Role of environmental and spatial processes in structuring anuran communities across a tropical rain forest

This study aimed to evaluate if anuran species distributions in riparian and non‐riparian areas are influenced by environmental factors (i.e. niche) and/or by spatial factors (i.e. dispersal). The environmental variables analysed were altitude, distance from the stream and leaf litter depth. Spatial factors were represented by the eigenvectors extracted from geographical coordinates by eigenfunction analysis. The study was conducted in 24 km² of terra‐firme forest in Central Amazonia, Manaus – Amazonas, Brazil. Between November 2008 and May 2009, three samples were taken from 41 plots, 21 plots being placed at non‐riparian areas and another 20 placed in riparian areas. We submitted the assemblage dataset to a partial redundancy analysis to evaluate the contributions of environmental and spatial variables (selected with a forward selection procedure). In addition, we tested if communities differ from riparian and non‐riparian areas using a db‐MANOVA. Species richness and species composition differed between riparian and non‐riparian plots. Some species were restricted to riparian areas. Altitude was the only significant variable (P = 0.005) explaining 21% of the total variance. When analysing the data from all plots using the partial redundancy analysis, 27% of the variance was explained by spatial and environmental variables. The environmental variables explained exclusively 4% of the variance in assemblage composition, and 13% was explained by environmental variables that were also structured in space (i.e. the shared fraction), while 10% was explained exclusively by spatial variables. In conclusion, our results showed differences between the assemblages of riparian and non‐riparian areas which can be explained by the distribution of anuran species along environmental gradients altitude and distance to streams, with little evidence of dispersal limitation.     

Landscape‐scale determinants of non‐native fish communities

Aim Predicting and preventing invasions depends on knowledge of the factors that make ecosystems susceptible to invasion. Current studies generally rely on non‐native species richness (NNSR) as the sole measure of ecosystem invasibility; however, species identity is a critical consideration, given that different ecosystems may have environmental characteristics suitable to different species. Our aim was to examine whether non‐native freshwater fish community composition was related to ecosystem characteristics at the landscape scale. Location United States. Methods We described spatial patterns in non‐native freshwater fish communities among watersheds in the Mid‐Atlantic region of the United States based on records of establishment in the U.S. Geological Survey’s Nonindigenous Aquatic Species Database. We described general relationships between non‐native species and ecosystem characteristics using canonical correspondence analysis. We clustered watersheds by non‐native fish community and described differences among clusters using indicator species analysis. We then assessed whether non‐native communities could be predicted from ecosystem characteristics using random forest analysis and predicted non‐native communities for uninvaded watersheds. We estimated which ecosystem characteristics were most important for predicting non‐native communities using conditional inference trees. Results We identified four non‐native fish communities, each with distinct indicator species. Non‐native communities were predicted based on ecosystem characteristics with an accuracy of 80.6%, with temperature as the most important variable. Relatively uninvaded watersheds were predicted to be invasible by the most diverse non‐native community. Main conclusions Non‐native species identity is an important consideration when assessing ecosystem invasibility. NNSR alone is an insufficient measure of invasibility because ecosystems with equal NNSR may not be equally invasible by the same species. Our findings can help improve predictions of future invasions and focus management and policy decisions on particular species in highly invasible ecosystems.     

No evidence for environmental and spatial processes in structuring phytoplankton communities

The relative importance of local and regional processes in shaping natural communities within a metacommunity context has been a focus of intense debate in recent years. Floodplain lakes provide a good system for testing this theoretical approach, as they undergo seasonal variations in physical, chemical and biological factors, as well as in their degree of connectivity. Here, we investigated how local phytoplankton communities in lakes of a tropical river-floodplain system (Araguaia River floodplain – Central Brazil) were affected by environmental and spatial (dispersal) predictors in two rainy and two dry seasons (two consecutive years). Partial redundancy analysis indicated that during the periods analyzed the effects of neither predictor were significant. Although we cannot exclude the possibility that these tropical phytoplankton communities could be regulated by stochastic events, we suggested that further studies will have greater explanatory power if they include other variables related to biotic interactions (e.g., abundance of grazers) and fine-scale environmental variation.     

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.     

Co‐varying impacts of land use and non‐native brown trout on fish communities in small streams

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Non‐native species modify the isotopic structure of freshwater fish communities across the globe

Multiple anthropogenic pressures including the widespread introductions of non‐native species threaten biodiversity and ecosystem functioning notably by modifying the trophic structure of communities. Here, we provided a global evaluation of the impacts of non‐native species on the isotopic structure (δ¹³C and δ¹⁵N) of freshwater fish communities. We gathered the stable isotope values (n = 4030) of fish species in 496 fish communities in lentic (lakes, backwaters, reservoirs) and lotic (running waters such as streams, rivers) ecosystems throughout the world and quantified the isotopic structure of communities. Overall, we found that communities containing non‐native species had a different isotopic structure than communities without non‐native species. However, these differences varied between ecosystem types and the trophic positions of non‐native species. In lotic ecosystems, communities containing non‐native species had a larger total isotopic niche than communities without non‐native species. This was primarily driven by the addition of non‐native predators at the top of the food chain that increased δ¹⁵N range without modifying the isotopic niche size of native species. In lentic ecosystems, non‐native primary consumers increased δ¹⁵N range and this was likely driven by an increase of resource availability for species at higher trophic levels, increasing food chain length. The introduction of non‐native secondary consumers at the centre of the isotopic niche of recipient communities decreased the core isotopic niche size, the δ¹³C range of recipient communities and the total isotopic niche of coexisting native species. These results suggested a modified contribution of the basal resources consumed (e.g. multi‐chain omnivory) and an increase level of competition with native species. Our results notably imply that, by affecting the isotopic structure of freshwater fish communities at a global scale, non‐native species represent an important source of perturbations that should be accounted for when investigating macro‐ecological patterns of community structure and biotic interactions.     

The role of non‐native plants and vertebrates in defining patterns of compositional dissimilarity within and across continents

Aim Human activities have led to the spread and establishment of increasing numbers of non‐native species. Here we assess whether non‐native plant and vertebrate species have affected species compositions within and across Europe and North America. We also assess the effects of intra‐continental species exchange using the example of vertebrates. Location European countries and North America (states in the contiguous United States and provinces of Canada). Methods We measured compositional dissimilarity of native and non‐native assemblages of vascular plants and vertebrates and related these patterns to climatic dissimilarity and geographical distance. We considered three categories of non‐native species (introduced after ad 1500), namely: those (1) originating outside of both continents, (2) native to one continent and non‐native to the other, and (3) native in a particular region of a continent but non‐native in another region. Results The presence of non‐native plants and vertebrates led to more homogeneous species compositions between continents and to less homogeneous species composition within Europe compared with the native assemblages. In North America, the presence of non‐native plants led to more homogeneous species compositions and the presence of non‐native vertebrates had no effect. Species compositions being more homogeneous than the native composition were found for the three categories of non‐native vertebrate species for both continents. Between continents, climate was a better predictor of compositional dissimilarity for non‐native plants, whereas for vertebrates the explanatory power of climate and geographical distance were comparable. By contrast, within continents, climate was a better predictor of compositional dissimilarity of both plants and vertebrates. Conclusions We found clear evidence for biotic homogenization as a consequence of species displacement. However, in relation to overall species richness this effect was rather small, indicating that floras and faunas are still quite distinct. Therefore, claiming that we already face homogeneous biotas might be premature, although clear indications are visible which should raise a note of caution, especially in the light of increasing globalization.     

Current knowledge on non‐native freshwater fish introductions

This review provides a contemporary account of knowledge on aspects of introductions of non‐native fish species and includes issues associated with introduction pathways, ecological and economic impacts, risk assessments, management options and impact of climate change. It offers guidance to reconcile the increasing demands of certain stakeholders to diversify their activities using non‐native fishes with the long‐term sustainability of native aquatic biodiversity. The rate at which non‐native freshwater fishes have been introduced worldwide has doubled in the space of 30 years, with the principal motives being aquaculture (39%) and improvement of wild stocks (17%). Economic activity is the principal driver of human‐mediated non‐native fish introductions, including the globalization of fish culture, whereby the production of the African cichlid tilapia is seven times higher in Asia than in most areas of Africa, and Chile is responsible for c. 30% of the world's farmed salmon, all based on introduced species. Consequently, these economic benefits need balancing against the detrimental environmental, social and economic effects of introduced non‐native fishes. There are several major ecological effects associated with non‐native fish introductions, including predation, habitat degradation, increased competition for resources, hybridization and disease transmission. Consideration of these aspects in isolation, however, is rarely sufficient to adequately characterize the overall ecological effect of an introduced species. Regarding the management of introduced non‐native fish, pre‐introduction screening tools, such as the fish invasiveness scoring kit (FISK), can be used to ensure that species are not introduced, which may develop invasive populations. Following the introduction of non‐native fish that do develop invasive populations, management responses are typified by either a remediation or a mitigation response, although these are often difficult and expensive to implement, and may have limited effectiveness.     

Soil pathogen communities associated with native and non‐native Phragmites australis populations in freshwater wetlands

Soil pathogens are believed to be major contributors to negative plant–soil feedbacks that regulate plant community dynamics and plant invasions. While the theoretical basis for pathogen regulation of plant communities is well established within the plant–soil feedback framework, direct experimental evidence for pathogen community responses to plants has been limited, often relying largely on indirect evidence based on above‐ground plant responses. As a result, specific soil pathogen responses accompanying above‐ground plant community dynamics are largely unknown. Here, we examine the oomycete pathogens in soils conditioned by established populations of native noninvasive and non‐native invasive haplotypes of Phragmites australis (European common reed). Our aim was to assess whether populations of invasive plants harbor unique communities of pathogens that differ from those associated with noninvasive populations and whether the distribution of taxa within these communities may help to explain invasive success. We compared the composition and abundance of pathogenic and saprobic oomycete species over a 2‐year period. Despite a diversity of oomycete taxa detected in soils from both native and non‐native populations, pathogen communities from both invaded and noninvaded soils were dominated by species of Pythium. Pathogen species that contributed the most to the differences observed between invaded and noninvaded soils were distributed between invaded and noninvaded soils. However, the specific taxa in invaded soils responsible for community differences were distinct from those in noninvaded soils that contributed to community differences. Our results indicate that, despite the phylogenetic relatedness of native and non‐native P. australis haplotypes, pathogen communities associated with the dominant non‐native haplotype are distinct from those of the rare native haplotype. Pathogen taxa that dominate either noninvaded or invaded soils suggest different potential mechanisms of invasion facilitation. These findings are consistent with the hypothesis that non‐native plant species that dominate landscapes may “cultivate” a different soil pathogen community to their rhizosphere than those of rarer native species.     

The role of local and regional processes in structuring larval dragonfly distributions across habitat gradients

Despite the importance of community-structuring processes operating at both local and regional scales, there is relatively little work examining both forces within a single system. I used a combination of observational and experimental approaches to examine the processes structuring larval dragonfly distributions in lentic habitats that encompass a gradient of both permanence and top predator type. I compared the relative vulnerability of species to predators from different portions of this gradient to assess the role of predation as a local force structuring communities. I also assessed the role of regional processes on species' distributions by examining species' propensity to disperse to and colonize artificial ponds distributed across a landscape. In both studies I contrasted habitat specialist species, which had larvae restricted to permanent lakes, with habitat generalist species, which had larvae that occur broadly across the habitat permanence and top predator transition. Results from this work suggest that dispersal and colonization behavior were critical mechanisms restricting the distributions of habitat specialist species, but that predation may act to reinforce this pattern. The habitat specialists dispersed less frequently, colonized artificial ponds less often when they did reach them, and most moved shorter distances than the habitat generalist species. Habitat specialists were also more vulnerable than habitat generalists to an invertebrate top predator with which they do not co-exist. Results from these studies suggest that species distributions can be shaped by processes operating at both regional and local spatial scales. The role of dispersal and recruitment limitation may be generally underestimated as a force shaping species distributions and community structure across habitat gradients in which there is a transition in both the biotic interactions and the disturbance interval across that gradient.     

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 specialist parasites in structuring host communities

Natural enemies can be a powerful force when structuring natural communities, and in facilitating or preventing species coexistence depending on the nature of the trophic interaction. In particular, “keystone” predators can promote species coexistence, provided they preferentially attack the competitively dominant species. However, it is not clear whether parasites can play a similar structuring role; parasites typically form chronic associations with their victims, reducing their fitness (i.e., fecundity) rather than survival, and allowing infected hosts to remain viable competitors within the community. Therefore the density-dependent suppression of the host is likely to be more subtle than that due to predation. Using a series of simple population-dynamic models we show that specialist parasites can facilitate species coexistence, although possibly less so than predators. These results contrast with those typically found with models of generalist parasites, which can reduce the likelihood of species coexistence through apparent competition. In addition, we show that the likelihood of parasite-facilitated species coexistence depends greatly on the specific type of parasite. In particular, macroparasites (e.g., parasitic helminths) may be less likely to facilitate species coexistence than microparasites (e.g., viruses or bacteria) due to their typically highly aggregated distribution amongst their hosts. Furthermore, species coexistence is more likely if the parasite is relatively benign to its host. Parasitism by apparently “harmless” specialist parasites may provide an important but overlooked factor in the maintenance of species diversity, facilitating species invasions into new communities and the emergence of novel infectious diseases.     

A multi-scale comparison of trait linkages to environmental and spatial variables in fish communities across a large freshwater lake

Species present in communities are affected by the prevailing environmental conditions, and the traits that these species display may be sensitive indicators of community responses to environmental change. However, interpretation of community responses may be confounded by environmental variation at different spatial scales. Using a hierarchical approach, we assessed the spatial and temporal variation of traits in coastal fish communities in Lake Huron over a 5-year time period (2001–2005) in response to biotic and abiotic environmental factors. The association of environmental and spatial variables with trophic, life-history, and thermal traits at two spatial scales (regional basin-scale, local site-scale) was quantified using multivariate statistics and variation partitioning. We defined these two scales (regional, local) on which to measure variation and then applied this measurement framework identically in all 5 study years. With this framework, we found that there was no change in the spatial scales of fish community traits over the course of the study, although there were small inter-annual shifts in the importance of regional basin- and local site-scale variables in determining community trait composition (e.g., life-history, trophic, and thermal). The overriding effects of regional-scale variables may be related to inter-annual variation in average summer temperature. Additionally, drivers of fish community traits were highly variable among study years, with some years dominated by environmental variation and others dominated by spatially structured variation. The influence of spatial factors on trait composition was dynamic, which suggests that spatial patterns in fish communities over large landscapes are transient. Air temperature and vegetation were significant variables in most years, underscoring the importance of future climate change and shoreline development as drivers of fish community structure. Overall, a trait-based hierarchical framework may be a useful conservation tool, as it highlights the multi-scaled interactive effect of variables over a large landscape.     

Changes in the roles of spatial and environmental processes in the structuring of rodent metacommunities

We studied the role of spatial (regional) and environmental (local) processes in the structuring of rodent metacommunities in three contiguous ecoregions that share the same species pool. The two northern ecoregions are mainly affected by anthropogenic processes (agriculture and urbanization) while the southern one is mainly affected by natural processes (flood and drought pulses). Local communities were described based on the analysis of 77 samples of barn owl pellets. To identify which processes (patch dynamics, species sorting, mass effect or neutral theory) structure each metacommunity we evaluated the percentage of variance explained by space (spatial arrangement of communities) and environment (topography, climate and land cover) in three Variation Partitioning Redundancy Analyses. The percentage of variance in rodent metacommunities composition explained by space and environment was between 38 and 61%, and was significant in all three analyses. The pure space fraction was significant for two of the three ecoregions, while the pure environmental fraction was significant for all three ecoregions. The processes that structure rodent metacommunities change across the region. In all three ecoregions the species sorting played a key role, while, mass effect was a structuring factor for northern metacommunities. These results can be explained by species-specific dispersal characteristics and environmental filtering.     

Disentangling environmental,spatial, and historical effects on ostracod communities in shallow lakes

Planktonic cladocerans of the genus Daphnia are an example of organisms whose ability to disperse among aquatic habitats is limited to dispersal of dormant eggs, encapsulated in protective structures called ephippia. In the present study, we aimed to quantify the production of floating and sinking ephippia in lakes to test the hypothesis that, even if they eventually sink, most of them are first floating at the surface. In addition, we checked the egg content status of the ephippia. The results of this study revealed numbers of ephippia floating at lakes surface reaching thousands per square metre at the time of ephippia production, and constituting substantial share of that production. In studied shallow lowland lakes and in a deep mountain lake, most ephippia were first floating at the water surface, while in deep lowland lakes the proportion was reversed. Approximately half of ephippia that appeared initially at the water surface sank during the six-week sampling period. The egg content status did not differ between floating and sinking ephippia. High numbers of ephippial females staying in the surface layers at night in a mountain lake, and a laboratory experiment with ephippial females indicated that ephippia are actively oviposited to the water surface by gravid females.     

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.