Metacommunity Structure Research Articles

Disentangling the role of niche-based and spatial processes on anuran beta diversity in temporary ponds along a forest–grassland transition

Evaluating beta diversity patterns along environmental gradients and different geographic ranges helps comprehend the roles of niche-based and spatial processes to metacommunity structure. Pond-breeding anuran composition depends on environmental predictors such as climate, vegetation type and habitat features as well as spatial processes (e.g., dispersal limitation or distances among patches). Here, we investigated the role of spatial and local (water chemistry and habitat structure) and regional (climate) environmental predictors to tadpole beta diversity (and its turnover and nestedness-resultant components) in temporary ponds distributed along a forest–grassland transition in the southern Brazilian Coastal Plain. Additionally, we investigated the role of the assessed predictors within forest and grassland sub-regions to test for congruence in their contribution across vegetation types. Turnover was the most relevant component of beta diversity. Spatial predictors alone explained beta diversity (and both components) at the broadest scale. However, the contribution of environmental and spatial predictors to beta diversity differed between sub-regions. Spatial and local environmental predictors independently explained beta diversity in the grassland, while in the forest sub-region, local predictors and spatially-structured climate were the most important. Our results revealed a complex interplay of niche-based and spatial assembly processes shaping anuran metacommunities. The different relationships of environmental and spatial predictors with beta diversity in the forest and grassland sub-regions suggest that the contribution of assembly processes to anuran metacommunity structure change with vegetation type.

Untangling the assembly of macrophyte metacommunities by means of taxonomic, functional and phylogenetic beta diversity patterns

Metacommunity ecology has broadened considerably with the recognition that measuring beta diversity beyond the purely taxonomic viewpoint may improve our understanding of the dispersal- and niche-based mechanisms across biological communities. In that perspective, we applied a novel multidimensional approach including taxonomic, functional and phylogenetic data to enhance our basic understanding of macrophyte metacommunity dynamics. For each beta diversity metric, we calculated the mean overall value and tested whether the mean value was different from that expected by chance using null models. We also employed evolutionary and spatially constrained models to first identify the degree to which the studied functional traits showed a phylogenetic signal, and then to estimate the relative importance of spatial and environmental effects on metacommunity structure. We first found that most individual ponds were inhabited by species that were merely random draws from the taxonomic and phylogenetic species pool available in the study region. Contrary to our expectations, not all measured traits were conserved along the phylogeny. We also showed that trait and phylogenetic dimensions strongly increased the amount of variation in beta diversity that can be explained by degree of environmental filtering and dispersal limitation. This suggests that accounting for functional traits and phylogeny in metacommunity ecology helps to explain idiosyncratic patterns of variation in macrophyte species distribution. Importantly, phylogenetic and functional analyses identified the influence of underlying mechanisms that would otherwise be missed in an analysis of taxonomic turnover. Together, these results let us conclude that macrophyte species have labile functional traits adapted to dispersal-based processes and some evolutionary trade-offs that drive community assembly via species sorting. Overall, our exploration of different facets of beta diversity showed how functional and phylogenetic information may be used with species-level data to test community assembly hypotheses that are more ecologically meaningful than assessments of environmental patterns based on the purely taxonomic viewpoint.

Future ocean climate homogenizes communities across habitats through diversity loss and rise of generalist species.

Predictions of the effects of global change on ecological communities are largely based on single habitats. Yet in nature, habitats are interconnected through the exchange of energy and organisms, and the responses of local communities may not extend to emerging community networks (i.e., metacommunities). Using large mesocosms and meiofauna communities as a model system, we investigated the interactive effects of ocean warming and acidification on the structure of marine metacommunities from three shallow-water habitats: sandy soft-bottoms, marine vegetation, and rocky reef substrates. Primary producers and detritus-key food sources for meiofauna-increased in biomass under the combined effect of temperature and acidification. The enhanced bottom-up forcing boosted nematode densities but impoverished the functional and trophic diversity of nematode metacommunities. The combined climate stressors further homogenized meiofauna communities across habitats. Under present-day conditions metacommunities were structured by habitat type, but under future conditions they showed an unstructured random pattern with fast-growing generalist species dominating the communities of all habitats. Homogenization was likely driven by local species extinctions, reducing interspecific competition that otherwise could have prevented single species from dominating multiple niches. Our findings reveal that climate change may simplify metacommunity structure and prompt biodiversity loss, which may affect the biological organization and resilience of marine communities.

Dispersal ability and niche breadth act synergistically to determine zooplankton but not phytoplankton metacommunity structure

AbstractIt is well known that both niche requirements and dispersal act together to structure communities; however, it remains unclear how the underlying mechanisms create the observed patterns in nature. In plankton communities, traits related to dispersal ability (e.g. body and propagule sizes) and niche breadth (i.e. habitat generalists and habitat specialists) have recently shown promise in this regard. Here, we hypothesized that body size (a proxy for dispersal ability) and niche breadth act together to determine limnetic plankton metacommunity structure in a tropical system of natural landlocked lakes in Brazil. Our hypotheses were partially supported for zooplankton community. Copepoda showed no relation to environmental variables, and the larger-bodied Calanoida was structured only by space. The Cladocera showed the largest spatial and environmental limitation. Rotifers showed the lowest association with space; however, its pure environmental fraction was higher than expected. The phytoplankton community was largely structured by both environment and space. The spatial limitation of this group was a clear divergence from our expectations. Lastly, we revealed that intragroup variation (when species were evaluated individually) was equal or larger than between group variation, indicating that group-specific traits may play a more important role in determining metacommunity structure.

Spatial and environmental factors are minor structuring forces in a soil Collembola metacommunity in a maize agroecosystem

Understanding the processes underlying species richness across multiple sites is a major challenge in ecology. Metacommunity theory assumes that local community assemblages are largely structured by spatial and environmental factors. However, the relative importance of these factors is not clear and has varied between studies. Most studies were conducted at sampling intervals that were short (<10 m), or long (>1 km). In this study, to address a dearth of data at intermediate sampling intervals, we examined the pattern of the soil Collembola metacommunity in an agroecosystem by sampling at 40 m intervals to try to disentangle the relative influence of spatial-environmental interactions on community composition. The soil Collembola community was sampled at 121 locations arranged regularly within a 400 m × 400 m corn field, over three months (August, September, and October 2015). We used standard geospatial analysis of metacommunity structure analytical framework to identify the pattern of soil Collembola metacommunity, with three essential elements (coherence, species turnover, and boundary clumping), to provide clues to the processes driving community assembly. We used variance partitioning analysis to determine the relative contributions of spatial and environmental factors (e.g., soil pH value, soil organic matter, soil total nitrogen and soil water content). From the results of the analysis, the soil Collembola metacommunity showed a nested pattern (rather than random, checkerboards, nested subsets, evenly spaced, Clementsian and Gleasonian patterns) on all three sampling dates. Soil Collembola metacommunities have a less restricted distribution along the environmental gradient, and connectivity may be important in shaping the community composition at this intermediate sampling interval. Based on the results of the variance partitioning analysis, spatial factors only explained 7.03–14.4% of the soil Collembola community variance, while the environmental factors were not significant at this scale. Our results suggest that both spatial and environmental factors are minor drivers of soil Collembola communities at an intermediate sampling frequency, compared to other studies conducted at shorter or longer sampling intervals.

Comparison of two maximum entropy models highlights the metabolic structure of metacommunities as a key determinant of local community assembly

The principle of Maximum Entropy (MaxEnt) promises a novel approach for understanding community assembly. Despite reproducing a variety of observed species abundance patterns, MaxEnt models in ecology have been hampered by disparate model assumptions and interpretations. A recurring challenge is that MaxEnt predictions are highly sensitive to the level of detail used to describe the community being modeled, and there seems to be no reason to prefer one level of detail over another. Here we present of formal unification of two previously developed MaxEnt models which differ in their level of detail, but which are otherwise mathematically similar. The less detailed model, “Maximum Entropy Theory of Ecology” (METE), does not resolve species identity or explicitly represent species-specific traits. The more detailed model, “Very Entropic Growth” (VEG), defines each separate species by its per capita metabolic rate ε and assumes a “density of species” function ρ(ε) representing the distribution of ε in the metacommunity. A formal comparison of METE and VEG then highlights ρ(ε) as a key determinant of local community assembly. In particular, appropriate choice of ρ(ε) in VEG can produce more realistic predictions for the metabolic-rank distribution of local communities than METE, which does not explicitly account for metacommunity structure. This opens new avenues of inquiry about what determines metacommunity structure in nature and suggests possible ways to improve METE.

Checkerboard metacommunity structure: an incoherent concept.

Checkerboards have emerged as a metaphor to (1) describe mutually exclusive patterns of co-occurrence for ecologically similar species that are geographically interspersed (i.e., checkerboard distributions), and (2) characterize relationships among species distributions along gradients that involve entire metacommunities (i.e., checkerboard metacommunity structure). Critical differences exist in the conceptual foundations that characterize these patterns. Checkerboard distributions are characterized by mutual exclusion of geographically interspersed species, usually pairs of ecologically similar species for which competition prevents syntopy. In contrast, checkerboard metacommunity structures are more restrictive: groups of species must exhibit mutually exclusive distributions, and each of these groups must be spatially independent of all other groups. Consequently, in a checkerboard metacommunity, competition defines one relationship for each species (i.e., that with its mutually exclusive partner), whereas independence characterizes all other interspecific associations. Consequently, a structure designed to be consistent with this concept will conclude that the metacommunity has random rather than checkerboard structure. Indeed, empirical checkerboard metacommunities are quite rare (7 of 766 reported empirical structures), and likely arise because of poor characterization of species ranges due to detection errors (i.e., a preponderance of rare or hard-to-detect species), rather than from underlying ecological mechanisms. Importantly, no ecological mechanism has been identified that is consistent with the concept of negative coherence. Consequently, the evaluation of checkerboards should be restricted to small sets of ecologically similar species for which interspecific interactions may lead to mutual exclusion, and coherence should be used only to evaluate if species distributions are more coherent than expected by chance (i.e., one-tailed tests).

Mass effects explain sapling community assembly in Araucaria mixed forest metacommunities

AbstractQuestionsWhat is the relative influence of: (a) traits related to dispersal (seed size) and growth (specific leaf area [SLA]), and; (b) regional species abundances, on the structure of local sapling communities? Which metacommunity models best explain the process of forest nucleation in grasslands?LocationWoody vegetation patches originating from nucleation and the continuous forest that neighbors native grasslands, in the southern Brazilian plateau (29°28′58.00″ S, 50°12′26.00″ W).MethodsWe calculated community‐weighted trait means (CWM) and functional diversity for woody saplings, considering species occurrences (FD) and abundances (FDis) separately for both seed size and SLA, in woody vegetation patches of different sizes and in the continuous forest. We tested whether mean SLA and mean seed size differed between habitat types, and whether values of FD and FDis were lower than expected by chance. We also evaluated whether the relative abundances of species in woody vegetation patches were determined by their abundances in the continuous forest, using CATS models (Community Assembly by Trait Selection). The results were then compared to the expectations from each metacommunity model.ResultsFunctional diversity of SLA and seed size did not differ from what was expected by chance in most sites. Species in communities of different habitat types did not differ in their mean SLA values, but we observed differences in mean seed size, with large‐seeded species being typical of areas with more shade. Species abundances in the regional species pool were more important factors than species traits for determining species abundances in the local communities.ConclusionsThe analytical framework employed in this study enables a straightforward test of different metacommunity structuring models. Mass effects were the metacommunity model that best explained the process of forest nucleation in the southern Brazilian plateau, which highlights the importance of dispersal for the structure of local communities.

Metacommunity patterns of Amazonian Odonata: the role of environmental gradients and major rivers.

BackgroundWe identified and classified damselfly (Zygoptera) and dragonfly (Anisoptera) metacommunities in Brazilian Amazonia, relating species distribution patterns to known biological gradients and biogeographical history. We expected a random distribution of both Zygoptera and Anisoptera within interfluves. At the Amazonian scale, we expected Anisoptera metacommunities to be randomly distributed due to their higher dispersal ability and large environmental tolerance. In contrast, we expected Zygoptera communities to exhibit a Clementsian pattern, limited by the large Amazonia rivers due to their low dispersal ability.MethodsWe used a dataset of 58 first-to-third order well-sampled streamlets in four Amazonian interfluves and applied an extension of the Elements of Metacommunity Structure (EMS) framework, in which we order Zygoptera and Anisoptera metacommunities by known spatial and biogeographic predictors.ResultsAt the Amazonian scale, both Zygoptera and Anisoptera presented a Clementsian pattern, driven by the same environmental and biogeographical predictors, namely biogeographic region (interfluve), annual mean temperature, habitat integrity and annual precipitation. At the interfluve scale, results were less consistent and only partially support our hypothesis. Zygoptera metacommunities at Guiana and Anisoptera metacommunities at Tapajós were classified as random, suggesting that neutral processes gain importance at smaller spatial scales.DiscussionOur findings were consistent with previous studies showing that environmental gradients and major rivers limit the distribution of Odonata communities, supporting that larger Amazonian rivers act as barriers for the dispersal of this group. In addition, the importance of habitat integrity indicates that intactness of riparian vegetation is an important filter shaping metacommunity structure of Amazonian stream Odonata.

Drivers of macroinvertebrate metacommunity structure in Tierra del Fuego rivers

Among the different mechanisms that shape metacommunity structure, species sorting and mass effects are the most studied and reported mechanisms. Species sorting is related to environmental filtering, while mass effects are related to high dispersal between communities, which ultimately overrides environmental controls. In this paper, we aimed at studying metacommunity patterns of stream macroinvertebrates on Tierra del Fuego Island at different spatial scales (province, ecoregion and catchment) and at different positions along the river network (upstream, mid-stream and downstream segments) to understand the mechanisms driving metacommunity structure. For this purpose, we used complementary approaches based on the analysis of beta diversity index as well as its turnover and nestedness components, which are associated with the two main underlying mechanisms of metacommunity structure (species sorting and mass effect). Our results indicate that species sorting is the preponderant mechanism structuring these macroinvertebrate metacommunities at larger spatial scales, with an increased importance of mass effects at smaller scales (e.g., catchment). Metacommunities at different positions on the river network exhibit similar total beta diversity values, turnover and nestedness components. On the other hand, turnover and total beta diversity increased with spatial extent, while nestedness remained constant. Those results suggest that at the catchment scale and along the longitudinal axis of the rivers other mechanisms, like randomness and species dispersal, are more important in shaping the macroinvertebrate metacommunity.

Freshwater Bacterioplankton Metacommunity Structure Along Urbanization Gradients in Belgium.

Urbanization is transforming and fragmenting natural environments worldwide, driving changes in biological communities through alterations in local environmental conditions as well as by changing the capacity of species to reach specific habitats. While the majority of earlier studies have been performed on higher plants and animals, it is crucial to increase our insight on microbial responses to urbanization across different spatial scales. Here, using a metacommunity approach, we evaluated the effects of urbanization on bacterioplankton communities in 50 shallow ponds in Belgium (Flanders region), one of the most urbanized areas in Northwest Europe. We estimated the relative importance of local environmental factors (35 abiotic and biotic variables), regional spatial factors and urbanization (built-up area) quantified at two spatial scales (200 m × 200 m and 3 km × 3 km). We show that urbanization at local or regional scales did not lead to strong changes in community composition and taxon diversity of bacterioplankton. Urbanization at regional scale (3 km × 3 km) explained only 2% of community composition variation while at local scale (200 m × 200 m), no effect was detected. Local environmental factors explained 13% (OTUs with relative abundance ≥ 0.1%) to 24% (12 dominant OTUs -≥ 1%) of community variation. Six local environmental variables significantly explained variation in bacterioplankton community composition: pH, alkalinity, conductivity, total phosphorus, abundance of Daphnia and concentration of copper (Cu), of which pH was partly mediated by urbanization. Our results indicate that environmental rather than spatial factors accounted for the variation in bacterioplankton community structure, suggesting that species sorting is the main process explaining bacterioplankton community assembly. Apparently, urbanization does not have a direct and strong effect on bacterioplankton metacommunity structure, probably due to the capacity of these organisms to adapt toward and colonize habitats with different environmental conditions and due to their fast adaptation and metabolic versatility. Thus, bacterioplankton communities inhabiting shallow ponds may be less affected by environmental conditions resulting from urbanization as compared to the impacts previously described for other taxa.

Metacommunity Structure of Helminths of Necromys Lasiurus (Rodentia: Sigmodontinae) in Different Land Use Areas in the Brazilian Cerrado

The ecology of host-parasite interactions can be studied in the infracommunity and component community scales, which may show different patterns in species distributions, interacting and affecting each other on a regional scale. Few studies have been carried out concerning the structure and variation of the helminth communities of wild rodents in Brazil. The rodent Necromys lasiurus is typical from the mammalian fauna of the Cerrado biome; however, the environmental disturbances are making this species occur in rural areas and in other biomes where it may act as host/reservoir of many diseases. This study aimed to describe the composition and structure of the helminth metacommunity in the rodent N. lasiurus in the Brazilian Cerrado, Uberlândia, State of Minas Gerais, using the analysis of metacommunity structure. Rodents were sampled in 3 habitat types: borders of soybean and corn plantations, pasturelands, and preserved areas of Cerrado borders. Adult helminths of 8 species were found in the gastrointestinal tract: Protospirura numidica criceticola, Physaloptera sp., Pterygodermatites ( Paucipectines) zygodontomis (Spirurida), Stilestrongylus freitasi (Rhabditida), Trichuris navonae (Trichurida) and Syphacia ( Syphacia) alata (Oxiurida) of the Phylum Nematoda; Rodentolepis akodontis (Cyclophyllidea) of the Phylum Platyhelminthes; and Moniliformis sp. (Moniliformida) of the Phylum Acanthocephala. Season and the kind of land use favored some helminths species in this rodent, especially in the plantation area, although diversity was not largely influenced by the land use. Plantation areas could provide an increase in the host abundance and the occurrence of other rodent species, favoring a higher rate of parasite exchange among different hosts. A checkerboard structure of metacommunity was found on the infracommunity scale, which suggests the existence of interspecific competition. A quasi-nested structure of metacommunity was observed on the component community scale showing that most species were influenced by the same environmental gradient and that the species-poor communities were subsets of species-rich communities. Syphacia alata, P. zygodontomis, S. freitasi, and R. akodontis were dominant species in all habitats and represented the core-species in the metacommunity.

Hydroperiod and species-sorting influence metacommunity composition of crustaceans in temporary rock pools in India

ABSTRACTMost studies on crustacean metacommunity ecology do not consider constituent higher-level taxa separately, despite ecological differences among them. This oversight, coupled with the fact that observed patterns can vary in different regions, prompted our study in the megadiverse, yet understudied, Western Ghats region in India. We studied temporary rock pools on a lateritic outcrop to identify factors influencing species richness, metacommunity structure, and spatial patterns of constituent crustacean groups. We used Spearman’s correlation, ordination-based methods, partial Mantel tests, distance-decay patterns, and β-diversity partitioning to analyse the influence of spatial and ecological factors on distribution of taxa and community composition. Overall crustacean species richness was strongly influenced by hydroperiod; some influence of other parameters such as maximum surface area and depth on species richness of individual taxa was also observed. Hydroperiod had the strongest influence on the community composition, and community dissimilarity increased with environmental and interpool distance. High turnover among the communities and significant influence of environmental factors indicated species-sorting processes were structuring the crustacean metacommunity. The communities of copepods and cladocerans showed higher nestedness while those of ostracods and large branchiopods showed higher turnover. We discuss the metacommunity structure observed at a small spatial scale in relation to the ecology of constituent crustacean groups, habitat, and landscape features.

Local habitat heterogeneity determines the differences in benthic diatom metacommunities between different urban river types

Benthic diatoms are useful indicators of the ecological state of river systems. To understand the factors determining benthic diatom metacommunity composition in urban rivers, we studied in situ surface sediment diatom communities from 23 rivers in Shanghai City. Based on our study results on the metacommunity structure of benthic diatoms and hierarchical cluster analysis (HCA), we found substantial differences between restored (G1) and unrestored rivers (G2–G4) in taxa richness, relative abundances, and dominant and indicator taxa of benthic diatoms. The epiphytic diatoms Cocconeis placentula and Amphora libyca var. baltica were representative of the restored rivers (G1), where aquatic macrophytes were more abundant and the water was clearer. The motile epipelic diatoms Navicula recens and Navicula germainii dominated the moderately polluted rivers (G2). The eutrophic taxa Cyclotella meneghiniana, Aulacoseira granulata, and Cyclostephanos tholiformis dominated in G3, which comprised relatively heavily polluted rivers with low organic matter sediment and high disturbance. The polysaprobic taxon Nitzschia palea and the halophilous taxon Fallacia pygmaea represented relatively heavily polluted rivers with a comparatively higher sedimentary salinity (SSal) (G4). Redundancy analysis (RDA) revealed that total phosphorus (TP), dissolved organic carbon (DOC), silicon dioxide (SiO2), dissolved oxygen (DO), Secchi depth (SD), SSal, and the ratio of carbon to nitrogen (C/N) in the sediment were important environmental factors explaining variation among benthic diatom metacommunity composition. Partial RDA (pRDA) implied that the relative importance of environmental factors in structuring benthic diatom metacommunity was much higher than spatial factors. Classification and regression trees (CART) further indicated that DOC, the sediment C/N ratio, and SSal were the key local environmental factors affecting grouping patterns of benthic diatom metacommunities. Our study proposes that benthic diatom metacommunities respond to the complex characteristics of local environment in urban rivers and provides useful knowledge for consideration in the ecological monitoring of urban river systems.

Phylogenetic and spatial analyses suggest minimum temperature as an environmental filter for turtle communities

AbstractAimThis study aims to evaluate phylogenetic structure of turtle communities and their potential correlates on a global scale. More specifically, we tested whether cold temperatures and low precipitation could act as potential environmental filters of turtle communities and whether the dispersal limitation caused by altitudinal range could generate phylogenetic clustered patterns in these communities.LocationGlobal.TaxonTurtles.MethodsWe used phylogenetic and distributional data of 257 species of turtles on a grid of 100 km × 100 km to generate measures of phylogenetic structure (mean nearest taxon distance and the mean pairwise distance, and their standardized effect sizes—SES) of turtle communities using three different species pools. We evaluated the relationship between SESs with climatic and altitudinal data in order to evaluate our hypotheses. Finally, we inspected whether there was phylogenetic signal in the climatic variables classified as potential filters, as it is a basic assumption for using phylogenetic structure metrics to infer assembly rules.ResultsPhylogenetic clustering was generally stronger than phylogenetic overdispersion in turtle communities. There was a positive relationship between minimum temperature and the phylogenetic structure metrics, while minimum precipitation and altitudinal range were negatively related to these metrics. We found a strong phylogenetic signal in the minimum temperature, while precipitation had a weak signal.Main conclusionsThe positive correlation between phylogenetic community structure metrics and minimum temperature and the phylogenetic signal present in this climatic variable are in accordance with our initial hypothesis regarding minimum temperature acting as an environmental filter for turtles. However, we did not find evidence to support minimum precipitation as a filter. Altitudinal range followed the expected patterns, suggesting that it increases dispersal limitation and allows a concentration of closely related species.

Spatial scale modulates the inference of metacommunity assembly processes.

The abundance and distribution of species across the landscape depend on the interaction between local, spatial, and stochastic processes. However, empirical syntheses relating these processes to spatiotemporal patterns of structure in metacommunities remain elusive. One important reason for this lack of synthesis is that the relative importance of the core assembly processes (dispersal, selection, and drift) critically depends on the spatial grain and extent over which communities are studied. To illustrate this, we simulated different aspects of community assembly on heterogeneous landscapes, including the strength of response to environmental heterogeneity (inherent to niche theory) vs. dispersal and stochastic drift (inherent to neutral theory). We show that increasing spatial extent leads to increasing importance of niche selection, whereas increasing spatial grain leads to decreasing importance of niche selection. The strength of these scaling effects depended on environment configuration, dispersal capacity, and niche breadth. By mapping the variation observed from the scaling effects in simulations, we could recreate the entire range of variation observed within and among empirical studies. This means that variation in the relative importance of assembly processes among empirical studies is largely scale dependent and cannot be directly compared. The scaling coefficient of the relative contribution of assembly processes, however, can be interpreted as a scale-integrative estimate to compare assembly processes across different regions and ecosystems. This emphasizes the necessity to consider spatial scaling as an explicit component of studies intended to infer the importance of community assembly processes.

Multiple stressors and the role of hydrology on benthic invertebrates in mountainous streams

Hydrological alteration of rivers is recognised as a major threat to lotic biodiversity acting at broad spatial scales, however, the effect size and pathways of hydrology are rarely quantified in comparison with other stressors such as land use and physico-chemistry. Here we present a multiple stressor study that aims to disentangle the effect sizes and pathways of hydrological alteration on benthic invertebrate community structure and functional metrics. Therefore, we analyse the following four multiple stressor groups: land use, hydrology, physical habitat structure, and physico-chemistry at 51 sites including 72 surveys in the German mountain range. Stressor data were contrasted to benthic invertebrate data using partial canonical correspondence analysis to quantify the community-level response and path analysis to investigate the cause-effect pathway structure of single stressors affecting benthic invertebrate metrics either directly or indirectly (i.e. mediated by other stressors). Hydrological stressors showed a strong impact on community structure, with its unique effects being more dominant than those of any other stressor group. Path analysis confirmed strong direct effects of hydrological stressors on biological metrics but revealed land use to be the most influential stressor group in terms of the sum of direct and indirect effects on biology. Notably, indirect land use effects are mediated by hydrology. Our findings suggest a key role of hydrological stressors in lotic ecosystem assessment, which, however, are rarely addressed in operational river monitoring and management. In light of the wide-spread availability of hydrological data from gauging stations throughout Europe, we plea for a better involvement of hydrological data in river basin management.

Planktonic diatom community dynamics in a tropical flood-pulse lake: the Tonle Sap (Cambodia)

It has been proposed that, within flood-pulse systems, seasonally dynamic conditions promote extremely high levels of biodiversity. In this paper, we aim to understand how variation in environmental conditions and habitat availability, across space and time, influences the structure and composition of plankton diatom communities within the Tonle-Sap Lake (Cambodia) during two flood-pulse cycles. We hypothesize that (i) communities vary considerably across space and time, (ii) environmental filtering accounts for a substantial portion of the variation in community composition, and (iii) shifts in meta-community structure occur seasonally, due to variation in both environmental conditions and habitat availability. A principal component analysis and a permutational multivariate analysis of variance were used to characterize spatio-temporal variations in environmental conditions, community structure and composition. Determinants of community variations across space were identified by redundancy analyses and variation partitioning, while seasonal changes in meta-community structure were assessed by investigating temporal changes in β-diversity. Our results indicate that (i) spatial variations in community structure and composition are largely influenced by the seasonal flood-pulse, (ii) environmental filtering is the most likely process driving the compositional changes, and (iii) changes in diatom life-form community provide insights into the hydrological functioning of the Tonle Sap. We conclude that the survey of diatom communities may constitute a relevant sensor of hydrological change in the system and could be used to derive regionally specific hypotheses about how global climate change is impacting the lake’s functioning.

An ancestral process with selection in an ecological community

An ecological community is a geographical area composed of two or more species. The ancestral histories of individuals from the same and different species in an ecological community may be interconnected due to direct and indirect interactions. Here, we present a model of the ancestral history of an ecological community that is built upon the framework of coalescent and ancestral graph theory. The model includes selection, whereby the fitness of an ancestral lineage is a function of both its abiotic environment and interactions with individuals from its biotic environment. The model also allows for metacommunity structure. We first define a forward-time percolation process characterizing the evolution of an ecological community and then present its corresponding backward-time graphical model in the limit of large population sizes. Next, we present expectations of properties of phenotypes in the graph. These expectations give insight into the structure of phenotypic variation and trait-environment covariances across local communities, including the effects of drift, intra and inter-species genealogical structure and the sampling effects of selection. In addition, we derive expectations for multivariate phenotypic diversity in a community assuming neutrality and compare this to expectations with stabilizing selection.

Do metacommunity theories explain spatial variation in fish assemblage structure in a pristine tropical river?

Abstract 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 in this system must emphasise maintenance of habitat heterogeneity and connectivity.