Elements of metacommunity structure and community-environment relationships in stream organisms (original) (raw)
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Oecologia, 2016
We examined variation in the composition of six freshwater organismal groups across various drainage basins in Finland. We first modelled spatial structures within each drainage basin using Moran eigenvector maps. Second, we partitioned variation in community structure among three groups of predictors using constrained ordination: (1) local environmental variables, (2) spatial variables, and (3) dummy variable drainage basin identity. Third, we examined turnover and nestedness components of multiple-site beta diversity, and tested the best fit patterns of our datasets using the "elements of metacommunity structure" analysis. Our results showed that basin identity and local environmental variables were significant predictors of community structure, whereas withinbasin spatial effects were typically negligible. In half of the organismal groups (diatoms, bryophytes, zooplankton), basin identity was a slightly better predictor of community structure than local environmental variables, whereas the opposite was true for the remaining three organismal groups (insects, macrophytes, fish). Both pure basin and local environmental fractions were, however, significant after accounting for the effects of the other predictor variable sets. All organismal groups exhibited high levels of beta diversity, which was mostly attributable to the turnover component. Our results showed consistent Clementsian type metacommunity structures, suggesting that subgroups of species responded similarly to environmental factors or drainage basin limits. We conclude that aquatic communities across large scales are mostly determined by environmental and basin effects, which leads to high beta diversity and prevalence of Clementsian community types.
A topic under intensive study in community ecology and biogeography is the degree to which microscopic, as well as macroscopic organisms, show spatially-structured variation in community characteristics. In general, unicellular microscopic organisms are regarded as ubiquitously distributed and, therefore, without a clear biogeographic signal. This view was summarized 75 years ago by Baas-Becking, who stated ''everything is everywhere, but, the environment selects''. Within the context of metacommunity theory, this hypothesis is congruent with the species sorting model. By using a broad-scale dataset on stream diatom communities and environmental predictor variables across most of Finland, our main aim was to test this hypothesis. Patterns of spatial autocorrelation were evaluated by Moran's I based correlograms, whereas partial regression analysis and partial redundancy analysis were used to quantify the relative importance of environmental and spatial factors on total species richness and on community composition, respectively. Significant patterns of spatial autocorrelation were found for all environmental variables, which also varied widely. Our main results were clear-cut. In general, pure spatial effects clearly overcame those of environmental effects, with the former explaining much more variation in species richness and community composition. Most likely, missing environmental variables cannot explain the higher predictive power of spatial variables, because we measured key factors that have previously been found to be the most important variables (e.g. pH, conductivity, colour, phosphorus, nitrogen) shaping the structure of diatom communities. Therefore, our results provided only limited support for the Baas-Becking hypothesis and the species sorting perspective of metacommunity theory.
Context dependency and metacommunity structuring in boreal headwater streams
Oikos, 2012
We studied the relative importance of spatial and environmental factors as determinants of algal, bryophyte, and macroinvertebrate metacommunities in two boreal drainage basins differing in spatial extent. We used eigenfunction spatial analysis to model the spatial relationships among sites and distance-based redundancy analysis to partition the variability in biotic communities between the spatial filters generated through spatial eigenfunction analysis and the environmental factors measured in the field. In the smaller study area, each metacommunity was structured mostly by environmental factors. This was evidenced by the fact that either the pure environmental effect was significant or environmental factors were strongly spatially structured. In the larger study area, only pure environmental effects were significant. These findings suggest that the environmental control prevails in boreal headwater streams. However, our findings also suggest that the specific details of the community-environment and community-space relationships are dependent on the focal organism group and drainage basin.
A comparative analysis of metacommunity types in the freshwater realm
Ecology and Evolution, 2015
Most metacommunity studies have taken a direct mechanistic approach, aiming to model the effects of local and regional processes on local communities within a metacommunity. An alternative approach is to focus on emergent patterns at the metacommunity level through applying the elements of metacommunity structure (EMS; Oikos, 97, 2002, 237) analysis. The EMS approach has very rarely been applied in the context of a comparative analysis of metacommunity types of main microbial, plant, and animal groups. Furthermore, to our knowledge, no study has associated metacommunity types with their potential ecological correlates in the freshwater realm. We assembled data for 45 freshwater metacommunities, incorporating biologically highly disparate organismal groups (i.e., bacteria, algae, macrophytes, invertebrates, and fish). We first examined ecological correlates (e.g., matrix properties, beta diversity, and average characteristics of a metacommunity, including body size, trophic group, ecosystem type, life form, and dispersal mode) of the three elements of metacommunity structure (i.e., coherence, turnover, and boundary clumping). Second, based on those three elements, we determined which metacommunity types prevailed in freshwater systems and which ecological correlates best discriminated among the observed metacommunity types. We found that the three elements of metacommunity structure were not strongly related to the ecological correlates, except that turnover was positively related to beta diversity. We observed six metacommunity types. The most common were Clementsian and quasi-nested metacommunity types, whereas Random, quasi-Clementsian, Gleasonian, and quasi-Gleasonian types were less common. These six metacommunity types were best discriminated by beta diversity and the first axis of metacommunity ecological traits, ranging from metacommunities of producer organisms occurring in streams to those of large predatory organisms occurring in lakes. Our results showed that focusing on the emergent properties of multiple metacommunities provides information additional to that obtained in studies examining variation in local community structure within a metacommunity.
Most bioassessment programs rest on the assumption that species have different niches, and that abiotic environmental conditions and changes therein determine community structure. This assumption is thus equivalent to the species sorting perspective (i.e. that species differ in their responses to environmental variation) in metacommunity ecology. The degree to which basing bioassessment on the species sorting perspective is reasonable is likely to be related to the spatial extent of a study and the characteristics of the organism groups (e.g. dispersal ability) with which the effects of anthropogenic changes are assessed. Recent findings in metacommunity research have stressed that community structure is determined not only by local abiotic environmental conditions but also by biotic interactions and dispersal-related effects. For example, dispersal limitation may prevent community structure recovery from the effects of a putative stressor, as organisms may not be able to disperse to all sites in a region. Mass effects (i.e. the presence of species in environmentally suboptimal sites due to high dispersal rates from environmentally suitable sites) may, in turn, obscure the effects of a stressor, as dispersal from source sites (e.g. an unaltered site) allows persistence at sink sites (e.g. an anthropogenically altered site). Better bioassessment should thus take both niche-and dispersal-related processes simultaneously into consideration, which can be accomplished by explicitly modelling spatial location as a proxy for dispersal effects. Such an integrated approach should be included in bioassessment programs using general multivariate approaches, predictive modelling, and multimetric indices.
Assembly rules and community models for unicellular organisms: patterns in diatoms of boreal streams
Freshwater Biology, 2005
1. Many studies have addressed either community models (e.g. Clementsian versus Gleasonian gradients) or assembly rules (e.g. nestedness, checkerboards) for higher plant and animal communities, but very few studies have examined different non-random distribution patterns simultaneously with the same data set. Even fewer studies have addressed generalities in the distribution patterns of unicellular organisms, such as diatoms. 2. We studied non-randomness in the spatial distribution and community composition of stream diatoms. Our data consisted of diatom surveys from 47 boreal headwater streams and small rivers in northern Finland. Our analytical approaches included ordinations, cluster analysis, null model analyses, and associated randomisation tests. 3. Stream diatom communities did not follow discrete Clementsian community types, where multiple species occur exclusively in a single community type. Rather, diatom species showed rather individualistic responses, leading to continuous Gleasonian variability in community composition. 4. Although continuous variability was the dominating pattern in the data, diatoms also showed significant nestedness and less overlap in species distribution than expected by chance. However, these patterns were probably only secondary signals from species' individualistic responses to the environment. 5. Although unicellular organisms, such as diatoms, differ from multicellular organisms in several biological characteristics, they nevertheless appear to show largely similar nonrandom distribution patterns previously found for higher plants and metazoans.
Journal of the North American Benthological Society, 2011
Explaining the mechanisms underlying patterns of species diversity and composition in riverine networks is challenging. Historically, community ecologists have conceived of communities as largely isolated entities and have focused on local environmental factors and interspecific interactions as the major forces determining species composition. However, stream ecologists have long embraced a multiscale approach to studying riverine ecosystems and have studied both local factors and larger-scale regional factors, such as dispersal and disturbance. River networks exhibit a dendritic spatial structure that can constrain aquatic organisms when their dispersal is influenced by or confined to the river network. We contend that the principles of metacommunity theory would help stream ecologists to understand how the complex spatial structure of river networks mediates the relative influences of local and regional control on species composition. From a basic ecological perspective, the concept is attractive because new evidence suggests that the importance of regional processes (dispersal) depends on spatial structure of habitat and on connection to the regional species pool. The role of local factors relative to regional factors will vary with spatial position in a river network. From an applied perspective, the long-standing view in ecology that local community composition is an indicator of habitat quality may not be uniformly applicable across a river network, but the strength of such bioassessment approaches probably will depend on spatial position in the network. The principles of metacommunity theory are broadly applicable across taxa and systems but seem of particular consequence to stream ecology given the unique spatial structure of riverine systems. By explicitly embracing processes at multiple spatial scales, metacommunity theory provides a foundation on which to build a richer understanding of stream communities.
Null model analyses have greatly improved our understanding of species co-occurrence. Null model analyses have shown, for example, that cold-blooded animals show less segregated distributions than warm-blooded animals. This topic has rarely been studied simultaneously across multiple metacommunities. We analysed data on 10 stream metacommunities (with 10 communities in each metacommunity) of a cold-blooded animal group, benthic macroinvertebrates, and examined co-occurrence within five ecological guilds. We found that the segregated species co-occurrence was not the rule in stream invertebrate guilds. This was evidenced by the finding that only 10% of the 50 guild matrices we analyzed showed significant segregation and no matrices showed significant aggregation in the within-stream analyses. However, in the across-streams analysis, all guilds showed significant segregation. We neither found differences in the degree of segregation among the guilds, the degree of species segregation did not increase with overall environmental heterogeneity, and there were no differences in the relationships between species segregation and overall environmental heterogeneity among the guilds. Expanding the spatial extent from single stream metacommunities (i.e. within each stream) to the whole study region (i.e. across the streams) increased significantly segregation in all guilds. Because environmental heterogeneity across streams was much higher than within single streams, overall environmental heterogeneity may nevertheless have effects on species segregation. It also seems that the effects of overall heterogeneity on species segregation were masked by mass effects in the within-stream analyses.
Hydrobiologia, 2014
Most studies characterize metacommunities based on a single snapshot of the spatial structure, which may be inadequate for taxa with high migratory behavior (e.g., fish). Here, we applied elements of metacommunity structure to examine variations in the spatial distributions of stream fishes over time and to explore possible structuring mechanisms. Although the major environmental gradients influencing species distributions remained largely the same in time, the best-fit pattern of metacommunity structure varied according to sampling occasion and whether or not we included non-native species in the analyses. Quasi-Clementsian and Clementsian structures were the predominant best-fit structures, indicating the importance of species turnover among sites and the existence of more or less discrete community boundaries. The environmental gradient most correlated with metacommunity structure was defined by altitude, area of artificial ponds in the catchment, and dissolved oxygen content. Our results suggest that the best-fit metacommunity structure of the native species can change in time in this catchment due to seasonal changes in distribution patterns. However, the distribution of non-native species throughout the landscape homogenizes the temporal variability in metacommunity structure of native species. Further studies are necessary from other regions to examine best-fit metacommunity structures of stream fishes within relatively short environmental gradients.
Streams within geothermal areas in Iceland that vary in a syndrome of temperature-linked variables, including discharge and potential grazing pressure (snails vs. chironomid larvae), provided a test of how microvegetation structure might change systematically with such drivers. We examined if such streams could form a parallel with biome sequences, for example with similar change in overall structure and organism traits found from tundra to Boreal forest. The warmer streams had an over-story of bryophytes with more open patches of epilithic algae and Cyanobacteria, and a grazer community of snails and blackfly larvae; the vegetation of the colder streams consisted mainly of epilithic algae and Cyanobacteria, dominated by grazing chironomid larvae. Diatom species comprising the epilithic assemblages were generally smaller in the warmer streams compared with the colder streams. Temperature and discharge did not significantly influence the diversity of diatoms (Shannon Index, Sørensen Index, Evenness, Species richness), although more species were observed in the colder streams compared with the warmer streams. Trait diversity was greatest in the coldest streams, but despite high grazing pressure only one growth form (attachment by a mucilage stalk) was predominant in the warmer regimes. Grazers may have influenced diatom species traits but did not significantly suppress the diatom biomass. Analogy with the sequence from tundra, through taiga to boreal forest, over a comparable temperature gradient, showed some parallel features but important differences. Microorganism systems may thus not always be the useful surrogates for larger systems that some ecologists have suggested.