Stochastic Community Assembly Processes Research Articles

Organic carbon negatively affects the diversity of soil nitrous oxide reducers in Chinese fir plantations at a regional scale

Large-scale planting of Chinese fir (Cunninghamia lanceolata) can bring considerable economic benefits but also seriously alters soil properties, disrupting soil microbial survival patterns and giving rise to distinct microbial communities. Nitrous oxide (N2O) reduction is the sole recognized biological sink for N2O, thereby playing a critical role in forest nitrogen cycling. However, the distribution of N2O reducers in Chinese fir plantation soils and the underlying factors at a regional scale remain unclear. Here, we assessed the abundance, diversity, and community structure of nosZ I-type N2O reducers in soil samples collected from nine state-owned forest farms in Fujian Province, China. The abundance of nosZ I-type reducers exhibited significant variation across sites, with altitude exerting the greatest influence, positively influencing their abundance. Soil organic carbon (SOC) and dissolved organic carbon (DOC) were the strongest predictors of alpha-diversity. Interestingly, SOC and DOC exhibited a negative association with nosZ I-type reducer alpha-diversity. Additionally, DOC played a significant role in influencing community structure. All nosZ I sequences were associated with Alphaprotebacteria and Betaproteobacteria, with Alphaproteobacteria dominating nosZ I-type reducer communities in all soils, accounting for over 90 % of the total sequences. Moreover, the modified stochasticity ratio values exceeded 0.5 at all sampling sites except for one, characterized by the lowest pH and relatively lower nutrient content, indicating the predominance of stochastic processes in community assembly. Taken together, our results provide novel evidence that the augmentation of organic carbon content could potentially reduce diversity and alter the structure of nosZ I-type reducer communities, with potential implications for N2O emissions in Chinese fir plantations.

Enhanced AHL-mediated quorum sensing accelerates the start-up of biofilm reactors by elevating the fitness of fast-growing bacteria in sludge and biofilm communities

Quorum sensing (QS)-based manipulations emerge as a promising solution for biofilm reactors to overcome challenges from inefficient biofilm formation and lengthy start-ups. However, the ecological mechanisms underlying how QS regulates microbial behaviors and community assembly remain elusive. Herein, by introducing different levels of N-acyl-homoserine lactones, we manipulated the strength of QS during the start-up of moving bed biofilm reactors and compared the dynamics of bacterial communities. We found that enhanced QS elevated the fitness of fast-growing bacteria with high ribosomal RNA operon (rrn) copy numbers in their genomes in both the sludge and biofilm communities. This led to notably increased extracellular substance production, as evidenced by strong positive correlations between community-level rrn copy numbers and extracellular proteins and polysaccharides (Pearson's r = 0.529−0.830, P < 0.001). Network analyses demonstrated that enhanced QS significantly promoted the ecological interactions among taxa, particularly cooperative interactions. Bacterial taxa with higher network degrees were more strongly correlated with extracellular substances, suggesting their crucial roles as public goods in regulating bacterial interactions and shaping network structures. However, the assembly of more cooperative communities in QS-enhanced reactors came at the cost of decreased network stability and modularity. Null model and dissimilarity-overlap curve analysis revealed that enhanced QS strengthened stochastic processes in community assembly and rendered the universal population dynamics more convergent. Additionally, these shaping effects were consistent for both the sludge and biofilm communities, underpinning the planktonic-to-biofilm transition. This work highlights that QS manipulations efficiently drive community assembly and confer specialized functional traits to communities by recruiting taxa with specific life strategies and regulating interspecific interactions. These ecological insights deepen our understanding of the rules governing microbial societies and provide guidance for managing engineering ecosystems.

Ecological determinants of 17α-ethynylestradiol biodegradation: Unveiling unique microbial community assemblages in lake sediments under nitrate or sulfate reduction

Nitrate and sulfate, as electron acceptors (EAs), influence microbial communities and the subsequent biodegradation of emerging contaminants, such as 17α-ethynylestradiol (EE2) in anaerobic sediments. Results of this study showed that nitrate amendment significantly raised nitrate reduction intensity within a short period (<5 days) and greatly influenced microbial community succession compared with sulfate amendment. Nitrate reduction induced deterministic selections that narrowed the niche widths of the microbial community, creating direct correlations between functional members, including denitrifiers and potential estrogen degraders like Pseudomonas. The facilitative coordination of microbes and increased denitrification resulted in greater EE2 biodegradation under nitrate-reducing (0.19 ± 0.02 μg g−1) and combined nitrate- and sulfate-reducing conditions (0.23 ± 0.01 μg g−1) after 120 days. Moreover, natural organic matter enhanced interspecies syntrophic cooperation (the number of direct connectors increased from 9 to 25 in the microbial network) in the microbial community and further improved EE2 biodegradation efficiency to 0.26 ± 0.03 μg g−1. Conversely, sulfate reduction required longer adaptation (>20 days) with a higher contribution of stochastic processes in community assembly, yielding fewer connections with estrogen degraders and weaker EE2 biodegradation (0.15 ± 0.03 μg g−1). These mechanistic understandings can guide EA selection in the bioremediation of emerging contaminants.

Biogeographic patterns of meio- and micro-eukaryotic communities in dam-induced river-reservoir systems.

Although the Three Gorges Dam (TGD) is the world's largest hydroelectric dam, little is known about the spatial-temporal patterns and community assembly mechanisms of meio- and micro-eukaryotes and its two subtaxa (zooplankton and zoobenthos). This knowledge gap is particularly evident across various habitats and during different water-level periods, primarily arising from the annual regular dam regulation. To address this inquiry, we employed mitochondrial cytochrome c oxidase I (COI) gene-based environmental DNA (eDNA) metabarcoding technology to systematically analyze the biogeographic pattern of the three communities within the Three Gorges Reservoir (TGR). Our findings reveal distinct spatiotemporal characteristics and complementary patterns in the distribution of meio- and micro-eukaryotes. The three communities showed similar biogeographic patterns and assembly processes. Notably, the diversity of these three taxa gradually decreased along the river. Their communities were less shaped by stochastic processes, which gradually decreased along the longitudinal riverine-transition-lacustrine gradient. Hence, deterministic factors, such as seasonality, environmental, and spatial variables, along with species interactions, likely play a pivotal role in shaping these communities. Environmental factors primarily drive seasonal variations in these communities, while hydrological conditions, represented as spatial distance, predominantly influence spatial variations. These three communities followed the distance-decay pattern. In winter, compared to summer, both the decay and species interrelationships are more pronounced. Taken together, this study offers fresh insights into the composition and diversity patterns of meio- and micro-eukaryotes at the spatial-temporal level. It also uncovers the mechanisms behind community assembly in various environmental niches within the dam-induced river-reservoir systems. KEY POINTS: • Distribution and diversity of meio- and micro-eukaryotes exhibit distinct spatiotemporal patterns in the TGR. • Contribution of stochastic processes in community assembly gradually decreases along the river. • Deterministic factors and species interactions shape meio- and micro-eukaryotic community.

Root-centric β diversity reveals functional homogeneity while phylogenetic heterogeneity in a subtropical forest.

Root-centric studies have revealed fast taxonomic turnover across root neighborhoods, but how such turnover is accompanied by changes in species functions and phylogeny (i.e., β diversity) remains largely unknown. As β diversity can reflect the degree of community-wide biotic homogenization, such information is crucial for better inference of below-ground assembly rules, community structuring, and ecosystem processes. We collected 2480 root segments from 625 0-30 cm soil profiles in a subtropical forest in China. Root segments were identified into 138 species with DNA-barcoding with six root morphological and architectural traits measured per species. By using the mean pairwise (Dpw ) and mean nearest neighbor distance (Dnn ) to quantify species ecological differences, we first tested the non-random functional and phylogenetic turnover of root neighborhoods that would lend more support to deterministic over stochastic community assembly processes. Additionally, we examined the distance-decay pattern of β diversity, and finally partitioned β diversity into geographical and environmental components to infer their potential drivers of environmental filtering, dispersal limitation, and biotic interactions. We found that functional turnover was often lower than expected given the taxonomic turnover, whereas phylogenetic turnover was often higher than expected. Phylogenetic Dpw (e.g., interfamily species) turnover exhibited a distance-decay pattern, likely reflecting limited dispersal or abiotic filtering that leads to the spatial aggregation of specific plant lineages. Conversely, both functional and phylogenetic Dnn (e.g., intrageneric species) exhibited an inverted distance-decay pattern, likely reflecting strong biotic interactions among spatially and phylogenetically close species leading to phylogenetic and functional divergence. While the spatial distance was generally a better predictor of β diversity than environmental distance, the joint effect of environmental and spatial distance usually overrode their respective pure effects. These findings suggest that root neighborhood functional homogeneity may somewhat increase forest resilience after disturbance by exhibiting an insurance effect. Likewise, root neighborhood phylogenetic heterogeneity may enhance plant fitness by hindering the transmission of host-specific pathogens through root networks or by promoting interspecific niche complementarity not captured by species functions. Our study highlights the potential role of root-centric β diversity in mediating community structures and functions largely ignored in previous studies.

Transformation of Biofilm to Carbon Sinks after Prolonged Droughts Linked with Algal Biodiversity Change.

Global climate change significantly increased the duration of droughts in intermittent rivers, impacting benthic microbial-mediated biogeochemical processes. However, the impact of prolonged droughts on the carbon contribution of intermittent rivers remains poorly understood. In this study, we investigated the potential effects of varying drought gradients (ranging from 20 to 130 days) on benthic biofilms community structure (algae, bacteria, and fungi) and their carbon metabolism functions (ecosystem metabolism and carbon dioxide (CO2) emission fluxes) using mesocosm experiments. Our findings indicate that longer drought durations lead to reduced alpha diversity and community heterogeneity, tighter interdomain networks, and an increased role of stochastic processes in community assembly, with a discernible threshold at around 60 days. Concurrently, the biofilm transforms into a carbon sink following a drought period of 60 days, as evidenced by the transformation of CO2 emission fluxes from 633.25 ± 194.69 to -349.61 ± 277.79 mg m-2 h-1. Additionally, the partial least-squares path model revealed that the resilience of algal communities and network stability may drive biofilm's transformation into a carbon sink, primarily through the heightened resilience of autotrophic metabolism. This study underscores the significance of the carbon contribution from intermittent rivers, as the shift in carbon metabolism functions with increasing droughts could lead to skewed estimations of current riverine carbon fluxes.

Linking process to pattern in community assembly in dry evergreen Afromontane forest of Hararghe highland, Southeast Ethiopia

Abstract&#x0D; &#x0D; Many scholars have attempted to identify the role of deterministic and stochastic processes in community assembly, but there is no consensus on which processes dominate and at what spatial scales they occur. To shed light on this issue, we tested two non-exclusive processes, scale-dependent hypotheses: (i) that limiting similarity dominates on small spatial scales; and (ii) that environmental filtering does so on a large scale. To achieve this, we studied the functional patterns of dry evergreen Afromontane forest communities along elevation gradients in southeastern Ethiopia using floristic and functional trait data from fifty-four 0.04 ha plots. We found evidence of functional overdispersion on small spatial scales, and functional clustering on large spatial scales. The observed clustering pattern, consistent with an environmental filtering process, was most evident when environmental differences between a pair of plots were maximized. To strengthen the link between the observed community functioning pattern and the underlying process of environmental filtering, we demonstrated differences in the topographical factors of the most abundant species found at lower and higher elevations and examined whether their abundance varied over time or changed with time along the elevation. We found (i) that the largest functional differences in the community (observed between lower and upper dry evergreen Afromontane forest assemblages) were primarily the result of strong topographical influence; and (ii) that the abundance of such species varied along the elevation gradient. Variation in stand structure and tree species diversity within the DAF plots shows that topography is among the important drivers of local species distribution and hence the maintenance of tree diversity in dry Afromontane forest.&#x0D; Our results support the conclusion that environmental filtering at large spatial scales is the primary mechanism for community merging, since functional grouping pattern was associated with species similarities in topographic variation, ultimately leading to changes in species abundances along the gradient. There was also evidence of competitive exclusion at more homogeneous and smaller spatial scales, where plant species compete effectively for resources.

Contrasting effects of two phenotypes of an alpine cushion plant on understory species drive community assembly

In alpine systems, cushion plants act as foundation species by ameliorating local environmental conditions. Empirical studies indicate that contrasting phenotypes of alpine cushion species have different effects on understory plant species, either facilitative or competitive. Furthermore, dependent species within each community type might also exhibit different responses to each cushion phenotype, which can be clustered into several “response groups”. Additionally, these species-groups specific responses to alpine cushion species phenotypes could alter community assembly. However, very few studies have assessed responses of dependent communities at species-group levels, in particular for both above- and below-ground communities. Here, we selected a loose and a tight phenotype of the alpine cushion species Thylacospermum caespitosum in two sites of northwest China, and use the relative intensity of interactions index to quantify cushion plant effects on subordinate communities of plants and soil fungi and bacteria. We assessed variations in responses of both above- and below-ground organisms to cushion plant effects at species-group level. Species-group level analyses showed that the effects of the phenotype varied among groups of each of the three community types, and different species-groups were composed by unique taxa. Additionally, we found that loose cushions enhanced stochastic processes in community assembly, for plants and soil fungi but not for soil bacteria. These variations of phenotypic effects on different species-group induced contrasting taxonomic composition between groups, and alter community assembly thereby. Our study highlights the occurrence of contrasting effects of two phenotypes of a foundation cushion plant on understory plants, soil fungi and bacteria community composition, but not necessarily on their richness. We also showed that assessing responses of understory species at the species-group level allows a more realistic and mechanistic understanding of biotic interactions both for above- and below-ground communities.

Aridity drives the loss of dung beetle taxonomic and functional diversity in three contrasting deserts

AbstractAimAridity gradients are of great interest for understanding the responses of biodiversity to water availability and water stress. However, little is known about the responses of many animal groups, which are crucial for assessing the effects of climate change. Here, we study the effects of aridity on dung beetle communities, a group with well‐known responses to large‐scale environmental gradients.LocationSahara, Kalahari and Chihuahuan deserts.TaxaDung beetles of the family Scarabaeidae.MethodsWe conducted standardized surveys along approximately 400 km aridity gradients in each of the three deserts, and measured species richness, abundance, evenness and three aspects of trait‐based functional diversity (functional richness, functional evenness and functional dispersion). By using randomization tests and linear mixed models, we compared observed with expected values for functional diversity indices from null models that hierarchically incorporate additional assembly constraints.ResultsOverall, we found a decrease of both taxonomic richness and functional dispersion along the three aridity gradients. Also, aridity seems to have mild effects on functional richness and functional evenness. Besides these general trends, we identified differences between deserts in the responses of both taxonomic and functional diversity.Main conclusionsAridity shows greater importance than competition and other processes of limiting similarity or stochastic processes in community assembly. Also, the functional hypervolume of dung beetle desert communities decreases with aridity not only due to species loss, but also because of selection of a few distinct phenotypes under harsh environmental conditions. Last, we observed that the different regional pools respond to aridity in different ways. Therefore, understanding future responses of dung beetle communities to the progressive decreases in water availability driven by climate change requires determining how the characteristics of the species in the regional pool interact with aridity‐driven assembly processes.

Brassica napus Bacterial Assembly Processes Vary with Plant Compartment and Growth Stage but Not between Lines.

ABSTRACTHolobiont bacterial community assembly processes are an essential element to understanding the plant microbiome. To elucidate these processes, leaf, root, and rhizosphere samples were collected from eight lines of Brassica napus in Saskatchewan over the course of 10 weeks. We then used ecological null modeling to disentangle the community assembly processes over the growing season in each plant part. The root was primarily dominated by stochastic community assembly processes, which is inconsistent with previous studies that suggest of a highly selective root environment. Leaf assembly processes were primarily stochastic as well. In contrast, the rhizosphere was a highly selective environment. The dominant rhizosphere selection process leads to more similar communities. Assembly processes in all plant compartments were dependent on plant growth stage with little line effect on community assembly. The foundations of assembly in the leaf were due to the harsh environment, leading to dominance of stochastic effects, whereas the stochastic effects in the root interior likely arise due to competitive exclusion or priority effects. Engineering canola microbiomes should occur during periods of strong selection assuming strong selection could promote beneficial bacteria. For example, engineering the microbiome to resist pathogens, which are typically aerially born, should focus on the flowering period, whereas microbiomes to enhance yield should likely be engineered postflowering as the rhizosphere is undergoing strong selection.IMPORTANCE In order to harness the microbiome for more sustainable crop production, we must first have a better understanding of microbial community assembly processes that occurring during plant development. This study examines the bacterial community assembly processes of the leaf, root, and rhizosphere of eight different lines of Brassica napus over the growing season. The influence of growth stage and B. napus line were examined in conjunction with the assembly processes. Understanding what influences the assembly processes of crops might allow for more targeted breeding efforts by working with the plant to manipulate the microbiome when it is undergoing the strongest selection pressure.

Spatial heterogeneity in temporal dynamics of Alpine bird communities along an elevational gradient

AbstractAimMountains are biodiversity hotspots and are among the most sensitive ecosystems to ongoing global change being thus of conservation concern. Under this scenario, assessing how biological communities vary over time along elevational gradients and the relative effects of niche‐based deterministic processes and stochastic events in structuring assemblages is essential. Here, we examined how the temporal trends of bird communities vary with elevation over a 20 year‐period (1999–2018). We also tested for differences in temporal dynamics among habitat types (among‐community variability) and functional groups (within‐community variability).Taxon97 species of common breeding birds.LocationSwiss Alps.MethodsWe used abundance data from the Swiss breeding bird survey to compute different temporal dynamic metrics (temporal turnover, synchrony, rate of community change and community‐level of covariance among species). We also examined the relative contribution of deterministic and stochastic processes in community assembly using the Raup‐Crick method and the normalized stochasticity ratio.ResultsWe found that, with greater elevation, temporal species turnover increased while the rate of overall community change over successive years decreased, suggesting that high‐elevation communities display more erratic dynamics with no clear trend. Despite this, we found a more deterministic assembly of alpine communities in comparison to those located at lower elevations. Deterministic processes had greater influence than stochastic processes on community assembly along the entire elevational gradient (80% of communities). Forest communities exhibited higher synchrony in comparison to the remaining habitats likely because they consisted of species with greater functional redundancy, whereas alpine communities were the least stable as a result of their low taxonomic richness (‘portfolio’ effect).Main conclusionsCommunity‐level synchrony was overall positive supporting the idea that compensatory mechanisms are rare in natural biological communities. Our results suggest that rather than competition, the existence of differences in the ecological strategies of species may have a stabilizing effect on bird communities by weakening the concordance of species responses to fluctuations in environmental conditions (i.e. enhanced interspecific temporal asynchrony). This study provides evidence that, although species turnover in metacommunities is frequent, a high temporal turnover does not necessarily imply the overriding importance of stochastic processes.

Linking patterns and processes of tree community assembly across spatial scales in tropical montane forests.

Many studies have tried to assess the role of both deterministic and stochastic processes in community assembly, yet a lack of consensus exists on which processes are more prevalent and at which spatial scales they operate. To shed light on this issue, we tested two nonmutually exclusive, scale-dependent hypotheses: (1) that competitive exclusion dominates at small spatial scales; and (2) that environmental filtering does so at larger ones. To accomplish this, we studied the functional patterns of tropical montane forest communities along two altitudinal gradients, in Ecuador and Peru, using floristic and functional data from 60 plots of 0.1ha. We found no evidence of either functional overdispersion or clustering at small spatial scales, but we did find functional clustering at larger ones. The observed pattern of clustering, consistent with an environmental filtering process, was more evident when maximizing the environmental differences among any pair of plots. To strengthen the link between the observed community functional pattern and the underlying process of environmental filtering, we explored differences in the climatic preferences of the most abundant species found at lower and higher elevations and examined whether their abundances shifted along the elevation gradient. We found (1) that greater community functional differences (observed between lower and upper tropical montane forest assemblies) were mostly the result of strong climatic preferences, maintained across the Neotropics; and (2) that the abundances of such species shifted along the elevational gradient. Our findings support the conclusion that, at large spatial scales, environmental filtering is the overriding mechanism for community assembly, because the pattern of functional clustering was linked to species' similarities in their climatic preferences, which ultimately resulted in shifts in species abundances along the gradient. However, there was no evidence of competitive exclusion at more homogeneous, smaller spatial scales, where plant species effectively compete for resources.

Stochastic and deterministic processes drive wetland community assembly across a gradient of environmental filtering

The role of deterministic and stochastic processes in community assembly is a key question in community ecology. We evaluated the effect of an abiotic filter (hydroperiod) on the partitioned diversity of three taxonomic groups (birds, vegetation, macroinvertebrates) from prairie pothole wetlands in Alberta, Canada, which naturally vary in water permanence. We observed that alpha and gamma diversity were higher in permanent than temporary wetlands (16–25% and 34–47% respectively, depending on the taxon). This suggests an influence of deterministic constraints on the number of species a wetland can support. Taxa which cannot persist in shallow, temporary wetlands are excluded by the deterministic constraints that a shortened hydroperiod imposes. In contrast, we observed that beta diversity was significantly higher (2–12%) in temporary wetlands than permanent ones, and temporary wetlands supported more unique combinations of community composition than permanent wetlands, despite having a smaller regional species pool. This observation contradicts prior mesocosm studies that found beta diversity mirrored the pattern in gamma diversity along an environmental filtering gradient. We conclude that deterministic processes are more influential in more stable permanent wetlands, whereas stochastic processes play a more important role in assembly in dynamic temporary wetlands that must disassemble and re‐establish annually. Considering three distinct taxonomic groups differing in their relative mobility, our large‐scale field study demonstrates that both stochastic and deterministic processes act together to influence the assembly of multiple communities and that the relative importance of the two processes varies consistently along a gradient of environmental filtering.

Patterns of phylogenetic community structure of sand dune plant communities in the Yucatan Peninsula: the role of deterministic and stochastic processes in community assembly

ABSTRACTBackground: Tropical sand dunes are ideal systems for understanding drivers of community assembly as dunes are subject to both deterministic and stochastic processes. However, studies that evaluate the factors that mediate plant community assembly in these ecosystems are few.Aims: We evaluated phylogenetic community structure to elucidate the role of deterministic and stochastic processes in mediating the assembly of plant communities along the north of the Yucatan Peninsula, Mexico.Methods: We used plastid genetic markers to evaluate phylogenetic relationships in 16 sand-dune communities. To evaluate the role of climate in shaping plant community structure we carried out linear regressions between climatic variables and mean phylogenetic distance. We estimated the Net Relatedness Index and Nearest Taxon Index to identify ecological processes mediating community assembly.Results: Observed phylogenetic structure was not different from random, suggesting that stochastic processes are the major determinants of community assembly. Climate was slightly correlated with phylogenetic diversity suggesting that abiotic environment plays a minimal role in community assembly.Conclusions: Random assembly appears to be the primary factor structuring the studied sand dune plant communities. Environmental filters may represent a secondary factor contributing to the observed phylogenetic structure. Thus, both processes may act simultaneously to mediate the assembly of sand-dune plant communities.

Extensive Overlap of Tropical Rainforest Bacterial Endophytes between Soil, Plant Parts, and Plant Species.

The extent to which distinct bacterial endophyte communities occur between different plant organs and species is poorly known and has implications for bioprospecting efforts. Using the V3 region of the bacterial 16S ribosomal RNA (rRNA) gene, we investigated the diversity patterns of bacterial endophyte communities of three rainforest plant species, comparing leaf, stem, and root endophytes plus rhizosphere soil community. There was extensive overlap in bacterial communities between plant organs, between replicate plants of the same species, between plant species, and between plant organ and rhizosphere soil, with no consistent clustering by compartment or host plant species. The non-metric multidimensional scaling (NMDS) analysis highlighted an extensively overlapping bacterial community structure, and the β-nearest taxon index (βNTI) analysis revealed dominance of stochastic processes in community assembly, suggesting that bacterial endophyte operational taxonomic units (OTUs) were randomly distributed among plant species and organs and rhizosphere soil. Percentage turnover of OTUs within pairs of samples was similar both for plant individuals of the same species and of different species at around 80-90%. Our results suggest that sampling extra individuals, extra plant organs, extra species, or use of rhizosphere soil, might be about equally effective for obtaining new OTUs for culture. These observations suggest that the plant endophyte community may be much more diverse, but less predictable, than would be expected from culturing efforts alone.

Quasi-neutral community assembly: Evidence from niche overlap, phylogenetic, and trait distribution analyses of a subtropical forest in South America

In spite of recent advances, the relative roles of deterministic and stochastic processes in community assembly remains disputed. We tested community assembly hypotheses in a disturbance gradient in a logged mixed conifer-hardwood forest complex in southern Brazil. If abiotic heterogeneity was strong enough (Selection hypothesis), we expected niche clustering and reduced niche overlap between species and between ecological groups, phylogenetic clustering, significant β-diversity departure from a random expectation after controlling for α-diversity variation, and non-random trait distribution due to abiotic filtering and limiting similarity. Null expectations were predicted under the Neutral hypothesis. Under the Quasi-Neutral hypothesis, we expected niche clustering and reduced niche overlap as in the Selection hypothesis, but random phylogenetic distribution, random β-diversity variation, and random trait distribution due to niche-based specialization but weak habitat matching resulting from dispersal limitation and drift. We used a series of null models to provide an integrative evaluation of community assembly including species abundances, environmental preferences, phylogenetic relatedness, and trait distribution. Results supported the Quasi-Neutral hypothesis due to a combination of reduced niche overlap, niche clustering along soil and disturbance gradients, evidence for abiotic filtering of functional traits but absent phylogenetic structure and random β-diversity variation. Support to the Quasi-Neutral hypothesis provides evidence for niche-based species selection along environmental gradients, coupled with poor matches between species distributions and environmental factors due to stochastic processes. It also highlights the importance of an integrative approach to the evaluation of community patterns indicative of assembly processes. Had only a subset of the approaches used been employed and different conclusions would have been reached: niche overlap and trait distribution analyses would have indicated a stronger role for selection, while phylogenetic and abundance analyses would have indicated stochastic community assembly. Future research should include different disturbance regimes along with environmental variation in order to assess the interplay of the different drivers that shape the community assembly process, as well as the potential of the Quasi-Neutral hypothesis as an explanation for the assembly process of complex and species-rich subtropical forests.

The number of limiting resources in the environment controls the temporal diversity patterns in the algal benthos.

The role of the number of limiting resources (NLR) on species richness has been the subject of much theoretical and experimental work. However, how the NLR controls temporal beta diversity and the processes of community assembly is not well understood. To address this knowledge gap, we initiated a series of laboratory microcosm experiments, exposing periphyton communities to a gradient of NLR from 0 to 3, generated by supplementation with nitrogen, phosphorus, iron, and all their combinations. We hypothesized that similarly to alpha diversity, shown to decrease with the NLR in benthic algae, temporal beta diversity would also decline due to filtering. Additionally, we predicted that the NLR would also affect turnover and community nestedness, which would show opposing responses. Indeed, as the NLR increased, temporal beta diversity decreased; turnover, indicative of competition, decreased; and nestedness, suggestive of complementarity, increased. Finally, the NLR determined the role of deterministic versus stochastic processes in community assembly, showing respectively an increasing and a decreasing trend. These results imply that the NLR has a much greater, yet still unappreciated influence on producer communities, constraining not only alpha diversity but also temporal dynamics and community assembly.

Disentangling the drivers of metacommunity structure across spatial scales.

Metacommunity theories attribute different relative degrees of importance to dispersal, environmental filtering, biotic interactions and stochastic processes in community assembly, but the role of spatial scale remains uncertain. Here we used two complementary statistical tools to test: (1) whether or not the patterns of community structure and environmental influences are consistent across resolutions; and (2) whether and how the joint use of two fundamentally different statistical approaches provides a complementary interpretation of results. Grassland plants in the French Alps. We used two approaches across five spatial resolutions (ranging from 1 km × 1 km to 30 km × 30 km): variance partitioning, and analysis of metacommunity structure on the site-by-species incidence matrices. Both methods allow the testing of expected patterns resulting from environmental filtering, but variance partitioning allows the role of dispersal and environmental gradients to be studied, while analysis of the site-by-species metacommunity structure informs an understanding of how environmental filtering occurs and whether or not patterns differ from chance expectation. We also used spatial regressions on species richness to identify relevant environmental factors at each scale and to link results from the two approaches. Major environmental drivers of richness included growing degree-days, temperature, moisture and spatial or temporal heterogeneity. Variance partitioning pointed to an increase in the role of dispersal at coarser resolutions, while metacommunity structure analysis pointed to environmental filtering having an important role at all resolutions through a Clementsian assembly process (i.e. groups of species having similar range boundaries and co-occurring in similar environments). The combination of methods used here allows a better understanding of the forces structuring ecological communities than either one of them used separately. A key aspect in this complementarity is that variance partitioning can detect effects of dispersal whereas metacommunity structure analysis cannot. Moreover, the latter can distinguish between different forms of environmental filtering (e.g. individualistic versus group species responses to environmental gradients).