Dispersal promotes compensatory dynamics and stability in forced metacommunities

Understanding the factors that govern the stability of populations and communities has gained increasing importance as habitat fragmentation and environmental perturbations continue to escalate due to human activities. Dispersal is commonly viewed as essential to the maintenance of diversity in spatially subdivided communities, but few experiments have explored how dispersal interacts with the spatiotemporal components of environmental perturbations to determine community-level stability. We examined these processes using an experimental planktonic system composed of three competing species of zooplankton. We subjected zooplankton metacommunities to varying levels of dispersal and pH perturbations that varied in their degree of spatial synchrony. We show that dispersal can reverse the destabilizing effects of environmental forcing when perturbations are spatially asynchronous. Asynchrony in pH perturbations generated spatially and temporally varying species refugia that promoted source-sink dynamics and allowed prolonged persistence of zooplankton species that were otherwise extirpated in synchronously varying metacommunities. This, in turn, increased local species diversity, promoted compensatory population dynamics, and enhanced local community-level stability. Our results indicate that patterns of spatial covariation in environmental variability are critical to predicting the effects of dispersal on the dynamics and persistence of communities.     

Phylogenetic turnover during subtropical forest succession across environmental and phylogenetic scales

Although spatial and temporal patterns of phylogenetic community structure during succession are inherently interlinked and assembly processes vary with environmental and phylogenetic scales, successional studies of community assembly have yet to integrate spatial and temporal components of community structure, while accounting for scaling issues. To gain insight into the processes that generate biodiversity after disturbance, we combine analyses of spatial and temporal phylogenetic turnover across phylogenetic scales, accounting for covariation with environmental differences. We compared phylogenetic turnover, at the species‐ and individual‐level, within and between five successional stages, representing woody plant communities in a subtropical forest chronosequence. We decomposed turnover at different phylogenetic depths and assessed its covariation with between‐plot abiotic differences. Phylogenetic turnover between stages was low relative to species turnover and was not explained by abiotic differences. However, within the late‐successional stages, there was high presence‐/absence‐based turnover (clustering) that occurred deep in the phylogeny and covaried with environmental differentiation. Our results support a deterministic model of community assembly where (i) phylogenetic composition is constrained through successional time, but (ii) toward late succession, species sorting into preferred habitats according to niche traits that are conserved deep in phylogeny, becomes increasingly important.     

Within‐species patterns challenge our understanding of the leaf economics spectrum

The utility of plant functional traits for predictive ecology relies on our ability to interpret trait variation across multiple taxonomic and ecological scales. Using extensive data sets of trait variation within species, across species and across communities, we analysed whether and at what scales leaf economics spectrum (LES) traits show predicted trait–trait covariation. We found that most variation in LES traits is often, but not universally, at high taxonomic levels (between families or genera in a family). However, we found that trait covariation shows distinct taxonomic scale dependence, with some trait correlations showing opposite signs within vs. across species. LES traits responded independently to environmental gradients within species, with few shared environmental responses across traits or across scales. We conclude that, at small taxonomic scales, plasticity may obscure or reverse the broad evolutionary linkages between leaf traits, meaning that variation in LES traits cannot always be interpreted as differences in resource use strategy.     

Seasonal strengths of the abiotic and biotic drivers of a zooplankton community

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The relative influence of local to regional drivers of variation in reef fishes

In this study, fishes and habitat attributes were quantified, four times over 1 year, on three reefs within four regions encompassing a c. 6° latitudinal gradient across south-western Australia. The variability observed was partitioned at these spatio-temporal scales in relation to reef fish variables and the influence of environmental drivers quantified at local scales, i.e. at the scale of reefs (the number of small and large topographic elements, the cover of kelp, fucalean and red algae, depth and wave exposure) and at the scale of regions (mean and maximum nutrient concentrations and mean seawater temperature) with regard to the total abundance, species density, species diversity and the multivariate structure of reef fishes. Variation in reef fish species density and diversity was significant at the regional scale, whereas variation in the total abundance and assemblage structure of fishes was also significant at local scales. Spatial variation was greater than temporal variation in all cases. A systematic and gradual species turnover in assemblage structure was observed between adjacent regions across the latitudinal gradient. The cover of red algae within larger patches of brown macroalgae (a biological attribute of the reef) and the number of large topographic elements (a structural attribute of the reef) were correlated with variation observed at local scales, while seawater temperature correlated with variation at the scale of regions. In conclusion, conservation efforts on reef fishes need to incorporate processes operating at regional scales with processes that shape local reef fish communities at local scales.     

A seasonal alternation of coherent and compensatory dynamics occurs in phytoplankton

Functional groups with diverse responses to environmental factors sum to produce communities with less temporal variability in their biomass than those lacking this diversity. The detection of these compensatory dynamics can be complicated by a spatio-temporal alternation in the environmental factors limiting growth (both abiotic and biotic), which restricts the occurrence of compensatory dynamics to certain periods or locations. Hence, resolving the spatio-temporal scale may uncover important spatial and/or temporal components in community variability. Using long-term data from Lake Constance (Bodensee), we find that a reduction in grazing pressure and relaxed competition for nutrients during winter and spring generates coherent dynamics among edible and less edible phytoplankton. During summer and fall, when both grazing pressure and nutrient limitation are present, edible and less edible phytoplankton exhibit compensatory dynamics. This study supports recent work suggesting that both abiotic and biotic interactions promote compensatory dynamics and to our knowledge, this is the first example of a system where compensatory and coherent dynamics seasonally alternate.     

Geographical distribution of zooplankton biodiversity in highly polluted running water ecosystems: Validation of fine‐scale species sorting hypothesis

Dispersal, rather than species sorting, is widely recognized as the dominant driver for determining meta‐community structure at fine geographical scales in running water ecosystems. However, this view has been challenged by a recently proposed “fine‐scale species sorting hypothesis,” where community structure can be largely determined by an environmental gradient formed by local pollution at fine scales. Here, we tested this hypothesis by studying community composition and geographical distribution of metazoan zooplankton in a heavily polluted river—the North Canal River in the Haihe River Basin, China. Analysis of similarity (ANOSIM) showed that the community composition of metazoan zooplankton differed significantly (p = .001) along the environmental gradient. Ammonium (NH₄‐N) was the leading factor responsible for changes in zooplankton community structure and geographical distribution, followed by total dissolved solid (TDS), Na, dissolved oxygen (DO) and temperature (T). Variation partitioning revealed a larger contribution of environmental variables (21.6%) than spatial variables (1.1%) to the total explained variation of zooplankton communities. Our results support that species sorting, rather than dispersal, played a key role in structuring communities. Threshold Indicator Taxa ANalysis (TITAN) also revealed significant change points at both taxon and community levels along the gradient of NH₄‐N, providing further support for the influence of environmental variables on zooplankton communities. Collectively, we validate the fine‐scale species sorting hypothesis when an environmental gradient exists in running water ecosystems at fine geographical scales. However, future studies on interactions between pollutants and zooplankton communities are still needed to better understand mechanisms responsible for the meta‐community dynamics.     

哀牢山亚热带中山湿性常绿阔叶林树种beta多样性格局形成的驱动力

Beta多样性通常指群落在时间和空间上物种组成的差异, 包括物种周转组分和物种丰富度差异组分。驱动beta多样性格局形成的生态过程决定了群落的时空动态, 然而关于beta多样性及其两个组分格局形成的驱动力还存在较多争议。以往研究表明, beta多样性的格局存在取样尺度的依赖性, 驱动其形成的生态过程在不同取样尺度下的相对重要性也随之改变。本研究以哀牢山亚热带中山湿性常绿阔叶林20 ha动态监测样地为研究对象, 在不同取样尺度上, 将样方间的Bray-Curtis指数分解为物种周转组分和物种丰富度差异组分, 通过典范冗余分析和方差分解的方法揭示环境过滤和扩散限制对于beta多样性及其两个组分格局形成的相对重要性及其尺度依赖性。结果表明: (1) beta多样性、物种周转组分和物种丰富度差异组分均随取样尺度的增大而减小。在不同取样尺度下, 物种周转组分对于beta多样性的贡献始终占主导地位。(2)随着取样尺度的增大, 环境过滤驱动beta多样性格局形成的相对重要性逐渐增加, 而扩散限制的相对重要性逐渐降低。本研究进一步证实了取样尺度在beta多样性格局形成及其驱动力定量评价中的重要性, 今后的研究需要进一步解析上述尺度效应的形成机制。     

Spatial Structure and Distribution of Small Pelagic Fish in the Northwestern Mediterranean Sea

Understanding the ecological and anthropogenic drivers of population dynamics requires detailed studies on habitat selection and spatial distribution. Although small pelagic fish aggregate in large shoals and usually exhibit important spatial structure, their dynamics in time and space remain unpredictable and challenging. In the Gulf of Lions (north-western Mediterranean), sardine and anchovy biomasses have declined over the past 5 years causing an important fishery crisis while sprat abundance rose. Applying geostatistical tools on scientific acoustic surveys conducted in the Gulf of Lions, we investigated anchovy, sardine and sprat spatial distributions and structures over 10 years. Our results show that sardines and sprats were more coastal than anchovies. The spatial structure of the three species was fairly stable over time according to variogram outputs, while year-to-year variations in kriged maps highlighted substantial changes in their location. Support for the McCall''s basin hypothesis (covariation of both population density and presence area with biomass) was found only in sprats, the most variable of the three species. An innovative method to investigate species collocation at different scales revealed that globally the three species strongly overlap. Although species often co-occurred in terms of presence/absence, their biomass density differed at local scale, suggesting potential interspecific avoidance or different sensitivity to local environmental characteristics. Persistent favourable areas were finally detected, but their environmental characteristics remain to be determined.     

Nitrogen addition does not reduce the role of spatial asynchrony in stabilising grassland communities

While nitrogen (N) amendment is known to affect the stability of ecological communities, whether this effect is scale‐dependent remains an open question. By conducting a field experiment in a temperate grassland, we found that both plant richness and temporal stability of community biomass increased with spatial scale, but N enrichment reduced richness and stability at the two scales considered. Reduced local‐scale stability under N enrichment arose from N‐induced reduction in population stability, which was partly attributable to the decline in local species richness, as well as reduction in asynchronous local population dynamics across species. Importantly, N enrichment did not alter spatial asynchrony among local communities, which provided similar spatial insurance effects at the larger scale, regardless of N enrichment levels. These results suggest that spatial variability among local communities, in addition to local diversity, may help stabilise ecosystems at larger spatial scales even in the face of anthropogenic environmental changes.     

Hierarchical Dynamics of Ecological Communities: Do Scales of Space and Time Match?

Theory posits that community dynamics organize at distinct hierarchical scales of space and time, and that the spatial and temporal patterns at each scale are commensurate. Here we use time series modeling to investigate fluctuation frequencies of species groups within invertebrate metacommunities in 26 boreal lakes over a 20-year period, and variance partitioning analysis to study whether species groups with different fluctuation patterns show spatial signals that are commensurate with the scale-specific fluctuation patterns identified. We identified two groups of invertebrates representing hierarchically organized temporal dynamics: one species group showed temporal variability at decadal scales (slow patterns of change), whilst another group showed fluctuations at 3 to 5-year intervals (faster change). This pattern was consistently found across all lakes studied. A spatial signal was evident in the slow but not faster-changing species groups. As expected, the spatial signal for the slow-changing group coincided with broad-scale spatial patterns that could be explained with historical biogeography (ecoregion delineation, and dispersal limitation assessed through a dispersal trait analysis). In addition to spatial factors, the slow-changing groups correlated with environmental variables, supporting the conjecture that boreal lakes are undergoing environmental change. Taken together our results suggest that regionally distinct sets of taxa, separated by biogeographical boundaries, responded similarly to broad-scale environmental change. Not only does our approach allow testing theory about hierarchically structured space-time patterns; more generally, it allows assessing the relative role of the ability of communities to track environmental change and dispersal constraints limiting community structure and biodiversity at macroecological scales.     

Reconciling the influence of global climate phenomena on macrofaunal temporal dynamics at a variety of spatial scales

Determining the relative importance of environmental forces on population dynamics is a fundamental question for ecologists. Growing concern over the ecological effects of climate change emphasizes the importance of defining whether broad-scale environmental forces uniformly act upon local populations (hierarchy theory) or cross-scale interactions influence local responses (multiscale theory). This study analyses 13 years of data on species abundances at six sites within a large harbour to determine the effect of the El Niño Southern Oscillation (ENSO). Environmental variables both directly and indirectly related to ENSO were observed to be important predictors of the temporal dynamics of abundance in many species, but the observed effects were not consistent across sites or species. While nearly all species were affected by large temporal and spatial scale variability, smaller temporal scale, location-specific environmental variables (such as wind-generated wave exposure and turbidity) were also generally important, increasing the variability explained by our models by up to 25%. As with many other broad-scale variables, generality of response to ENSO is affected by interactions across time and space with smaller scale heterogeneity. This study therefore suggests that the degree of interaction between broad-scale climatic factors, such as ENSO, with smaller scale variability, will determine the consistency of responses over large spatial scales, and control our ability to predict effects of climate change on coastal and estuarine communities.     

Frog community composition-environment relationships vary over time: Are snapshot studies of metacommunity dynamics useful?

There have been important advances in understanding the relative importance of environmental and spatial processes for the variation in species composition across a set of local communities linked by dispersal (i.e. metacommunities). However, community composition-environment relationships change over time, and the mechanisms shaping such temporal variation in metacommunities encompassing large environmental gradients remain poorly understood. If the ability of statistical models to predict community composition-environment relationships depends on the sampling year, snapshot metacommunity studies would have limited implications, both theoretical and applied. Here, we partitioned the variation in compositional data of frog communities and asked if the relative importance of environmental and spatial components change over years at broad spatial scales (hereafter, protected areas in coastal and inland regions) of southeastern Brazil. These regions have marked differences in environmental characteristics as well as the size and composition of their regional species pool. Our results showed that the factors explaining the temporal variability in community composition-environment relationships were congruent for the inland region, which is less productive and characterized by harsh environmental conditions. In contrast, the relative importance of environmental and spatial components changed over years in the coastal region, which has more productive environments and benign conditions. Although snapshot studies will continue to provide important information about metacommunity dynamics, researchers have to be better able to incorporate the temporal variation inherent in community composition-environment relationships, which may be especially important in productive environments.     

Effect of (a)synchronous light fluctuation on diversity,functional and structural stability of a marine phytoplankton metacommunity

Disentangling the mechanisms that maintain the stability of communities and ecosystem properties has become a major research focus in ecology in the face of anthropogenic environmental change. Dispersal plays a pivotal role in maintaining diversity in spatially subdivided communities, but only a few experiments have simultaneously investigated how dispersal and environmental fluctuation affect community dynamics and ecosystem stability. We performed an experimental study using marine phytoplankton species as model organisms to test these mechanisms in a metacommunity context. We established three levels of dispersal and exposed the phytoplankton to fluctuating light levels, where fluctuations were either spatially asynchronous or synchronous across patches of the metacommunity. Dispersal had no effect on diversity and ecosystem function (biomass), while light fluctuations affected both evenness and community biomass. The temporal variability of community biomass was reduced by fluctuating light and temporal beta diversity was influenced interactively by dispersal and fluctuation, whereas spatial variability in community biomass and beta diversity were barely affected by treatments. Along the establishing gradient of species richness and dominance, community biomass increased but temporal variability of biomass decreased, thus highest stability was associated with species-rich but highly uneven communities and less influenced by compensatory dynamics. In conclusion, both specific traits (dominance) and diversity (richness) affected the stability of metacommunities under fluctuating conditions.     

A comprehensive framework for global patterns in biodiversity

The present study proposes to reconcile the different spatial and temporal scales of regional species production and local constraint on species richness. Although interactions between populations rapidly achieve equilibrium and limit membership in ecological communities locally, these interactions occur over heterogeneous environments within large regions, where the populations of species are stably regulated through competition and habitat selection. Consequently, exclusion of species from a region depends on long‐term regional‐scale environmental change or evolutionary change among interacting populations, bringing species production and extinction onto the same scale and establishing a link between local and regional processes.     

Environmental and biotic drivers of soil microbial β‐diversity across spatial and phylogenetic scales

Soil microbial communities play a key role in ecosystem functioning but still little is known about the processes that determine their turnover (β‐diversity) along ecological gradients. Here, we characterize soil microbial β‐diversity at two spatial scales and at multiple phylogenetic grains to ask how archaeal, bacterial and fungal communities are shaped by abiotic processes and biotic interactions with plants. We characterized microbial and plant communities using DNA metabarcoding of soil samples distributed across and within eighteen plots along an elevation gradient in the French Alps. The recovered taxa were placed onto phylogenies to estimate microbial and plant β‐diversity at different phylogenetic grains (i.e. resolution). We then modeled microbial β‐diversities with respect to plant β‐diversities and environmental dissimilarities across plots (landscape scale) and with respect to plant β‐diversities and spatial distances within plots (plot scale). At the landscape scale, fungal and archaeal β‐diversities were mostly related to plant β‐diversity, while bacterial β‐diversities were mostly related to environmental dissimilarities. At the plot scale, we detected a modest covariation of bacterial and fungal β‐diversities with plant β‐diversity; as well as a distance–decay relationship that suggested the influence of ecological drift on microbial communities. In addition, the covariation between fungal and plant β‐diversity at the plot scale was highest at fine or intermediate phylogenetic grains hinting that biotic interactions between those clades depends on early‐evolved traits. Altogether, we show how multiple ecological processes determine soil microbial community assembly at different spatial scales and how the strength of these processes change among microbial clades. In addition, we emphasized the imprint of microbial and plant evolutionary history on today's microbial community structure.     

Effects of nonnative species on the stability of riverine fish communities

Despite the increasing ubiquity of biological invasions worldwide, little is known about the scale-dependent effects of nonnative species on real-world ecological dynamics. Here, using an extensive time series dataset of riverine fish communities across different biogeographic regions of the world, we assessed the effects of nonnative species on the temporal variability and synchrony in abundance at different organizational levels (population, metapopulation, community and metacommunity) and spatial scales (stream reach and river basin). At the reach scale, we found that populations of nonnative species were more variable over time than native species, and that this effect scaled up to the community level – significantly destabilizing the dynamics of riverine fish communities. Nonnative species not only contributed to reduced community stability, but also increased variability of native populations. By contrast, we found no effect of nonnative species dominance on local interspecific synchrony among native species. At the basin scale, nonnative metapopulations were again more variable than the native ones. However, neither native metapopulations nor metacommunities showed differences in temporal variability or synchrony as nonnative species dominance increased basin-wide. This suggests a ‘dilution effect’ where the contribution to regional stability of local native populations from sites displaying low levels of invasion reduced the destabilizing effects of nonnative species. Overall, our results indicate that accounting for the destabilizing effect of nonnative species is critical to understanding native species persistence and community stability.     

Zooplankton community structure along a pollution gradient at fine geographical scales in river ecosystems: The importance of species sorting over dispersal

The release of anthropogenic pollution into freshwater ecosystems has largely transformed biodiversity and its geographical distribution patterns globally. However, for many communities including ecologically crucial ones such as zooplankton, it is largely unknown how different communities respond to environmental pollution. Collectively, dispersal and species sorting are two competing processes in determining the structure and geographical distribution of zooplankton communities in running water ecosystems such as rivers. At fine geographical scales, dispersal is usually considered as the dominant factor; however, the relative role of species sorting has not been evaluated well, mainly because significant environmental gradients rarely exist along continuously flowing rivers. The Chaobai River in northern China represents a rare system, where a significant environmental gradient exists at fine scales. Here, we employed high‐throughput sequencing to characterize complex zooplankton communities collected from the Chaobai River, and tested the relative roles of dispersal and species sorting in determining zooplankton community structure along the pollution gradient. Our results showed distinct patterns of zooplankton communities along the environmental gradient, and chemical pollutant‐related factors such as total phosphorus and chlorophyll‐a were identified as the major drivers for the observed patterns. Further partial redundancy analyses showed that species sorting overrode the effect of dispersal to shape local zooplankton community structure. Thus, our results reject the dispersal hypothesis and support the concept that species sorting caused by local pollution can largely determine the zooplankton community structure when significant environmental gradients exist at fine geographical scales in highly polluted running water ecosystems.     

Scale-dependent effects of natural environmental gradients,industrial emissions and dispersal processes on zooplankton metacommunity structure: Implications for the bioassessment of boreal lakes

Environmental controls were traditionally considered as sole determinants of community assembly for freshwater bioassessment studies, whereas potential importance of dispersal processes and spatial scale have received limited attention. We conducted a bioassessment of lakes across northeast Alberta, Canada using crustacean zooplankton to develop a framework for evaluating if and how atmospheric emissions from the nearby Athabasca Oil Sands Region could impact their community assemblages. We quantified the effects of environmental gradients and spatially contingent dispersal processes for determining zooplankton community composition of 97 lakes at two spatial scales (regional and sub-regional) using constrained ordination, spatial modeling and variance partitioning techniques. Our findings indicated that effects of both environmental gradients and dispersal processes on species composition were scale-dependent. Zooplankton community composition was significantly correlated to environmental parameters that are directly and indirectly sensitive to industrial deposition including nitrate, sulphate, dissolved organic carbon, base cation, chloride, trace metal concentrations and predation regime, indicating their potential to track future environmental impacts. The relative importance of these environmental predictors varied with spatial scale, yet unraveling the effects of natural environmental heterogeneity vs. industrial deposition on this scale-dependency was not possible due to lack of regional baseline information. Dispersal processes were not important in shaping zooplankton communities at the sub-regional scale, but had limited, yet significant influence on species composition at the regional scale, emphasizing the need for cautious interpretation of broad-scale community patterns. Beyond establishing crucial regional baselines, our study highlights the necessity for explicit incorporation of dispersal effects and spatial scale in bioassessment of lakes across landscapes.