness of all species collectively and of autogenic species and allogenic species separately. A positive relationship was found for allogenic species, whereas there was no association for all species, nor was there an association when the study was restricted to autogenic species. Mantel tests were used to determine the relationship between geographic distance and community dissimilarity for all species and for autogenic and allogenic species independently. Total community dissimilarity and allogenic dissimilarity were found to be influenced by geographic distance, whereas autogenic dissimilarity was random with regard to interpond distances. These findings serve to illustrate the importance of the autogenic-allogenic dichotomy and demonstrate that dispersal ability can influence commonly observed ecological patterns.     

The impact of ecological differentiation and dispersal limitation on species turnover and phylogenetic structure of inselberg's plant communities

Phylogenetic structure analysis is a novel way to address the relative importance of stochastic and deterministic processes governing species assemblages. Here we investigate the phylogenetic structure of the vegetation of inselbergs located in the African rain forest. Inselbergs combine strong ecological gradients at the local scale due to soil depth variation and insular properties at the regional scale. They are therefore ideal models to assess the influence of ecological sorting and dispersal limitation on the phylogenetic structure of plant communities. On 21 inselbergs separated by up to 200 km where five microhabitat-types were recognized, 311 vegetation plots were inventoried. We found that floristic similarity between plots depended on both microhabitat differentiation and spatial distance, while phylogenetic clustering (i.e. excess of phylogenetic similarity between species from a same plot) only appeared between plots from differentiated microhabitats and increased with ecological distance. Within a microhabitat-type, the absence of phylogenetic structure between inselbergs indicates that species turnover is probably due to dispersal limitation rather than to regional-scale variations in environmental factors. Hence, phylogenetic structure analysis can help disentangle the effects of ecological sorting and dispersal limitation on species assemblages. To estimate the time-scale of the processes generating the phylogenetic structure, we investigated how lineage similarity changes with increasing age in the phylogenetic tree. High lineage similarity levels between ecologically very differentiated plots were only reached at the proximity of the root of the phylogenetic tree. This was observed even when considering only plots sharing no species and indicates that phylogenetic niche conservatism has been important for generating the observed phylogenetic structure. Hence, ancient diversification exerts an impact on the assembly of current plant communities.     

Topographic and spatial controls of palm species distributions in a montane rain forest, southern Ecuador

The northern Andes harbour a flora that is as species-rich or even richer than the 18-times larger lowland Amazon basin. Gaining an understanding of how the high species richness of the Andean region is generated and maintained is therefore of particular interest. Environmental sorting due to elevational gradients in climate has been emphasized as a driver of vegetation distribution and plant community assembly in tropical mountain areas such as the Andes for two centuries, while alternative mechanisms have been little studied. Here, we investigated the importance of topography and spatial location as factors controlling species distributions in a palm community in a montane rain forest landscape in the Andes of southern Ecuador (1900–2150 m above sea level). Eleven species were present: Aiphanes verrucosa, Ceroxylon parvifrons, Chamaedorea pinnatifrons, Dictyocaryum lamarckianum, Euterpe precatoria, Geonoma densa, Geonoma orbignyana, Geonoma paradoxa, Prestoea acuminata and Wettinia aequatorialis. To study their spatial distribution, forty 250 m² (5 × 50 m²) plots were laid out perpendicular to four paths that were categorized into three areas and two topographic units (ridges and gullies). Mantel tests and indicator species analysis showed that both topography and spatial location imposed strong controls on palm species distributions at the study site. Our results suggest that species distributions in the studied montane forest landscape were partly determined by the species’ habitat requirements, but also by unknown spatial effects. Although a number of possible explanations exist for the latter, such as unmeasured environmental variables and historical disturbance events, we believe dispersal limitation is likely to be involved. Furthermore, although the gully- or ridge-association of some species corresponded to their general elevational ranges in southern Ecuador, this was not the case for other species. Based on such considerations, we conclude that elevational climatic gradients are likely to only form part of the explanation for the topographic effects on palm species distributions at the study site. Other factors must also be involved, notably wind-exposure and hydrology, as discussed for lowland palm communities. Our results show that to understand plant community assembly in the tropical montane forests of the Andes it is too simple to focus just on environmental sorting by elevational climatic gradients.     

Contrasting patterns and mechanisms of spatial turnover for native and exotic freshwater fish in Europe

Aim We compare the distribution patterns of native and exotic freshwater fish in Europe, and test whether the same mechanisms (environmental filtering and/or dispersal limitation) govern patterns of decrease in similarity of native and exotic species composition over geographical distance (spatial species turnover). Locations Major river basins of Europe. Methods Data related to geography, habitat diversity, regional climate and species composition of native and exotic freshwater fish were collated for 26 major European river basins. We explored the degree of nestedness in native and exotic species composition, and quantified compositional similarity between river basins according to the beta‐sim (independent of richness gradient) and Jaccard (dependent of richness gradient) indices of similarity. Multiple regression on distance matrices and variation‐partitioning approaches were used to quantify the relative roles of environmental filtering and dispersal limitation in shaping patterns of decreasing compositional similarity over geographical distance. Results Native and exotic species exhibited significant nested patterns of species composition, indicating that differences in fish species composition between river basins are primarily the result of species loss, rather than species replacement. Both native and exotic compositional similarity decreased significantly with increasing geographical distance between river basins. However, gradual changes in species composition with geographical distance were found only for exotic species. In addition, exotic species displayed a higher rate of similarity decay (higher species turnover rate) with geographical distance, compared with native species. Lastly, the majority of explained variation in exotic compositional similarity was uniquely related to geography, whereas native compositional similarity was either uniquely explained by geography or jointly explained by environment and geography. Main conclusions Our study suggests that large‐scale patterns of spatial turnover for exotic freshwater fish in Europe are generated by human‐mediated dispersal limitation, whereas patterns of spatial turnover for native fish result from both dispersal limitation relative to historical events (isolation by mountain ranges, glacial history) and environmental filtering.     

Environmental drivers of species composition and functional diversity of dung beetles along the Atlantic Forest–Pampa transition zone

Human activities are causing a rapid loss of biodiversity, which impairs ecosystem functions and services. Therefore, understanding which processes shape how biodiversity is distributed along spatial and environmental gradients is a first step to guide conservation and management efforts. We aimed to determine the relative explanatory importance of biogeographic, environmental, landscape and spatial variables on assemblage dissimilarities and functional diversity of dung beetles along the Atlantic Forest–Pampa (i.e. forest–grassland) transition zone located in Southeast South America. We described each site according to their biogeographic position, environmental conditions, landscape features and spatial patterns. The compositional dissimilarity was partitioned into turnover and nestedness components of β‐diversity. Mantel tests and generalised dissimilarity models were used to relate β‐diversity and its components to biogeographic, environmental, landscape and spatial variables. Variation partitioning analysis was used to estimate the pure and shared variation in species composition and functional diversity explained by the four categories of predictors. Biome domain was the main factor causing dung beetle compositional dissimilarity, with a high species replacement between Atlantic Forest and Pampa. Biogeographic, environmental, landscape and spatial distances also affected the patterns of dung beetle dissimilarity and β‐diversity components. The shared effects of the four sets of predictors explained most of the variation in dung beetle composition. A similar response pattern was found for dung beetle functional diversity, which excluded biogeographic effects. Only the pure effects of environmental and spatial predictors were significant for species composition and functional diversity. Our results indicate that dung beetle species composition and functional diversity are jointly driven by environmental, landscape and spatial predictors with higher pure environmental and spatial effects. The forest–grassland transition zone promotes a strong species and trait replacement highly influenced both by environmental filtering and dispersal limitation.     

The effects of contemporary selection and dispersal limitation on the community assembly of acidophilic microalgae

Extremophilic microalgae are primary producers in acidic habitats, such as volcanic sites and acid mine drainages, and play a central role in biogeochemical cycles. Yet, basic knowledge about their species composition and community assembly is lacking. Here, we begin to fill this knowledge gap by performing the first large‐scale survey of microalgal diversity in acidic geothermal sites across the West Pacific Island Chain. We collected 72 environmental samples in 12 geothermal sites, measured temperature and pH, and performed rbcL amplicon‐based 454 pyrosequencing. Using these data, we estimated the diversity of microalgal species, and then examined the relative contribution of contemporary selection (i.e., local environmental variables) and dispersal limitation on the assembly of these communities. A species delimitation analysis uncovered seven major microalgae (four red, two green, and one diatom) and higher species diversity than previously appreciated. A distance‐based redundancy analysis with variation partitioning revealed that dispersal limitation has a greater influence on the community assembly of microalgae than contemporary selection. Consistent with this finding, community similarity among the sampled sites decayed more quickly over geographical distance than differences in environmental factors. Our work paves the way for future studies to understand the ecology and biogeography of microalgae in extreme habitats.     

The relative role of rivers,environmental heterogeneity and species traits in driving compositional changes in southeastern Amazonian bird assemblages

Amazonian rivers have been proposed to act as geographic barriers to species dispersal, either driving allopatric speciation or defining current distribution limits. The strength of the barrier varies according to the species’ ecological characteristics and the river's physical properties. Environmental heterogeneity may also drive compositional changes but has not been well assessed in Amazonia. Aiming to understand the contributions of riverine barriers and environmental heterogeneity in shaping compositional changes in Amazonian forest bird assemblages, we focus on the Tapajós River. We investigate how spatial variation in species composition is related to physical barriers (Tapajós and Jamanxim rivers), species’ ecological characteristics (distinct guilds), and environmental heterogeneity (canopy reflectance, soils, and elevation). We sampled birds through point-counts and mist nets on both banks of the Tapajós and Jamanxim rivers. To test for relationships between bird composition and environmental data, we used Mantel and partial Mantel tests, NMDS, and ANOVA + Tukey HSD. The Mantel tests showed that the clearest compositional changes occurred across the Tapajós River, which seems to act unequally as a significant barrier to the bird guilds. The Jamanxim River was not associated with differences in bird communities. Our results reinforce that the Tapajós River is a biogeographical boundary for birds, while environmental heterogeneity influences compositional variation within interfluves. We discuss the combined influence of geographical barriers, environmental heterogeneity, and ecological characteristics of species in shaping species distributions and community composition and the complexity of extrapolating the patterns found for birds to other Amazonian organisms. Abstract in Portuguese is available with online material.     

Biogeographical patterns of freshwater micro‐ and macroorganisms: a comparison between phytoplankton,zooplankton and fish in the eastern Mediterranean

Aim We investigated the biogeographical patterns of phytoplankton, zooplankton and fish in freshwater ecosystems. We tested whether spatial distance or environmental heterogeneity act as potential factors controlling community composition. Location Northern and central Greece, eastern Mediterranean. Method Data on 310 phytoplankton, 72 zooplankton and 37 fish species were collected from seven freshwater systems. Species occurrence data were used to generate similarity matrices describing community composition. We performed Mantel tests to compare spatial patterns in community composition of phytoplankton, zooplankton and fish. Next, we examined the correlation between geographical distance and the degree of similarity in community composition. The analysis was repeated for different taxonomic, trophic and size‐based groups of the microorganisms studied. We assessed different environmental variables (topographic and limnological) as predictors of community composition. Results Phytoplankton community composition showed a strong positive correlation with environmental heterogeneity but was not correlated with the geographical distance between systems. Zooplankton community composition was unrelated to geographical distance and was only weakly correlated with environmental variables. In contrast, fish community similarity decayed significantly with distance. We found no relationship along all pairwise comparisons of the compositional matrices of the three groups. The pairwise comparisons of the different taxonomic, trophic and size‐based groups of the microorganism communities studied were in accordance with the results for the entire microorganism community. Main conclusions Our results support the proposition that the biogeography of microorganisms does not demonstrate a distance–decay pattern and further suggest that, in reality, the drivers of distribution depend on the specific community examined. In contrast, the biogeography of macroorganisms was affected by geographical distance. These differences reflect the dispersal abilities of the different organisms. The microorganisms exhibit passive dispersal through the air, with local environmental conditions structuring their community composition. On the other hand, for macroorganisms such as fish, the terrestrial environment could pose barriers to their dispersal; with fish structuring distinctive communities over greater distances. Overall, we suggest that the biogeography of freshwater phytoplankton and zooplankton reflects contemporary environmental conditions, while the biogeographical patterns for fish inhabiting the same systems are related to factors affecting their dispersal ability.     

The relative importance of local conditions and regional processes in structuring aquatic plant communities

1. The structure of biological communities reflects the influence of both local environmental conditions and processes such as dispersal that create patterns in species’ distribution across a region. 2. We extend explicit tests of the relative importance of local environmental conditions and regional spatial processes to aquatic plants, a group traditionally thought to be little limited by dispersal. We used partial canonical correspondence analysis and partial Mantel tests to analyse data from 98 lakes and ponds across Connecticut (northeastern United States). 3. We found that aquatic plant community structure reflects the influence of local conditions (pH, conductivity, water clarity, lake area, maximum depth) as well as regional processes. 4. Only 27% of variation in a presence/absence matrix was explained by environmental conditions and spatial processes such as dispersal. Of the total explained, 45% was related to environmental conditions and 40% to spatial processes. 5. Jaccard similarity declined with Euclidean distance between lakes, even after accounting for the increasing difference in environmental conditions, suggesting that dispersal limitation may influence community composition in the region. 6. The distribution of distances among lakes where species occurred was associated with dispersal‐related functional traits, providing additional evidence that dispersal ability varies among species in ways that affect community composition. 7. Although environmental and spatial variables explained a significant amount of variation in community structure, a substantial amount of stochasticity also affects these communities, probably associated with unpredictable colonisation and persistence of the plants.     

Similar fish species composition despite larger environmental heterogeneity during severe hypoxia in a coastal ecosystem

Environmental heterogeneity is one of the most influential factors that create compositional variation among local communities. Greater compositional variation is expected when an environmental gradient encompasses the most severe conditions where species sorting is more likely to operate. However, evidence for stronger species sorting at severer environment has typically been obtained for less mobile organisms and tests are scarce for those with higher dispersal ability that allows individuals to sensitively respond to environmental stress. Here, with the dynamics of fish communities in a Japanese bay revealed by environmental DNA metabarcoding analyses as a model case, we tested the hypothesis that larger environmental heterogeneity caused by severe seasonal hypoxia (lower concentration of oxygen in bottom waters in summer) leads to larger variation of species composition among communities. During summer, fish species richness was lower in the bottom layer, suggesting the severity of the hypoxic bottom water. In contrast to the prediction, we found that although the environmental parameters of bottom and surface water was clearly distinct in summer, fish species composition was more similar between the two layers. Our null model analysis suggested that the higher compositional similarity during hypoxia season was not a result of the sampling effect reflecting differences in the alpha or gamma diversity. Furthermore, a shift in the species occurrence from bottom to surface layers was observed during hypoxia season, which was consistent across species, suggesting that the severe condition in the bottom adversely affected fish species irrespective of their identity. These results suggest that larger environmental heterogeneity does not necessarily lead to higher compositional variation once the environmental gradient encompasses extremely severe conditions. This is most likely because individual organisms actively avoided the severity quasi‐neutrally, which induced mass effect‐like dispersal and lead to the mixing of species composition across habitats. By showing counter evidence against the prevailing view, we provide novel insights into how species sorting by environment acts in heterogeneous and severe conditions.     

Assembly mechanisms determining high species turnover in aquatic communities over regional and continental scales

Niche and neutral processes drive community assembly and metacommunity dynamics, but their relative importance might vary with the spatial scale. The contribution of niche processes is generally expected to increase with increasing spatial extent at a higher rate than that of neutral processes. However, the extent to what community composition is limited by dispersal (usually considered a neutral process) over increasing spatial scales might depend on the dispersal capacity of composing species. To investigate the mechanisms underlying the distribution and diversity of species known to have great powers of dispersal (hundreds of kilometres), we analysed the relative importance of niche processes and dispersal limitation in determining beta‐diversity patterns of aquatic plants and cladocerans over regional (up to 300 km) and continental (up to 3300 km) scales. Both taxonomic groups were surveyed in five different European regions and presented extremely high levels of beta‐diversity, both within and among regions. High beta‐diversity was primarily explained by species replacement (turnover) rather than differences in species richness (i.e. nestedness). Abiotic and biotic variables were the main drivers of community composition. Within some regions, small‐scale connectivity and the spatial configuration of sampled communities explained a significant, though smaller, fraction of compositional variation, particularly for aquatic plants. At continental scale (among regions), a significant fraction of compositional variation was explained by a combination of spatial effects (exclusive contribution of regions) and regionally‐structured environmental variables. Our results suggest that, although dispersal limitation might affect species composition in some regions, aquatic plant and cladoceran communities are not generally limited by dispersal at the regional scale (up to 300 km). Species sorting mediated by environmental variation might explain the high species turnover of aquatic plants and cladocerans at regional scale, while biogeographic processes enhanced by dispersal limitation among regions might determine the composition of regional biotas.