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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.

For over 200 years, scientists have recognized the nearly ubiquitous poleward decline of species richness, but none of the theories explaining its occurrence has been widely accepted. In this continental study of U.S. running waters, I report an exception to this general pattern, i.e., a U-shaped latitudinal distribution of diatom richness (DR), equally high in subtropical and temperate regions. This gradient is linked unequivocally to corresponding trends in basin and stream properties with impact on resource supply. Specifically, DR distribution was related to wetland area, soil composition, and forest cover in the watershed, which affected iron, manganese, and macronutrient fluxes into streams. These results imply that the large-scale biodiversity patterns of freshwater protists, which are seasonal, highly dispersive, and sheltered by their environment from extreme temperature fluctuations, are resource driven in contrast to more advanced, perennial, and terrestrial organisms with biogeography strongly influenced by climate. The finding that wetlands, through iron export, control DR in streams has important environmental implications. It suggests that wetlands loss, already exceeding 52 million hectares in the conterminous United States alone, poses a threat not only to local biota, but also to biodiversity of major stream producers with potentially harmful consequences for the entire ecosystem.

Biodiversity of both terrestrial ecosystems and lacustrine phytoplankton increases with niche dimensionality, which can be determined by the number of limiting resources (NLR) in the environment. In the present continental study, I tested whether niche dimensionality and, with this species, richness scale positively with NLR in running waters. Diatom richness in 2,426 benthic and 383 planktonic communities from 760 and 127 distinct localities, respectively, was examined as a function of NLR, including basic cations, silica, iron, ammonia, nitrate, and dissolved phosphorus. The patterns found in the two communities were opposite: as more resources became limiting, diatom richness declined in the benthos but increased in the phytoplankton. The divergence of benthic from both planktonic and terrestrial communities is attributed to the complex spatial organization of the benthos, generating strong internal resource gradients. Differential stress tolerance among benthic diatoms allows substantial overgrowth, which greatly reduces nutrient transport to the biofilm base and can be supported only by high ambient resource levels. Therefore, niche dimensionality in the benthos increases with the number of resources at high supply. These findings provide a mechanistic explanation of the well documented phenomenon of increased species richness after fertilization in freshwater as opposed to terrestrial ecosystems. Clearly, however, new theoretical approaches, retaining resource availability as an environmental constraint but incorporating a trade-off between tolerance and spatial positioning, are necessary to address coexistence in one of the major producer communities in streams, the algae.

Among the most studied relationships in ecology are those of population density with (1) body size and (2) species distribution. The first relationship, in conjunction with metabolic rate, determines the energy flows through species communities, whereas the second relationship shows how local communities are influenced by the species history of dispersal and establishment. Traditionally, these two relationships have been examined separately. Here, I explored how diatom density was affected by cell size (biovolume) and species distribution in benthic and planktonic stream habitats all the way from individual localities and hydrologic systems (regions) to the entire United States. At all scales, density was predominantly a negative function of biovolume and a positive function of distribution. Biovolume was more strongly related to density in the benthos than in the phytoplankton. Partial regressions revealed that biovolume, by itself, explained a substantially higher percentage of the variance in density at local than at regional and continental scales. Conversely, species distribution was a much more important descriptor of density at larger scales and a slightly better predictor than biovolume at local scales. At large scales density was explained primarily by distribution and, to a lesser extent and only in the benthos, by the covariance of distribution and biovolume, whereas biovolume was a marginal predictor in all habitats. This discovery suggests that the strong relationships between density and body size, reported for populations ranging from unicellular algae to mammals, may be less direct than previously thought but mediated by large-scale species distributions.

In this continental-scale study, we show that in major benthic and planktonic stream habitats, algal biovolume--a proxy measure of biomass--is a unimodal function of species richness (SR). The biovolume peak is observed at intermediate to high SR in the benthos but at low richness in the phytoplankton. The unimodal nature of the biomass-diversity relationship implies that a decline in algal biomass with potential harmful effects on all higher trophic levels, from invertebrates to fish, can result from either excessive species gain or species loss, both being common consequences of human-induced habitat alterations. SR frequency distributions indicate that the most frequent richness is habitat-specific and significantly higher in the benthos than in the plankton. In all studied stream environments, the most frequent SR is lower than the SR that yields the highest biovolume, probably as a result of anthropogenic influences, but always within one standard deviation from it, i.e. they are statistically indistinguishable. This suggests that algal communities may be driven toward maximum biomass.

The species–area relationship (SAR) has over a 150‐year‐long history in ecology, but how its shape and origins vary across scales and organisms remains incompletely understood. This is the first subcontinental freshwater study to examine both these properties of the SAR in a spatially explicit way across major organismal groups (diatoms, insects, and fish) that differ in body size and dispersal capacity. First, to describe the SAR shape, we evaluated the fit of three commonly used models, logarithmic, power, and Michaelis–Menten. Second, we proposed a hierarchical framework to explain the variability in the SAR shape, captured by the parameters of the SAR model. According to this framework, scale and species group were the top predictors of the SAR shape, climatic factors (heterogeneity and median conditions) represented the second predictor level, and metacommunity properties (intraspecific spatial aggregation, γ‐diversity, and species abundance distribution) the third predictor level. We calculated the SAR as a sample‐based rarefaction curve using 60 streams within landscape windows (scales) in the United States, ranging from 160,000 to 6,760,000 km2. First, we found that all models provided good fits (R2 ≥ 0.93), but the frequency of the best‐fitting model was strongly dependent on organism, scale, and metacommunity properties. The Michaelis–Menten model was most common in fish, at the largest scales, and at the highest levels of intraspecific spatial aggregation. The power model was most frequent in diatoms and insects, at smaller scales, and in metacommunities with the lowest evenness. The logarithmic model fit best exclusively at the smallest scales and in species‐poor metacommunities, primarily fish. Second, we tested our framework with the parameters of the most broadly used SAR model, the log–log form of the power model, using a structural equation model. This model supported our framework and revealed that the SAR slope was best predicted by scale‐ and organism‐dependent metacommunity properties, particularly spatial aggregation, whereas the intercept responded most strongly to species group and γ‐diversity. Future research should investigate from the perspective of our framework how shifts in metacommunity properties due to climate change may alter the SAR. [ABSTRACT FROM AUTHOR]

Aim: Biodiversity on Earth is threatened by climate change. Despite the vulnerability of freshwater habitats to human impacts, most climate change projections have focused on terrestrial systems. Here, we examined how the current distributions and biodiversity of stream taxa might change under mitigated, stabilizing and increasing greenhouse gas emissions. Location: Conterminous USA. Time period: Present day to 2070. Major taxa studied: Stream diatoms, insects and fish. Methods: We developed species distribution models for 336 freshwater taxa from 1,227 distinct stream localities using water chemistry, watershed and climatic variables. Models based only on climate were used to project changes in the distributions and biodiversity of cold‐ versus warm‐water taxa under representative concentration pathways (RCPs) ranging from 2.6 to 8.5 W/m2. Results: In all three organismal groups, climate emerged as the strongest predictor of species distributions, providing comparable explanatory power to water chemistry and watershed variables combined. The RCP‐based projections suggested a widespread expansion of warm‐water taxa, outpacing the decline of cold‐water taxa. Consequently, overall species richness would increase, but beta diversity would decrease drastically with the severity of climate change. A closer look at individual taxa and functional guilds revealed that vulnerable cold‐water taxa included: (a) diatom guilds forming the base and bulk of the biofilm; (b) environmentally sensitive insects, characteristic of unimpacted streams; and (c) ecologically and recreationally important salmonids, which were forecast to diminish dramatically in source habitats. Warm‐water fish projected to increase their distributions include bait bucket release minnows and dominant predators. Main conclusions: Our results suggest potentially devastating impacts of climate change on stream ecosystems, with the restructuring of diatom, insect and fish communities, diminished distributions of functionally important taxa and widespread expansion of warm‐water taxa, giving rise to biotic homogenization. Given that the magnitude of these biotic shifts depends on the severity of climate change, appropriate current policy decisions are necessary to preserve freshwater ecosystems. [ABSTRACT FROM AUTHOR]

Aim: Severity and heterogeneity of stress are major constraints of beta diversity, but their relative influence is poorly understood. Here, we addressed this question by examining the patterns of beta diversity in stress‐sensitive versus stress‐tolerant stream diatoms and their response to local versus regional factors along gradients of stress severity and heterogeneity. Location: The Adirondack region of New York. Methods: Beta diversity was measured as multivariate dispersion of communities across high stress, low stress, and high + low stress (heterogeneous) environments, encompassing 200 stream samples. Null models were implemented to assess community similarity relative to randomly assembled communities and the importance of local assembly processes versus the regional species pool. Results: The overall beta diversity was influenced by a combination of severity and heterogeneity of stress, while beta diversity of sensitive species increased with heterogeneity. Beta diversity of tolerant species did not vary with either severity or heterogeneity of stress. Heterogeneity decreased community similarity relative to the null expectation in all groups of species. Stress reduced the importance of local assembly mechanisms for the overall beta diversity and sensitive species beta diversity. In contrast, the importance of local assembly mechanisms increased with stress regarding beta diversity of tolerant species. Main conclusions: Beta diversity responded to both severity and heterogeneity of stress, but turnover along these gradients was mostly driven by sensitive species. The overall beta diversity and beta diversity of sensitive species became more constrained by the depauperate regional species pool, as opposed to local assembly mechanisms. While heterogeneous stress contributed to beta diversity, severe stress suppressed beta diversity through elimination of sensitive species. Therefore, an increase in beta diversity in an environmentally‐stressed region may serve as a forewarning for future loss of sensitive species, should the stress continue to intensify. [ABSTRACT FROM AUTHOR]

1. Understanding the mechanisms that drive beta diversity (i.e. β-diversity), an important aspect of regional biodiversity, remains a priority for ecological research. β-diversity and its components can provide insights into the processes generating regional biodiversity patterns. We tested whether environmental filtering or dispersal related processes predominated along the stream watercourse by analysing the responses of taxonomic and functional diatom β-diversity to environmental and spatial factors. 2. We examined the variation in total β-diversity and its components (turnover and nestedness) in benthic diatom species and ecological guilds (motile, planktonic, low- and high profile) with respect to watercourse position (up-, mid- and downstream) in 2,182 sites throughout France. We tested the effects of pure environmental and pure spatial factors on β-diversity with partial Mantel tests. Environmental factors included eight physicochemical variables, while geographical distances between sites were used as spatial factors. We also correlated α and γ-diversity, and the degree of nestedness (NODF metric) with environmental variables. 3. Total β-diversity and its turnover component displayed higher values upstream than mid- and downstream. The nestedness component exhibited low values, even when NODF values increased from up- to downstream. Pure environmental factors were highly significant for explaining total β-diversity and turnover regardless of watercourse position, but pure spatial factors were mostly significant mid- and downstream, with geographical distance being positively correlated with β-diversity. Across sites, nutrient enrichment decreased turnover but increased the degree of nestedness. Motile and low profile diatoms comprised the most abundant guilds, but their β-diversity patterns varied in an opposite way. The lowest guild β-diversity was observed upstream for low profile species, and downstream for motile species. 4. In conclusion, environmental filtering seemed to play a major role in structuring metacommunities irrespective of site watercourse position. Filtering promoted strong turnover patterns, especially in disconnected upstream sites. The greater role of spatial factors mid- and downstream was consistent with mass effects rather than neutral processes because these sites had lower total β-diversity than upstream sites. Motile species were most strongly affected by mass effects processes, whereas low profile species were primarily influenced by environmental conditions. Collectively, these findings suggest that partitioning of total β-diversity into its components and the use of diatom ecological guilds provide a useful framework for assessing the mechanisms underlying metacommunity patterns along the stream watercourse. [ABSTRACT FROM AUTHOR]

The current paradigm that stream producers are under exclusive macronutrient control was recently challenged by continental studies, demonstrating that iron supply constrained diatom biodiversity and energy flows. Using algal abundance and water chemistry data from the National Water-Quality Assessment Program, we determined for the first time community thresholds along iron gradients in non-acidic running waters, i.e. 30-79.5μgL-1 and 70-120 μg L-1 in oligotrophic and eutrophic streams, respectively. Given that Fe concentrations fell below both thresholds in 50% of US streams, and below the eutrophic threshold in 75% of US streams, we suggest that Fe limitation is potentially widespread and attribute it to the restricted distribution of wetlands. We also report results from the first laboratory experiments on algal-iron interactions in streams, revealing that iron supplementation leads to significant biovolume and biodiversity increase in both nitrogen fixing and non-nitrogen fixing algae. Therefore, the progressive brownification of freshwaters due to rising dissolved organic carbon and iron levels can have a stimulating influence on microbial producers with cascading effects along the trophic hierarchy. Future research in running waters should focus on the role of iron in algal physiology and biofilm functions, including accumulation of biomass, fixing atmospheric nitrogen and improving water quality. [ABSTRACT FROM AUTHOR]

For over 40 years, acid deposition has been recognized as a serious international environmental problem, but efforts to restore acidified streams and biota have had limited success. The need to better understand the effects of different sources of acidity on streams has become more pressing with the recent increases in surface water organic acids, or 'brownification,' associated with climate change and decreased inorganic acid deposition. Here, we carried out a large scale multi-seasonal investigation in the Adirondacks, one of the most acid-impacted regions in the United States, to assess how acid stream producers respond to local and watershed influences and whether these influences can be used in acidification remediation. We explored the pathways of wetland control on aluminum chemistry and diatom taxonomic and functional composition. We demonstrate that streams with larger watershed wetlands have higher organic content, lower concentrations of acidic anions, and lower ratios of inorganic to organic monomeric aluminum, all beneficial for diatom biodiversity and guilds producing high biomass. Although brownification has been viewed as a form of pollution, our results indicate that it may be a stimulating force for biofilm producers with potentially positive consequences for higher trophic levels. Our research also reveals that the mechanism of watershed control of local stream diatom biodiversity through wetland export of organic matter is universal in running waters, operating not only in hard streams, as previously reported, but also in acid streams. Our findings that the negative impacts of acid deposition on Adirondack stream chemistry and biota can be mitigated by wetlands have important implications for biodiversity conservation and stream ecosystem management. Future acidification research should focus on the potential for wetlands to improve stream ecosystem health in acid-impacted regions and their direct use in stream restoration, for example, through stream rechanneling or wetland construction in appropriate hydrologic settings. [ABSTRACT FROM AUTHOR]

The accumulation of new and taxonomically diverse species is a marked feature of community development, but the role of the environment in this process is not well understood. To address this problem, we subjected periphyton in laboratory streams to low (10-cm·s-1), high (30-cm·s-1), and variable (9- to 32-cm·s-1) current velocity and lowversus high-nutrient inputs. We examined how current velocity and resource supply constrained (i) the rates of species accumulation, a measure of temporal beta-diversity, and (ii) the rates of diversification of higher taxonomic categories, defined here as the rate of higher taxon richness increase with the increase of species richness. Temporal biofilm dynamics were controlled by a strong nutrient-current interaction. Nutrients accelerated the rates of accumulation of new species, when flow velocity was not too stressful. Species were more taxonomically diverse under variable than under low-flow conditions, indicating that flow heterogeneity increased the niche diversity in the high-nutrient treatments. Conversely, the lower diversification rates under highthan under low-nutrient conditions at low velocity are explained with finer resource partitioning among species, belonging to a limited number of related genera. The overall low rates of diversification in high-current treatments suggest that the ability to withstand current stress was conserved within closely related species. Temporal heterogeneity of disturbance has been shown to promote species richness, but here we further demonstrate that it also affects two other components of biodiversity, i.e., temporal beta-diversity and diversification rate. Therefore, management efforts for preserving the inherent temporal heterogeneity of natural ecosystems will have detectable positive effects on biodiversity. [ABSTRACT FROM AUTHOR]

The relationships of local population density ( N) with body size ( M) and distribution ( D) have been extensively studied because they reveal how ecological and historical factors structure species communities; however, a unifying model explaining their joint behaviour, has not been developed. Here, I propose a theory that explores these relationships hierarchically and predicts that: (1) at a metacommunity level, niche breadth, population density and regional distribution are all related and size-dependent and (2) at a community level, the exponents b and d of the relationships N ~ M b and N ~ D d are functions ( f) of the environment and, consequently, species richness ( S), allowing the following reformulation of the power laws: N ~ M f(S) and N ~ D f(S). Using this framework and continental data on stream environment, diatoms, invertebrates and fish, I address the following fundamental, but unresolved ecological questions: how do species partition their resources across environments, is energetic equivalence among them possible, are generalists more common than specialists, why are locally abundant species also regionally prevalent, and, do microbes have different biogeography than macroorganisms? The discovery that community scaling behaviour is environmentally constrained calls for better integration of macroecology and environmental science. [ABSTRACT FROM AUTHOR]

The cover image is based on the Biodiversity Research Beta diversity response to stress severity and heterogeneity in sensitive versus tolerant stream diatoms by Katrina L. Pound et al., DOI: 10.1111/ddi.12865. Image Credit: MAREK MIS/SCIENCE PHOTO LIBRARY. [ABSTRACT FROM AUTHOR]