The role of environmental and spatial processes in structuring native and non‐native fish communities across thousands of lakes

Quantifying the role of spatial patterns is an important goal in ecology to further understand patterns of community composition. We quantified the relative role of environmental conditions and regional spatial patterns that could be produced by environmental filtering and dispersal limitation on fish community composition for thousands of lakes. A database was assembled on fish community composition, lake morphology, water quality, climatic conditions, and hydrological connectivity for 9885 lakes in Ontario, Canada. We utilized a variation partitioning approach in conjunction with Moran's Eigenvector Maps (MEM) and Asymmetric Eigenvector Maps (AEM) to model spatial patterns that could be produced by human‐mediated and natural modes of dispersal. Across 9885 lakes and 100 fish species, environmental factors and spatial structure explained approximately 19% of the variation in fish community composition. Examining the proportional role of spatial structure and environmental conditions revealed that as much as 90% of the explained variation in native species assemblage composition is governed by environmental conditions. Conversely on average, 67% of the explained variation in non‐native assemblage composition can be related to human‐mediated dispersal. This study highlights the importance of including spatial structure and environmental conditions when explaining patterns of community composition to better discriminate between the ecological processes that underlie biogeographical patterns of communities composed of native and non‐native fish species.     

The relative importance of dispersal and local processes in structuring phytoplankton communities in a set of highly interconnected ponds

1. The recognition that both local and regional processes act together in shaping local communities makes determining their relative roles in natural communities central to understanding patterns in community structure. 2. We investigated the relative influence of these processes on the phytoplankton communities of a highly interconnected pond system. We sampled the phytoplankton communities of 28 ponds concurrently with 20 local environmental variables. 3. We found that phytoplankton community variation, in terms of both phytoplankton community composition (PCC) and diversity, was only significantly explained by local environmental variables. These were mainly associated with the contrasting clear‐water and turbid ecological states of the shallow ponds studied. Clear‐water conditions favoured only a few taxa, resulting in a significantly lower taxon diversity and richness under these conditions. 4. The failure to explain variation in PCC by a dispersal model based on the water flow between ponds points at very effective species sorting. This is attributed to the high population turn‐over rates and sensitivity to environmental conditions of phytoplankton communities. Some evidence was found, however, that dispersal influences local communities through mass effects between neighbouring ponds. 5. Overall, our results emphasize both the strong selection pressure that components of the food web exert on phytoplankton communities and the high potential of these communities to respond to such environmental change, thereby effectively opposing the homogenizing effects of continuous dispersal.     

An experimental model for the spatial structuring and selection of bacterial communities

Community-level selection is an important concept in evolutionary biology and has been predicted to arise in systems that are spatially structured. Here we develop an experimental model for spatially-structured bacterial communities based on coaggregating strains and test their relative fitness under a defined selection pressure. As selection we apply protozoan grazing in a defined, continuous culturing system. We demonstrate that a slow-growing bacterial strain Blastomonas natatoria 2.1, which forms coaggregates with Micrococcus luteus, can outcompete a fast-growing, closely related strain Blastomonas natatoria 2.8 under conditions of protozoan grazing. The competitive benefit provided by spatial structuring has implications for the evolution of natural bacterial communities in the environment.     

The importance of crustacean zooplankton in structuring rotifer and phytoplankton communities; an enclosure study

During the spring clear-water phase of 1993, an enclosure experimentwas performed in the mesotrophic Schöhsee (Plön, FRG)in order to assess the impact of crustacean zooplankton on therotifer and phytoplankton community. Among the crustacean plankton,calanoid and cyclopoid cope-pods were abundant, but Daphnia‘longispina’ reached the highest densities in thisexperiment. The colonial rotifer Conochilus unicomis was notaffected by crustacean plankton. The two most abundant species,Synchaeta peclinata and Keratella cochlearis, increased exponentiallywhen macrozooplankton had been excluded from the enclosures,but did not increase when crustaceans were present. Birth anddeath rates of K.cochlearis could be reliably determined inthis field experiment, suggesting that this rotifer specieswas mainly controlled by exploitative competition rather thanby mechanical interference or predation. Daphnia ‘longispina’generally grazed selectively on the smaller ciliates and algae,thus depriving the rotifers of their phytoplankton resources.The dominant alga, the chrysophycean Dinobryon, increased, whethercrustaceans were present or not, but appeared to be grazed uponto a certain extent despite its considerable cell size and colonialorganization.     

Salinity gradient and nutrients as major structuring factors of the phytoplankton communities in salt marshes

This study investigated the major environmental factors structuring, for a year, phytoplankton assemblages in the Sfax saltern (Tunisia): salinity and nutrients. A STATICO analysis based on 11 environmental variables and the abundances of 64 phytoplanktonic species was conducted. STATICO is used to analyze the stable part of the relationships between the environment and species, and then to determine how these relationships change over time. The analysis confirmed that the salinity gradient had a considerable influence on the composition of the phytoplanktonic communities. Bacillariophyceae and Dinophyceae dominated in the least salty ponds, whereas Chlorophyceae and Cyanophyceae dominated in the saltiest ponds, in accordance with the halotolerance level estimated for each species by calculating the optimum salinity and salt tolerance. Nitrogen (N) and phosphorus (P) ions could have a secondary influence on the phytoplankton distribution and its dynamics. Dinophyceae seem to be favored by high ammonium (NH₄ ⁺) concentrations, whereas diatoms seem to be favored by high orthophosphates (PO₄ ^3?) and nitrates (NO₃ ^?) values. The Chlorophyceae Dunalliela salina thrived in the saltiest ponds when the NO₃ ^? concentrations increased.     

Processes structuring communities: evidence for trait-mediated indirect effects through induced polymorphisms

Trait-mediated indirect effects (TMIs) are changes in the density of one species that are caused by induced changes in one or more traits of an intervening species. For example, induced defense in a prey species may alter the nature of indirect effects that are mediated through that prey species. In the present study, we investigated the TMIs that stem from an interaction between a carnivorous whelk ( Acanthina angelica ) and an intertidal barnacle ( Chthamalus anisopoma) . Depending on the timing of the interaction, the predator either kills the barnacle or induces a predation-resistant morph. Based on previous work that addressed the direct interactions between Chthamalus and other species in the community we predicted and subsequently found that community structure varies as a function of these differing effects. Specifically, we found that Acanthina has a positive indirect effect on mussels when it interacts with (kills) undefended adult barnacles. In contrast, the predator has a positive indirect effect on an encrusting algae ( Ralfsia ) when it comes into contact with juvenile barnacles, causing the induction of the predator-resistant morph. We suggest that further research should consider the role of environmentally induced polymorphisms in structuring communities.     

Seasonal and spatial functional shifts in phytoplankton communities of five tropical reservoirs

Trait-based approaches have become increasingly important and valuable in understanding phytoplankton community assembly and composition. These approaches allow for comparisons between water bodies with different species composition. We hypothesize that similar changes in environmental conditions lead to similar responses with regard to functional traits of phytoplankton communities, regardless of trophic state or species composition. We studied the phytoplankton (species composition, community trait mean and diversity) of five reservoirs in Brazil along a trophic gradient from ultra-oligotrophic to meso-eutrophic. Samples at two seasons (summer/rainy and winter/dry) with a horizontal and vertical resolution were taken. Using multivariate analysis, the five reservoirs separated, despite some overlap, according to their environmental variables (mainly total phosphorus, conductivity, pH, chlorophyll a). However, between the seasonal periods, the reservoirs shifted in a similar direction in the multi-dimensional space. The seasonal response of the overall phytoplankton community trait mean differed between the ultra-oligotrophic and the other reservoirs, with three reservoirs exhibiting a very similar community trait mean despite considerable differences in species composition. Within-season differences between different water layers were low. The functional diversity was also unrelated to the trophic state of the reservoirs. Thus, seasonal environmental changes had strong influence on the functional characteristics of the phytoplankton community in reservoirs with distinct trophic condition and species composition. These results demonstrate that an ataxonomic trait-based approach is a relevant tool for comparative studies in phytoplankton ecology.     

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.     

Integrating environmental and spatial processes in ecological community dynamics

The processes controlling the abundances of species across multiple sites form the cornerstone of modern ecology. In these metacommunities, the relative importance of local environmental and regional spatial processes is currently hotly debated, especially in terms of the validity of neutral model. I collected 158 published data sets with information on community structure, environmental and spatial variables. I showed that approximately 50% of the variation in community composition is explained by both environmental and spatial variables. The majority of the data sets were structured by species-sorting dynamics (SS), followed by a combination of SS and mass-effect dynamics. While neutral processes were the only structuring process in 8% of the collected natural communities, disregarding neutral dispersal processes would result in missing important patterns in 37% of the studied communities. Moreover, metacommunity characteristics such as dispersal type, habitat type and spatial scale predicted part of the detected variation in metacommunity structure.     

Variation in coral-associated cryptofaunal communities across spatial scales and environmental gradients

Coral Reefs - Most of the diversity on coral reefs is in the cryptofauna, the hidden organisms that inhabit the interstitial spaces of corals and other habitat-forming benthos. However, little is...     

The role of spatial scale and area in determining richness-altitude gradients in Swedish lake phytoplankton communities

Lake phytoplankton richness data were analyzed to reveal the effects of area and scale on the richness-elevation relationship. Lakes provide a unique opportunity to accomplish this because of their well-defined boundaries, which clearly define local communities and their habitat area. Local phytoplankton richness (alpha diversity) was found to decrease with increasing elevation, even when the effect of lake area was accounted for. This decrease coincided with a decrease in lake productivity with increasing elevation. Additionally, pairwise dissimilarity calculations suggested that isolation by distance was less important in structuring local phytoplankton richness than isolation by either elevation or environmental distance. In contrast, phytoplankton richness calculated for elevation bands (gamma diversity) showed a hump-shaped dependence on elevation, with a mid-elevation maximum, consistent with the predictions of a mid-domain null model. Moreover, mean pairwise dissimilarity within elevation bands shows compositional dissimilarity to be highest at high elevations and low environmental distances, emphasizing the isolated character of high-elevation lakes. We suggest that the form of the phytoplankton richness-elevation relationship is scale-dependent, and that geometric constraints, differences in the possibility of horizontal and vertical dispersal, environmental heterogeneity and an underlying monotonic trend in productivity, are likely to be responsible for the patterns in alpha and gamma diversity observed along elevation gradients.     

Genetic diversity of marine Synechococcus and co-occurring cyanophage communities: evidence for viral control of phytoplankton

Unicellular cyanobacteria of the genus Synechococcus are a major component of the picophytoplankton and make a substantial contribution to primary productivity in the oceans. Here we provide evidence that supports the hypothesis that virus infection can play an important role in determining the success of different Synechococcus genotypes and hence of seasonal succession. In a study of the oligotrophic Gulf of Aqaba, Red Sea, we show a succession of Synechococcus genotypes over an annual cycle. There were large changes in the genetic diversity of Synechococcus, as determined by restriction fragment length polymorphism analysis of a 403- bp rpoC1 gene fragment, which was reduced to one dominant genotype in July. The abundance of co-occurring cyanophage capable of infecting marine Synechococcus was determined by plaque assays and their genetic diversity was determined by denaturing gradient gel electrophoresis analysis of a 118-bp g20 gene fragment. The results indicate that both abundance and genetic diversity of cyanophage covaried with that of Synechococcus. Multivariate statistical analyses show a significant relationship between cyanophage assemblage structure and that of Synechococcus. These observations are consistent with cyanophage infection being a major controlling factor in picophytoplankton succession.     

Relative influence of deterministic processes on structuring marsh plant communities varies across an abiotic gradient

Understanding the processes determining community structure is one of the major goals of ecological research. Both deterministic and stochastic processes may shape community structure. The challenge is to understand the relative influence of each type of process across different environmental conditions. We investigated the influence of deterministic and stochastic processes on plant community assembly in tidal marshes across a strong abiotic (salinity) gradient in three estuaries in Georgia, USA using probabilistic Raup–Crick community dissimilarity. Our results indicated that deterministic processes had an increasingly important influence on structuring plant communities in salt and brackish marshes, probably due to high heterogeneity of microhabitats produced by the interplay between abiotic stress and biotic interactions. In contrast, the influence of deterministic processes on plant community assembly decreased in tidal freshwater marshes, suggesting an increasingly important role of stochastic processes in plant community assembly in tidal freshwater marshes, probably due to the higher species richness, higher recruitment from seed, and lower levels of abiotic stress in these habitats. At the estuarine scale (across tidal freshwater, brackish and salt marshes in each estuary), our results suggested that deterministic processes also had a relatively important influence on shaping plant community structure. Our results illustrated that plant community structure in tidal marshes is influenced by both deterministic and stochastic processes, but that the relative influence of these two types of processes varies across estuarine landscapes.     

Concordance among assemblages of upland Amazonian lakes and the structuring role of spatial and environmental factors

Conservation planning and biodiversity monitoring rely exclusively on the use of surrogate groups. However, the effectiveness of these groups in representing unknown biodiversity is rarely tested. These tests can be based on cross-taxon concordance (or congruence) analysis. In a second step, the main mechanisms underlying concordance can be investigated. Here, we evaluated the relative influence of local environmental factors and spatial processes on the patterns of beta-diversity exhibited by phytoplankton, zooplankton and benthic invertebrates and the strength of assemblage concordance among these assemblages in 38 upland Amazonian lakes (northern Brazil) over four years. Zooplankton and benthic invertebrates responded exclusively to environmental variables. These results are in accordance with the expected under the species sorting model, where local factors are the main mechanisms shaping assemblage structure. In general, patterns of concordance between the assemblages were weak and varied through time, indicating that the use of surrogates in freshwater ecosystems may be a flawed approach.     

The roles of environmental conditions and spatial factors in controlling stream macroalgal communities

In the last three decades, several studies have suggested that the structure of stream macroalgal communities is shaped by local environmental variables, but some recent papers have shown that the relevance of the environment on these communities may be overestimated. Using Partial Redundancy Analysis (pRDA), we analyzed macroalgal communities (considering all macroalgae and Phyla Chlorophyta, Cyanobacteria, and Rhodophyta individually) from 105 streams in southern Brazil to test the hypothesis that the relative contributions of the environment and space on the taxonomic composition of these communities is mainly determined by the biological traits and dispersal mechanisms typical for each group. The pRDA showed that the taxonomic composition of the entire community and green algae were explained by both space and environment, whereas for cyanobacteria, only the environment was significant, and for red algae, only space was significant. These divergences in the relative contribution among algal phyla were consistent with our initial hypothesis and can be ascribed to the differences in the ecological features of each group. Our results also support the idea that the community structure of organisms with low dispersal is influenced more significantly by spatial processes, whereas for organisms with high dispersal the local environmental variables are more influential.     

Origin of phytoplankton and the environmental factors governing the structure of microalgal communities in lowland streams

Information on the structure of microalgal assemblages in the epiphyton and epilithon is necessary to understand the origin of phytoplankton in lowland rivers. To this end, we carried out concurrent investigations on phytoplankton, epiphyton and epilithon in 18 reaches of three Estonian rivers during the midsummers of 2002 and 2003. A total of 251 taxa was recorded, of which 192 were epiphyton species, 158 were epilithon species and 150 were phytoplankton species. Canonical correspondence analysis (CCA), based on the 31 most abundant taxa, indicated differences in the structure of the algal assemblages between the different biotopes (phytoplankton, epiphyton and epilithon) as well as between the studied rivers. The composition of the phytoplankton clearly differed from that of the other biotopes, with prevailing small flagellates, a chrysophyte (Synura uvella) and cryptophytes (Rhodomonas lacustris and Cryptomonas erosa). The epiphyton was characterized by a large number of diatoms, while the epilithic community contained filamentous cyanobacteria (Phormidium tergestinum and Planktolyngya sp.) and a green alga (Stigeoclonium tenue) in addition to diatoms. Based on redundancy analysis (RDA), phosphorous was the most relevant parameter determining the distribution of species in the phytoplankton assemblages. Shading by trees on the river bank, dissolved oxygen concentration and water temperature as well as river width determined the distribution of species in the epiphyton. The data set on the epilithon did not reveal any significant relationships between species distribution and the measured environmental parameters.     

Cell death in lake phytoplankton communities

1. The fraction of living and dead phytoplankton cells in seven Florida lakes was assessed by using the cell digestion assay, a non‐staining membrane permeability test. The cell digestion assay is an effective method to analyse cell viability in complex natural phytoplankton communities. 2. The lakes examined ranged widely in phytoplankton abundance and community composition. The variability in the percentage of living cells (% LC) was high among the taxonomic groups forming the different phytoplankton communities, ranging from 19.7% to 98% LC. 3. All cells within single cyanobacteria filaments were determined to be either dead or alive, suggesting physiological integration of the cells within colonies. 4. Within each lake, the dominant taxa generally exhibited the highest proportion of living cells. A high proportion of living cells was found to be a characteristic of the different taxa forming the communities of eutrophic lakes. The average value for the % LC for all groups comprising the phytoplankton communities in each of the lakes ranged from 29.9 ± 7.2 to 80.4 ± 4.0 (mean ± SE) and varied strongly and positively with chlorophyll a concentration. 5. These results suggest phytoplankton cell death to be an important process structuring phytoplankton communities in lakes, particularly in oligotrophic ones.     

Biomass partitioning in Florida phytoplankton communities

Comparison of the partition of biomass among genera within phytoplanktoncommunities in 165 lakes demonstrated a shift from communitieswith a relatively even distribution of biomass among taxa tocommunity where one, or a few taxa, capitalized most of thebiomass, as communities biomass increased. This pattern suggeststhat interference among co-existing phytoplankton taxa increasesas community biomass increases, and provides additional evidencefor the biomass-dependence of phytoplankton community structure.