The relative importance of local conditions and regional processes in structuring aquatic plant communities

1. The structure of biological communities reflects the influence of both local environmental conditions and processes such as dispersal that create patterns in species’ distribution across a region. 2. We extend explicit tests of the relative importance of local environmental conditions and regional spatial processes to aquatic plants, a group traditionally thought to be little limited by dispersal. We used partial canonical correspondence analysis and partial Mantel tests to analyse data from 98 lakes and ponds across Connecticut (northeastern United States). 3. We found that aquatic plant community structure reflects the influence of local conditions (pH, conductivity, water clarity, lake area, maximum depth) as well as regional processes. 4. Only 27% of variation in a presence/absence matrix was explained by environmental conditions and spatial processes such as dispersal. Of the total explained, 45% was related to environmental conditions and 40% to spatial processes. 5. Jaccard similarity declined with Euclidean distance between lakes, even after accounting for the increasing difference in environmental conditions, suggesting that dispersal limitation may influence community composition in the region. 6. The distribution of distances among lakes where species occurred was associated with dispersal‐related functional traits, providing additional evidence that dispersal ability varies among species in ways that affect community composition. 7. Although environmental and spatial variables explained a significant amount of variation in community structure, a substantial amount of stochasticity also affects these communities, probably associated with unpredictable colonisation and persistence of the plants.     

A comparative hierarchical analysis of bacterioplankton and biofilm metacommunity structure in an interconnected pond system

It is unknown whether bacterioplankton and biofilm communities are structured by the same ecological processes, and whether they influence each other through continuous dispersal (known as mass effects). Using a hierarchical sampling approach we compared the relative importance of ecological processes structuring the dominant fraction (relative abundance ≥0.1%) of bacterioplankton and biofilm communities from three microhabitats (open water, Nuphar and Phragmites sites) at within‐ and among‐pond scale in a set of 14 interconnected shallow ponds. Our results demonstrate that while bacterioplankton and biofilm communities are highly distinct, a similar hierarchy of ecological processes is acting on them. For both community types, most variation in community composition was determined by pond identity and environmental variables, with no effect of space. The highest β‐diversity within each community type was observed among ponds, while microhabitat type (Nuphar, Phragmites, open water) significantly influenced biofilm communities but not bacterioplankton. Mass effects among bacterioplankton and biofilm communities were not detected, as suggested by the absence of within‐site covariation of biofilm and bacterioplankton communities. Both biofilm and plankton communities were thus highly structured by environmental factors (i.e., species sorting), with among‐lake variation being more important than within‐lake variation, whereas dispersal limitation and mass effects were not observed.     

Disturbance from pond management obscures local and regional drivers of assemblages of primary producers

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Disentangling the effects of local and regional processes on biodiversity patterns through taxon‐contingent metacommunity network analysis

Metacommunity theory, which has gained a central position in ecology, accounts for the role of migration in patterns of diversity among communities at different scales. Community isolation has a main role in this theory, but is difficult to estimate empirically, partly due to the taxon‐dependent nature of dispersal. Landscapes could be perceived as either fragmented or connected for organisms with contrasting dispersal abilities. Indeed, the dispersal ability of a taxon, and the spatial scale at which eco‐evolutionary processes shape local diversity, determine a taxon‐dependent metacommunity network. In this paper, we introduce a methodology using graph theory to define this taxon‐dependent metacommunity network and then to estimate the isolation of local communities. We analyzed the relative importance of local conditions versus community isolation as determinants of community richness for 25 taxa inhabiting 18 temporary ponds. Although local factors have been the foci of most previous empirical and theoretical considerations, we demonstrate that the metacommunity network is an equally important contributor to local diversity. We also found that the relative effect of local conditions and the metacommunity network depend on body size and taxon abundance. Local diversity of larger species was more affected by patch isolation, while taxon abundances were associated with positive or negative effects of isolation. Our results provide empirical support for the proposed role of metacommunity networks as determinants of community diversity and show the taxon‐dependent nature of these networks.     

Intraspecific phenotypic variation in a fish predator affects multitrophic lake metacommunity structure

Contemporary insights from evolutionary ecology suggest that population divergence in ecologically important traits within predators can generate diversifying ecological selection on local community structure. Many studies acknowledging these effects of intraspecific variation assume that local populations are situated in communities that are unconnected to similar communities within a shared region. Recent work from metacommunity ecology suggests that species dispersal among communities can also influence species diversity and composition but can depend upon the relative importance of the local environment. Here, we study the relative effects of intraspecific phenotypic variation in a fish predator and spatial processes related to plankton species dispersal on multitrophic lake plankton metacommunity structure. Intraspecific diversification in foraging traits and residence time of the planktivorous fish alewife (Alosa pseudoharengus) among coastal lakes yields lake metacommunities supporting three lake types which differ in the phenotype and incidence of alewife: lakes with anadromous, landlocked, or no alewives. In coastal lakes, plankton community composition was attributed to dispersal versus local environmental predictors, including intraspecific variation in alewives. Local and beta diversity of zooplankton and phytoplankton was additionally measured in response to intraspecific variation in alewives. Zooplankton communities were structured by species sorting, with a strong influence of intraspecific variation in A. pseudoharengus. Intraspecific variation altered zooplankton species richness and beta diversity, where lake communities with landlocked alewives exhibited intermediate richness between lakes with anadromous alewives and without alewives, and greater community similarity. Phytoplankton diversity, in contrast, was highest in lakes with landlocked alewives. The results indicate that plankton dispersal in the region supplied a migrant pool that was strongly structured by intraspecific variation in alewives. This is one of the first studies to demonstrate that intraspecific phenotypic variation in a predator can maintain contrasting patterns of multitrophic diversity in metacommunities.     

Bioassessment in a metacommunity context: Are diatom communities structured solely by species sorting?

Aquatic ecosystems face a variety of anthropogenic pressures, urging the development of efficient biological indicators. In addition to local environmental conditions, the community structure of indicator organisms is affected by spatial processes, such as high and limited dispersal rates. Understanding the relative roles of environmental factors and spatial processes for ecological communities should thus be associated with bioassessment practices. We examined the main drivers, both environmental and spatial, influencing community structure and several indices derived from diatom communities. We sampled 81 stony littoral sites in a large boreal lake system (305 km²), where relatively large gradients in water chemistry (35 variables measured) exist, but no dispersal limitation can be expected. Instead, high dispersal rates should interfere with species sorting. Our response variables, including commonly-used diatom indices, diversity indices and taxonomic distinctness indices, were better explained by pure effects of spatial variables and shared effects of spatial and environmental variables than by pure effects of environmental variables. Thus, high dispersal rates between sites are likely to interfere with environmental filtering and can result in clear spatial structures in index values used in bioassessment. Bioassessment should thus acknowledge the importance of spatial processes and not take it for granted that only local environmental conditions determine index values. Failure to consider high dispersal rates may lead to biased information about the state of freshwater ecosystems. The same idea should also be considered in systems with similarly highly-connected sets of bioassessment sites, such as marine coastal systems and stream networks.     

Lateral hydrological connectivity differentially affects the community characteristics of multiple groups of aquatic invertebrates in tropical wetland pans in South Africa

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Community assembly in Lake Tanganyika cichlid fish: quantifying the contributions of both niche‐based and neutral processes

The cichlid family features some of the most spectacular examples of adaptive radiation. Evolutionary studies have highlighted the importance of both trophic adaptation and sexual selection in cichlid speciation. However, it is poorly understood what processes drive the composition and diversity of local cichlid species assemblages on relatively short, ecological timescales. Here, we investigate the relative importance of niche‐based and neutral processes in determining the composition and diversity of cichlid communities inhabiting various environmental conditions in the littoral zone of Lake Tanganyika, Zambia. We collected data on cichlid abundance, morphometrics, and local environments. We analyzed relationships between mean trait values, community composition, and environmental variation, and used a recently developed modeling technique (STEPCAM) to estimate the contributions of niche‐based and neutral processes to community assembly. Contrary to our expectations, our results show that stochastic processes, and not niche‐based processes, were responsible for the majority of cichlid community assembly. We also found that the relative importance of niche‐based and neutral processes was constant across environments. However, we found significant relationships between environmental variation, community trait means, and community composition. These relationships were caused by niche‐based processes, as they disappeared in simulated, purely neutrally assembled communities. Importantly, these results can potentially reconcile seemingly contrasting findings in the literature about the importance of either niche‐based or neutral‐based processes in community assembly, as we show that significant trait relationships can already be found in nearly (but not completely) neutrally assembled communities; that is, even a small deviation from neutrality can have major effects on community patterns.     

A truce with neutral theory: local deterministic factors, species traits and dispersal limitation together determine patterns of diversity in stream invertebrates

1. Studies seeking to explain local patterns of diversity have typically relied on niche explanations, reflected in correlations with local environmental conditions, or neutral theory, invoking dispersal processes and speciation. 2. We used macroinvertebrate community data from 10 streams that varied independently in local ecological conditions and spatial proximity. Neutral theory predicts that similarity in communities will be negatively associated with distance between sites, while niche theory suggests that community similarity will be positively associated with similarity in local ecological conditions. 3. Similarity in total invertebrate, grazer and predator assemblages showed negative relationships with distance and, for grazers and predators, positive relationships with local ecological conditions. However, the best model predicting community similarity in all three cases included aspects of both local ecological conditions and distance between sites. 4. When assemblages were analysed according to dispersal ability, high-dispersal species were shown to be freely accessing all sites and community similarity was not well predicted by either local ecology or spatial separation. Assemblages of species with low and moderate dispersal ability were best predicted by combined models, including distance between sites and local ecological factors. 5. The results suggest that the perceived dichotomy between neutral and local environmental processes in determining local patterns of diversity may not be useful. Neutral and niche processes structured these communities differentially depending on trophic level and species traits. 6. We emphasize the potential for both dispersal processes and local environmental conditions to explain local patterns of diversity.     

Functional traits reveal environmental constraints on amphibian community assembly in a subtropical dry forest

The relationships between functional traits and environmental gradients are useful to identify different community assembly processes. In this work, we used an approach based on functional traits to analyse if changes in hydroperiod and tree covers of ponds are relevant for local amphibian community assembly processes. Ephemeral ponds with low vegetation cover are expected to impose constraints on different species with particular trait combinations and, therefore, to exhibit communities with lower functional diversity than more stable ponds with greater tree cover. Sampling was conducted in 39 temporary ponds located along vegetation and hydroperiod gradients in the most arid portion of the Chaco ecoregion. Seven functional traits were measured in each species present in the regional pool. Associations between these traits and environmental gradients were detected using multivariate ordination techniques and permutation test (RLQ and fourth‐corner analyses respectively). Functional diversity indices were then calculated and related to variations in the environmental gradients. The results obtained allowed us to identify different sets of traits associated with hydroperiod and tree cover, suggesting that these environmental variables are relevant for structuring amphibian communities according to interspecific variations in functional traits from both, larval and adult stages. Contrary to our expectations, communities associated with more stable ponds and with greater tree cover exhibited lower functional diversity than expected by chance (and were the ponds with highest species richness). This result indicates that the reduction in relative importance of environmental restrictions imposed by a very short hydroperiod and the lack of tree cover, favours different species of the regional pool that are similar in several functional traits. Accordingly, communities associated with stable ponds with high tree cover exhibited high functional redundancy.     

The effect of local environmental variables on the helminth parasite communities of the pointedbelly frog Leptodactylus podicipinus from ponds in the Pantanal wetlands

Understanding the patterns of species distribution and abundance has been at the core of ecology. In general, these patterns are determined by species dispersion as well as by abiotic and biotic environmental conditions. Similarly, host-parasite relations and the structure of parasite assemblages are also shaped by environmental conditions and landscape composition. Herein, we assessed the influence of environmental variables and parasite species dispersion on the structure of helminth parasites communities in the frog Leptodactylus podicipinus. We sampled 10 ponds and recorded area, depth, altitude, pH, dissolved oxygen, salinity, temperature, and extent of soil, water, and vegetation cover as well as the distances between the ponds. We collected 121 frogs and found 9 helminth taxa; 2 of them were core species (prevalence higher than 50%), which contributed to the relatively high similarity observed among the ponds. Most of the helminths showed some variation in the frequencies of occurrence among communities from different ponds. The change in species composition among ponds was explained by the environmental variables but not by the distance between the ponds. Moreover, the results indicated that local processes (variation in environmental conditions) were more important than the regional processes (species distribution) in determining the structure of parasite communities. The variation in helminth communities among ponds in response to moderate differences in pond environmental characteristics points to the potential of helminth species as indicators of environmental conditions.     

Ignoring spatial effects results in inadequate models for variation in littoral macroinvertebrate diversity

Studies focusing on the effects of spatial processes versus environmental filtering on aquatic metacommunities have so far been focused almost entirely on relatively isolated systems, such as sets of different lakes or streams. In contrast, metacommunity patterns and underlying processes within a single aquatic system have received less attention. In this study, we aimed to examine how strongly variations in different diversity indices are affected by spatial processes (dispersal) versus local environmental conditions (species sorting) within a large lake system. Modern biodiversity research focuses on multiple diversity facets because different indices may be uncorrelated within and between facets, and they may thus describe different phenomena. We investigated the relationship of littoral macroinvertebrate diversity with environmental and spatial factors using 10 indices of species, functional and taxonomic diversity. Using spatial factors as proxies of dispersal, we decomposed variation in diversity indices into fractions attributable to environmental and spatial factors. Our results highlighted generally equal or higher importance of spatial processes in controlling the variation in diversity indices when compared to local environmental variables. Local environmental conditions accounted for higher proportion of variation only in a single index (i.e. taxonomic diversity). These findings suggest that the effects of high dispersal rates (mass effects) may override the influences of local environmental conditions (species sorting) on the diversity in highly‐connected aquatic system, such as large lakes and marine coastal systems. Our results further suggest that biodiversity assessment and environmental monitoring in highly‐connected systems cannot rely solely on the idea of environmental control. We hence recommend that the roles of both environmental and spatial processes should be integrated in basic and applied ecological research of aquatic systems.     

Plankton metacommunities in floodplain wetlands under contrasting hydrological conditions

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Biogeographical patterns of freshwater micro‐ and macroorganisms: a comparison between phytoplankton,zooplankton and fish in the eastern Mediterranean

Aim We investigated the biogeographical patterns of phytoplankton, zooplankton and fish in freshwater ecosystems. We tested whether spatial distance or environmental heterogeneity act as potential factors controlling community composition. Location Northern and central Greece, eastern Mediterranean. Method Data on 310 phytoplankton, 72 zooplankton and 37 fish species were collected from seven freshwater systems. Species occurrence data were used to generate similarity matrices describing community composition. We performed Mantel tests to compare spatial patterns in community composition of phytoplankton, zooplankton and fish. Next, we examined the correlation between geographical distance and the degree of similarity in community composition. The analysis was repeated for different taxonomic, trophic and size‐based groups of the microorganisms studied. We assessed different environmental variables (topographic and limnological) as predictors of community composition. Results Phytoplankton community composition showed a strong positive correlation with environmental heterogeneity but was not correlated with the geographical distance between systems. Zooplankton community composition was unrelated to geographical distance and was only weakly correlated with environmental variables. In contrast, fish community similarity decayed significantly with distance. We found no relationship along all pairwise comparisons of the compositional matrices of the three groups. The pairwise comparisons of the different taxonomic, trophic and size‐based groups of the microorganism communities studied were in accordance with the results for the entire microorganism community. Main conclusions Our results support the proposition that the biogeography of microorganisms does not demonstrate a distance–decay pattern and further suggest that, in reality, the drivers of distribution depend on the specific community examined. In contrast, the biogeography of macroorganisms was affected by geographical distance. These differences reflect the dispersal abilities of the different organisms. The microorganisms exhibit passive dispersal through the air, with local environmental conditions structuring their community composition. On the other hand, for macroorganisms such as fish, the terrestrial environment could pose barriers to their dispersal; with fish structuring distinctive communities over greater distances. Overall, we suggest that the biogeography of freshwater phytoplankton and zooplankton reflects contemporary environmental conditions, while the biogeographical patterns for fish inhabiting the same systems are related to factors affecting their dispersal ability.     

Macroecological drivers of zooplankton communities across the mountains of western North America

Disentangling the environmental and spatial drivers of biological communities across large scales increasingly challenges modern ecology in a rapidly changing world. Here, we investigate the hierarchical and trait‐based organization of regional and local factors of zooplankton communities at a macroscale of 1240 mountain lakes and ponds spanning western North America (California, USA, to Yukon Territory, Canada). Variation partitioning was used to test the hypothesized importance of climate, connectivity, catchment features, and exotic sportfish to zooplankton beta‐diversity in the context of key functional traits (body size and reproductive dispersal potential) given the pronounced environmental heterogeneity (e.g. thermal gradients), topographic barriers, and legacy of stocked fish in mountainous regions. Dispersal limitation was inferred from multispecies patch connectivity estimates based on nearest and average distances to occupied patches. Environmental heterogeneity best explained community composition as catchment/lake features (morphometry, land cover, and lithology) collectively captured greater variation than did climate (temperature, precipitation, and solar radiation), local stocking, or connectivity; however, single climatic variables captured the most variation individually. Macrospatial variation by larger obligate sexual species was better explained than that by smaller cyclically parthenogenetic asexual species. Our results provide several novel insights into the macroecology of zooplankton of the North American Cordillera, demonstrating their stronger associations to climatically driven aquatic‐terrestrial habitat coupling than dynamics arising from introduced salmonids, human land‐use, or species dispersal. These findings highlight the clear and important role of these communities as bioindicators of the limnological impacts of accelerating rates of climate change, as their responses appear relatively not confounded by local human perturbations or dispersal limitation.     

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.     

Environmental heterogeneity among lakes promotes hyper β‐diversity across phytoplankton communities

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Using functional diversity patterns to explore metacommunity dynamics: a framework for understanding local and regional influences on community structure

The metacommunity concept, describing how local and regional scale processes interact to structure communities, has been successfully applied to patterns of taxonomic diversity. Functional diversity has proved useful for understanding local scale processes, but has less often been applied to understanding regional scale processes. Here, we explore functional diversity patterns within a metacommunity context to help elucidate how local and regional scale processes influence community assembly. We detail how each of the four metacommunity perspectives (species sorting, mass effects, patch dynamics, neutral) predict different patterns of functional beta‐ and alpha‐diversity and spatial structure along two key gradients: dispersal limitation and environmental conditions. We then apply this conceptual model to a case study from alpine tundra plant communities. We sampled species composition in 17 ‘sky islands’ of alpine tundra in the Colorado Rocky Mountains, USA that differed in geographic isolation and area (key factors related to dispersal limitation) and temperature and elevation (key environmental factors). We quantified functional diversity in each site based on specific leaf area, leaf area, stomatal conductance, plant height and chlorophyll content. We found that colder high elevation sites were functionally more similar to each other (decreased functional beta‐diversity) and had lower functional alpha‐diversity. Geographic isolation and area did not influence functional beta‐ or alpha‐diversity. These results suggest a strong role for environmental conditions structuring alpine plant communities, patterns consistent with the species sorting metacommunity perspective. Incorporating functional diversity into metacommunity theory can help elucidate how local and regional factors structure communities and provide a framework for observationally examining the role of metacommunity dynamics in systems where experimental approaches are less tractable.     

Meta‐community theory and stream restoration: evidence that spatial position constrains stream invertebrate communities in a mine impacted landscape

Mining activities, particularly acid mine drainage, often result in adverse effects on stream diversity and ecosystem functioning, and increased concern about these effects has generated a focus on restoration of mine‐impacted waterways. However, many stream restoration projects have not led to increased stream diversity and ecological recovery. One reason for this failure may be that restoration practitioners focus on local environmental conditions and fail to consider the importance of dispersal as a driver of stream invertebrate composition. To test this hypothesis, we used a meta‐community analysis to compare the influence of the local stream conditions with the regional supply of colonists. Invertebrate communities and physico‐chemical conditions were sampled in 37 streams across a mine‐impact gradient on the Stockton Plateau, West Coast of New Zealand's South Island. We found that pH, temperature, dissolved metals, and sediment significantly influenced invertebrate community composition. Furthermore, the spatial location of streams was a good predictor of stream diversity and invertebrate communities, independent of local environmental conditions. This result indicates an important role for regional dispersal barriers in determining stream invertebrate communities. Consequently, consideration of both the locations and strategic preservation of future colonist source streams and potential dispersal barriers during mine planning would enhance post‐mining restoration.     

Assembly mechanisms determining high species turnover in aquatic communities over regional and continental scales

Niche and neutral processes drive community assembly and metacommunity dynamics, but their relative importance might vary with the spatial scale. The contribution of niche processes is generally expected to increase with increasing spatial extent at a higher rate than that of neutral processes. However, the extent to what community composition is limited by dispersal (usually considered a neutral process) over increasing spatial scales might depend on the dispersal capacity of composing species. To investigate the mechanisms underlying the distribution and diversity of species known to have great powers of dispersal (hundreds of kilometres), we analysed the relative importance of niche processes and dispersal limitation in determining beta‐diversity patterns of aquatic plants and cladocerans over regional (up to 300 km) and continental (up to 3300 km) scales. Both taxonomic groups were surveyed in five different European regions and presented extremely high levels of beta‐diversity, both within and among regions. High beta‐diversity was primarily explained by species replacement (turnover) rather than differences in species richness (i.e. nestedness). Abiotic and biotic variables were the main drivers of community composition. Within some regions, small‐scale connectivity and the spatial configuration of sampled communities explained a significant, though smaller, fraction of compositional variation, particularly for aquatic plants. At continental scale (among regions), a significant fraction of compositional variation was explained by a combination of spatial effects (exclusive contribution of regions) and regionally‐structured environmental variables. Our results suggest that, although dispersal limitation might affect species composition in some regions, aquatic plant and cladoceran communities are not generally limited by dispersal at the regional scale (up to 300 km). Species sorting mediated by environmental variation might explain the high species turnover of aquatic plants and cladocerans at regional scale, while biogeographic processes enhanced by dispersal limitation among regions might determine the composition of regional biotas.