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Ecological disturbances are often hypothesized to alter community assembly processes that influence variation in community composition (β‐diversity). Disturbance can cause convergence in community composition (low β‐diversity) by increasing niche selection of disturbance‐tolerant species. Alternatively, disturbance can cause divergence in community composition (high β‐diversity) by increasing habitat filtering across environmental gradients. However, because disturbance may also influence β‐diversity through random sampling effects owing to changes in the number of individuals in local communities (community size) or abundances in the regional species pool, observed patterns of β‐diversity alone cannot be used to unambiguously discern the relative importance of community assembly mechanisms. We compared β‐diversity of woody plants and inferred assembly mechanisms among unburned forests and forests managed with prescribed fires in the Missouri Ozarks, USA. Using a null‐model approach, we compared how environmental gradients influenced β‐diversity after controlling for differences in local community size and regional species abundances between unburned and burned landscapes. Observed β‐diversity was higher in burned landscapes. However, this pattern disappeared or reversed after controlling for smaller community size in burned landscapes. β‐diversity was higher than expected by chance in both landscapes, indicating an important role for processes that create clumped species distributions. Moreover, fire appeared to decrease clumping of species at broader spatial scales, suggesting homogenization of community composition through niche selection of disturbance‐tolerant species. Environmental variables, however, explained similar amounts of variation in β‐diversity in both landscapes, suggesting that disturbance did not alter the relative importance of habitat filtering. Our results indicate that contingent responses of communities to fire reflect a combination of fire‐induced changes in local community size and scale‐dependent effects of fire on species clumping across landscapes. Synthesis. Although niche‐based mechanisms of community assembly are often invoked to explain changes in community composition following disturbance, our results suggest that these changes also arise through random sampling effects owing to the influence of disturbance on community size. Comparative studies of these processes across disturbed ecosystems will provide important insights into the ecological conditions that determine when disturbance alters the interplay of deterministic and stochastic processes in natural and human‐modified landscapes.

There is increasing recognition that community assembly theory can offer valuable insights for ecological restoration. We studied community assembly processes following tropical forest restoration efforts, using dung beetles (Scarabaeinae) as a focal taxon to investigate taxonomic and functional patterns of biodiversity recovery. We evaluated the relative importance of the local environment (i.e., canopy cover, understory cover, tree basal area, and soil texture), landscape context (i.e., habitat patch proximity and availability and percentage of surrounding area classified as natural forest or Eucalyptus spp. plantation), and space (i.e., spatial proximity of the study areas to estimate dispersal limitation or unmeasured spatially structured processes) on dung beetle species and functional trait composition across a gradient of 15 restoration sites in Brazilian Atlantic Forest. We also assessed which factors were the primary determinants in the establishment of individual dung beetle functional groups, classified according to size, food relocation habit, diet, and period of flight activity. Both species and functional trait composition were most strongly influenced by the local environment, indicating that assembly was predominantly driven by niche-based processes. Most of the variation explained by space was co-explained by local environment and landscape context, ruling out a strong influence of dispersal limitation and random colonization on assembly following restoration. In addition, nearly all of the variance explained by landscape context was coexplained by local environment, suggesting that arrival and establishment at a site depends on both local and landscape-scale environmental factors. Despite strong evidence for niche-based assembly, a large amount of variation remained unexplained in all models, suggesting that stochastic processes and/or unmeasured environmental variables also play an important role. The relative importance of local environment, landscape context, and space changed considerably when analyzing the assembly mechanisms of each functional group separately. Therefore, to recover distinct functional traits in restoration sites, it may be necessary to manipulate different components of the local environment and surrounding landscape. Overall, this study shows that assembly rules can help to better understand recovery processes, enabling improvement of future restoration efforts.

Understanding the dynamics of plant-associated microbial communities within agriculture is well documented. However, the ecological processes that assemble the plant microbiome are not well understood. This study elucidates the relative dominance of assembly processes across plant compartments (root, stem, and leaves) and developmental stages (emergence, growth, flowering, and maturation). Bacterial community composition and assembly processes were assessed using 16S rRNA gene amplicon sequencing. Null models that couple phylogenetic community composition and species distribution models were used to evaluate ecological assembly processes of bacterial communities. All models highlighted that the balance between the assembly process was modulated by compartments and developmental stages. Dispersal limitation dominated amongst the epiphytic communities and at the maturation stage. Homogeneous selection dominated assembly across plant compartments and development stages. Overall, both sets of models were mostly in agreement in predicting the prevailing assembly processes. Our results show, for the first time, that even though niche-based processes dominate in the plant environment, the relative influence of dispersal limitation in community assembly is important.

Aim: Spatial patterns in biodiversity along environmental gradients are a central theme in ecology. However, the ways in which local assembly processes control changes in species turnover (β-diversity) along broader gradients have been less well documented. In this study, we aimed to elucidate factors and processes governing the altitudinal gradients in the β-diversity of woody plants and ground-dwelling oribatid mites. Location: Shiretoko National Park in Hokkaido, Japan. Methods: The diversity of plants and oribatids was investigated in seven plots (each containing 10 subplots) at different altitudes, and the β-diversity of the two organism groups was calculated for each altitude. The dependence of β-diversity on the size of the species pool (γ-diversity) is an issue of long-standing importance. We therefore used null modelling, which randomly shuffles individuals among subplots while preserving the γ-diversity, the relative abundance of each species per plot and the number of individuals per subplot. This approach enabled us to estimate how much the observed β-diversity deviates from the expected β-diversity under stochastic assembly processes. Environmental data were collected to evaluate the possible effects of habitat condition/heterogeneity on community processes. Results: In plants, deterministic processes dominated in the low-productivity, high-altitude stands because of the finer-scale niche partitioning seen among small individuals within less-stratified stands. In the structurally developed, low-altitude stands, the community structure was more strongly affected by stochasticity, probably resulting from one-sided competition such that the canopy trees intercept the majority of light, a primary resource for plants, and therefore the small understorey individuals had limited access to light. Among the oribatids, the altitudinal gradient of β-diversity was less evident than among the studied plants. However, this nonlinearity does not support the notion that local assembly processes contribute little to the spatial pattern of β-diversity. Indeed, local-scale environmental heterogeneity favoured a more deterministic assembly of oribatids at a given altitude. Main conclusion: The biogeographical patterns of β-diversity are not independent of community processes and, in reality, are shaped by local stochastic/deterministic factors that change within a landscape.

AIM: We used a hierarchical fractal‐based sampling design to test how sampling grain influences (1) beta diversity of and (2) inferences from the modelled contribution of niche‐ versus dispersal‐based assembly processes in structuring tree and bird assemblages. LOCATION: Coastal forest fragments, South Africa. METHODS: We surveyed 103 tree plots and 267 bird points within eight forest fragments and partitioned beta diversity (βₛₒᵣ) into its turnover (βₛᵢₘ) and nestedness (βₙₑₛ) components. We evaluated how sampling at fine, intermediate and coarse scales influenced beta diversity components, and compared how tree and bird beta diversity responded to sampling grain variation. We then explored the relative contributions of niche‐ and dispersal‐based assembly processes in explaining spatial turnover as a function of sampling grain and/or study taxon, by using multiple regression modelling on distance matrices and variance partitioning. RESULTS: The βₛₒᵣ of trees and birds was mainly explained by βₛᵢₘ at all sampling scales. For both taxonomic groups, βₛₒᵣ and βₛᵢₘ decreased as sampling scale increased. Beta diversity differed among trees and birds at fine, but not at coarse, sampling scales. Dispersal‐based assembly processes were the best predictors of community assembly at fine scales, whereas niche‐based assembly processes were the best predictors at coarse scales. However, most of the variation in tree community composition was explained at fine scales (by dispersal‐based assembly processes), while most of the variation in bird community composition was explained at coarse scales (by niche‐based assembly processes). MAIN CONCLUSIONS: Our study shows that inferences from beta diversity are scale dependent. By matching the grain of the data with the grain at which predictor variables and associated processes are likely to operate, multi‐scale sampling approaches can help improve planning for biodiversity conservation and should be part of initiatives aimed at ecological conservation plans.

In the last 50 years, intensive agriculture has replaced large tracts of rainforests. Such changes in land use are driving niche-based ecological processes that determine local community assembly. However, little is known about the relative importance of these anthropogenic niche-based processes, in comparison to climatic niche-based processes and spatial processes such as dispersal limitation. In this study, we use a variation partitioning approach to determine the relative importance of land-use change (ranked value of forest loss), climatic variation (temperature and precipitation), and distance between transects, on bird beta diversity at two different spatial scales within the Western Ghats–Sri Lanka biodiversity hotspot. Our results show that the drivers of local community assembly are scale dependent. At the larger spatial scale, distance was more important than climate and land use for bird species composition, suggesting that dispersal limitation over the Palk Strait, which separates the Western Ghats and Sri Lanka, is the main driver of local community assembly. At the smaller scale, climate was more important than land use, suggesting the importance of climatic niches. Therefore, to conserve all species in a biodiversity hotspot, it is important to consider geographic barriers and climatic variation along with land-use change.

We applied a step-down factor analysis (SDFA) and multi-site generalised dissimilarity modelling (MS-GDM) to local flea communities harboured by small mammals (i.e., collected at small sampling sites over a short time period) in two South American regions (Patagonia and the Northwestern Argentina) with the aim of understanding whether these communities were assembled via niche-based or dispersal-based processes. The SDFA allows us to determine whether clusters of flea assemblages across different types of climates, vegetation and soils can be distinguished (suggesting niche-based assembly). MS-GDM allows us to determine whether a substantial proportion of the variation in flea species turnover is explained by specific climate-associated, vegetation-associated and soil-associated variables (indicating niche-based assembly) or host turnover (indicating dispersal-based assembly). Mapping of assemblages on climate, vegetation and soil maps, according to their loadings on axis 1 or axis 2 of the SDFA, did not provide clear-cut results. Clusters of similar loadings could be recognized within some, but not other, climate, vegetation and soil types. However, MS-GDM demonstrated that the effect of environmental variables (especially air temperature) on flea compositional turnover was much stronger than that of host turnover, indicating the predominance of niche-based processes in local community assembly. A comparison of our results with those on the mechanisms that drive species assembly in regional communities allows us to conclude that local and regional communities result from the joint action of niche-based and dispersal-based processes, with the former more important at a smaller spatial scale and the latter at a larger spatial scale.

Aim: The local- and regional-based forms of anthropogenic change reducing grassland diversity are generally identified, but these scale-dependent processes tend to co-occur with unclear interactive effects. Here, we explicitly test how common local and regional perturbations simultaneously affect plant alpha and beta diversity in a multiyear community assembly experiment using fragments of grassland habitat of various sizes. We hypothesized that local disturbances and decreasing patch size would interact, suppressing local diversity while homogenizing composition among patches. Location: North America. Methods: We conducted a three-year grassland assembly experiment, factorially manipulating local perturbation (nitrogen addition and mowing) and patch area for 36 patches over 13 ha. We quantified the individual and interactive effects of these local and regional factors on plant alpha and beta diversity within (quadrat scale) and among patches (patch scale). We also used a null model approach to disentangle between stochastic- and niche-based assembly mechanisms. Results: We detected a gradient of assembly outcomes driven by two non-interacting factors-the effects of N fertilization on alpha (negative) and beta (positive) diversity regardless of spatial scale and the scale-dependant effect of increasing patch size on alpha (positive) and beta (positive) diversity. These effects unfolded over time, with the constraints on richness and composition shifting from dispersal-based during the first sampling year to perturbation-and size-based factors at year two and three. Fertilization effects were driven by a mixture of deterministic (i.e., selection at the species level) and stochastic (i.e., random extinctions) processes resulting in a decline in local richness but an increase in spatial heterogeneity in species composition. Area appeared to influence alpha diversity mainly via stochastic \"sampling effect\"—larger patches represented a larger sample of the regional pool. Niche-based processes, however, led to convergence in beta diversity among smaller patches driving a positive overall effect of area on beta diversity. Main conclusion: Our results illustrate how diversity regulation in contemporary grasslands can be simultaneously shaped by local and regional factors acting additively but via contrasting assembly mechanisms that operate at different spatial and temporal scales.

Background Improved understanding of the processes shaping the assembly of tropical tree communities is crucial for gaining insights into the evolution of forest communities and biological diversity. The climate is thought to be the first order determinant of abundance and distribution patterns of tree species with contrasting traits such as evergreen and deciduous leaf phenology. However, the relative role of neutral, and niche-based processes in the evolution of these patterns remain poorly understood. Methods Here, we perform an integrated analysis of the data on tree species abundance, functional traits and community phylogeny from a network of 96 forest plots, each 1 ha in size, distributed along a broad environmental gradient in the central Western Ghats, India. Then, we determine the relative importance of various process in assembly and structuring of tropical forest communities with evergreen and deciduous leaf phenology. Results The deciduous leaf phenological trait has repeatedly evolved among multiple distantly related lineages. Tree communities in dry deciduous forests were phylogenetically clustered and showed a low range and variance of functional traits related to light harvesting, reproduction, and growth suggesting niche-based processes such as environmental filtering play a vital role in the assembly of tree communities in these forests. The external factors such as human-mediated disturbance also significantly, but to a lesser extent, influences the species and phylogenetic turnover. Conclusions These findings revealed that the environmental filtering plays a significant role in assembly of tree communities in the biologically diverse tropical forests in the Western Ghats biodiversity hotspot.

We evaluated metacommunity hypotheses of landscape arrangement (indicative of dispersal limitation) and environmental gradients (hydroperiod and nutrients) in structuring macroinvertebrate and fish communities in the southern Everglades. We used samples collected at sites from the eastern boundary of the southern Everglades and from Shark River Slough, to evaluate the role of these factors in metacommunity structure. We used eigenfunction spatial analysis to model community structure among sites and distance-based redundancy analysis to partition the variability in communities between spatial and environmental filters. For most animal communities, hydrological parameters had a greater influence on structure than nutrient enrichment, however both had large effects. The influence of spatial effects indicative of dispersal limitation was weak and only periphyton infauna appeared to be limited by regional dispersal. At the landscape scale, communities were well-mixed, but strongly influenced by hydrology. Local-scale species dominance was influenced by water-permanence and nutrient enrichment. Nutrient enrichment is limited to water inflow points associated with canals, which may explain its impact in this data set. Hydroperiod and nutrient enrichment are controlled by water managers; our analysis indicates that the decisions they make have strong effects on the communities at the base of the Everglades food web.