Composition of Endemic Fish Assemblages in Relation to Exotic Species and River Regulation in a Temperate Stream

Spatial variation in the fish community of the regulated Raia stream (Portugal) was examined during the summer of 1995. Variation in the native fish community (abundance of species-size combinations) was explicitly related to both the abundance of exotic species and habitat variables. The fish community changed in space from assemblages characterised by the high relative abundance of Leuciscus pyrenaicus, Leuciscus alburnoides and the smaller size classes of Barbus bocagei (all Iberian endemics with total length, TL, < 100mm to assemblages characterised by the high relative abundance of Chondrostoma polylepis (Iberian endemic) and B. bocagei with TL > 200mm and of exotics Micropterus salmoides and Lepomis gibbosus (> 100mm TL). The former assemblages used shallow sites far from downstream dams with some current whereas the latter assemblages used deeper sites closer to downstream dams without current velocity and with abundant floating macrophytes. Both exotic species and habitat variables were significant correlates of endemic assemblage composition in the Raia stream and the abundance of M. salmoides > 150mm in TL was the best biotic predictor of endemic assemblage composition. The total variation in the community of endemic fish was partitioned into four components: (i) associated uniquely with exotic species - 12.6%, (ii) associated uniquely with habitat variables - 27.6%, (iii) associated both with exotic species and habitat variables - 14.5%, and (iv) that unexplained - 45.3%. A significant association of exotic species with the endemic fish community remained after accounting for the selected environmental variables and a strong (habitat) x (exotic species) interaction was indicated.     

Effect of field sampling design on variation partitioning in a dendritic stream network

Variation partitioning is one of the most frequently used method to infer the importance of environmental (niche based) and spatial (dispersal) processes in metacommunity structuring. However, the reliability of the method in predicting the role of the major structuring forces is less known. We studied the effect of field sampling design on the result of variation partitioning of fish assemblages in a stream network. Along with four different sample sizes, a simple random sampling from a total of 115 stream segments (sampling objects) was applied in 400 iterations, and community variation of each random sample was partitioned into four fractions: pure environmentally (landscape variables) explained, pure spatially (MEM eigenvectors) explained, jointly explained by environment and space, and unexplained variance. Results were highly sensitive to sample size. Even at a given sample size, estimated variance fractions had remarkable random fluctuation, which can lead to inconsistent results on the relative importance of environmental and spatial variables on the structuring of metacommunities. Interestingly, all the four variance fractions correlated better with the number of the selected spatial variables than with any design properties. Sampling interval proved to be a fundamentally influential sampling design property because it affected the number of the selected spatial variables. Our findings suggest that the effect of sampling design on variation partitioning is related to the ability of the eigenvectors to model complex spatial patterns. Hence, properties of the sampling design should be more intensively considered in metacommunity studies.     

Spatial and temporal variation in fish assemblages of drying stream pools: The role of abiotic and biotic factors

Droughts and summer drying create unusual temporary aquatic habitats in the form of isolated pools in many small streams around the world. To examine spatial and temporal variation in fish community structure of drying stream pools, their relation to abiotic environmental variables, and associations among species, fish were sampled during summer 1995 and 1996 from pools of four streams in the Ozark mountains, Arkansas, USA. Redundancy analysis of physical-chemical variables showed significant differences among stream sites, but no significant difference between years or stream site by year interaction. Stream sites separated consistently along axes one (habitat heterogeneity) and two (temperature/canopy cover) in both years. Redundancy analysis of fish species-size class densities showed a significant stream site by year interaction. Groupings of stream sites based on fish assemblages were not well explained by physical-chemical variables measured at the pool scale, but were related to location within the drainage basin, and these groupings differed between years. There were 27 (15.8%) and 10 (5.8%) significant associations found among fish species-size classes in 1995 and 1996, respectively, and all but two significant associations in 1995 were positive. Pool depth, habitat heterogeneity, pool size and dissolved oxygen/canopy cover were important local abiotic factors depending on response variables examined. In both years, large fish total density, large central stoneroller density (80 mm TL), and small sunfish (<80 mm TL) density were positively related to pool depth. Otherwise, there was no consistent relationship between physical-chemical variables and dependent variables (fish density and species richness) within a year or between years for a given dependent variable. These results support the hypothesis that local abiotic factors are important in structuring fish assemblages in harsh environments, but the importance of those factors varies temporally, and regional influences appear to override local abiotic conditions as factors structuring fish assemblages in drying stream pools. Predation by terrestrial vertebrates may also be an important factor structuring these fish assemblages that has been largely overlooked.     

Associations between fish assemblages and environmental factors for Mediterranean-type rivers during summer

The structure of summer fish assemblages was examined along longitudinal gradients in 31 Mediterranean-type rivers of the middle Guadiana basin (south-west Iberian Peninsula), using data from 157 sites including small streams to deep rivers. An ordination analysis, based on 16 variables, was applied to species presence, using principal component and canonical correspondence analysis. The results for the habitat data were compared with those for the biological data using a Mantel analysis, and the agreement was highly significant. Spatial structure was considered by partitioning the total variability among the environmental and geographical variables. The fish assemblages showed longitudinal zoning during the summer, with species distributed over gradients of habitat size (depth), water quality (current and physico-chemical variables), and cover (substratum and vegetation), according to their adult size and life history. The size of the habitat that remained available in summer had the greatest biological effect, being the most important factor explaining fish species distribution and assemblage structure during this stressing season. Strictly spatial variation was low, but there was still a high residual variation. Habitat associations and life-history strategies are discussed for native and exotic species.     

Influence of ionic strength and conductivity on benthic diatom communities in a tropical river (Monjolinho), S?o Carlos-SP, Brazil

Benthic diatoms are important indicators of ecological conditions in lotic systems. The objective of this study was to elucidate the confounding effects of eutrophication, organic pollution and ionic strength and conductivity on benthic diatom communities. Benthic diatoms and water quality sampling was done at 10 sites during summer base flow period (2008 and 2009). Detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) were used to determine environmental gradients along which species vary with respect to ionic strength and conductivity and other environmental variables. Using variance partitioning, we assessed the individual importance of a set of environmental variables (eutrophication and organic pollution) versus ionic strength and conductivity on diatom community structure. The effects of ionic strength and conductivity and organic pollution, eutrophication and other environmental variables were integrated into overall resultant benthic diatom communities. Through partial CCA, we partitioned the variance in diatom data between two sets of exploratory variables, i.e. ionic strength and conductivity (26.9%); other variables, particularly eutrophication and organic pollution (23.0%); shared variance (11.3%) and unexplained variance (38.8%). Due to the interaction of the effects of ionic strength and conductivity and other variables in this study, laboratory experiments must be performed to confirm the observed effects of ionic strength and conductivity.     

Tropical paradox: a multi-scale analysis of the invasion paradox within Miami Rock Ridge tropical hardwood hammocks

The invasion paradox describes the scale dependence of native-exotic richness relationships (NERRs), where NERRs are negative at neighborhood scales and positive at landscape scales. However, a lack of tropical surveys and past failures to isolate potential confounding variables contribute to significant gaps in our understanding of the processes producing these patterns. We surveyed the vascular flora of 13 tropical hardwood hammocks for community characteristics (e.g., native and exotic species richness, vegetative cover) with a hierarchical sampling design. Using model selection, we determined which variables best predicted patterns of exotic species richness at each spatial scale of consideration. We found that native and exotic species richness were positively correlated at neighborhood scales, but negatively correlated at landscape scales. The latter result stands in stark opposition to the patterns published in the literature thus far. We found that natural disturbance history (as approximated by vegetative cover) was positively correlated with exotic species richness at intermediate and landscape scales only. Overall, hammock identity was the most important factor driving exotic species richness patterns at all spatial scales. Hammocks with highly-disturbed hydrologies, brought about by water management, had fewer native species and more exotic species than hammocks with more natural hydrological conditions. Our results are among the first from examination of subtropical communities, and may support the hypothesis that tropical and subtropical communities are subject to more intense biotic interactions. However, given our unique sampling design, our results do not reject the hypothesis that environmental heterogeneity drives the relationship between native and exotic species richness patterns.     

Metacommunity ecology meets biogeography: effects of geographical region,spatial dynamics and environmental filtering on community structure in aquatic organisms

Metacommunity patterns and underlying processes in aquatic organisms have typically been studied within a drainage basin. 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 within-basin 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.     

Patterns in the distribution of fish assemblages in Río Amacuzac,Mexico: influence of abiotic factors and biotic factors

Patterns in spatial and seasonal distribution of fish communities were analyzed in the Río Amacuzac, Mexico, and their relationship to environmental variables and habitat characteristics. The PCA biplot of the study sites and environmental factors showed the first two axes accounting for 52.93% of the variance. The diagram showed the study sites ordination in environmental gradients. The first axis explained variables related to habitat characteristics and temperature (36.30%) and second axis arranged the sites in physicochemical and water quality environmental gradients (conductivity, dissolved oxygen, orthophosphates, ammonium, pH) displaying seasonal variation. Fifteen species were recorded, eight of them are exotic. One new record appeared for the Río Amacuzac: Pterygoplichthys disjunctivus is exotic. Study sites with highest species richness were: 5, 7 and 9 (twelve species each one); while the study sites with low species richness were 1, 2, 3 and 6 (eight species each one). Six of the species were distributed throughout the whole river. Based on the composition of the fish fauna, the study sites form two groups and the analysis of fish species displays three groups according to the Bray–Curtis index. The diagram of the canonical correspondence analysis relates environmental parameters to the abundance of fish species and showed that the first two axes exhibit 78.31% of the explained variance. Species richness had a spatial pattern associated to the introduction of exotic species for ornamental uses. According to the results of the importance value index (IVI), the dominant species were the poecilids Poeciliopsis gracilis and Heterandria bimaculata, small fishes that were introduced in the river. Río Amacuzac has a biotic alteration in the structure of fish communities due to the invasion of exotic species, representing risks to the integrity of the native fish fauna.     

Darwin's naturalization conundrum: dissecting taxonomic patterns of species invasions

Darwin acknowledged contrasting, plausible arguments for how species invasions are influenced by phylogenetic relatedness to the native community. These contrasting arguments persist today without clear resolution. Using data on the naturalization and abundance of exotic plants in the Auckland region, we show how different expectations can be accommodated through attention to scale, assumptions about niche overlap, and stage of invasion. Probability of naturalization was positively related to the number of native species in a genus but negatively related to native congener abundance, suggesting the importance of both niche availability and biotic resistance. Once naturalized, however, exotic abundance was not related to the number of native congeners, but positively related to native congener abundance. Changing the scale of analysis altered this outcome: within habitats exotic abundance was negatively related to native congener abundance, implying that native and exotic species respond similarly to broad scale environmental variation across habitats, with biotic resistance occurring within habitats.     

Choice of biota in stream assessment and monitoring programs in tropical streams: A comparison of diatoms,macroinvertebrates and fish

The objective of this study was to compare the responses of diatoms, macroinvertebrates and fish to agriculture, urbanization and mining in the Manyame River Basin. Water quality sampling and benthic diatom, macroinvertebrate and fish community data were collected in April (end of the rain season) and September (dry season) 2013 at 44 sampling stations spread out across three land-use categories: commercial agricultural, communal agricultural and urban-mining areas. Commercial agricultural areas were relatively pristine as they were characterized by mature deciduous riparian forest strips which acted as riparian buffers thus protecting water resources from nonpoint source pollution. In communal agricultural areas a combination of poor agricultural practices (stream bank cultivation, overgrazing, soil erosions) and high human population densities had negative effects on water quality of streams draining these areas. Streams in urban-mining areas were highly stressed, being impacted primarily by physical habitat degradation and both point and nonpoint sources of pollution. A suite of environmental variables that varied with land-use pattern was assessed to find the combination of variables that best explained patterns of biota community composition. Community metrics i.e. the Trophic Diatom Index (TDI) based on diatoms, the South African Scoring system version 5 (SASS 5) based on macroinvertebrates and the Fish Assemblage Integrity Index (FAII) were used to determine the ecological status of study streams in relation to human-induced stressors. Data were also subjected to multivariate statistical techniques; canonical correspondence analysis (CCA), mantel test and cluster analysis to determine environmental gradients along which the diatom, macroinvertebrate and fish assemblages were distributed as well as to elucidate hypothesized differences in response to stressors among communities per land-use type. Using CCA, we assessed the individual importance of a set of environmental variables on each biotic community structure. ANOVA, showed a significant difference (p < 0.05) in physical and chemical variables among commercial agricultural, communal agricultural and urban-mining sampling stations with no significant differences (p > 0.05) between the 2 sampling periods. Based on CCAs carried out using individual variables, the strengths of relationships between diatoms and macroinvertebrates was generally high for nutrient levels, organic and metal pollution and other variables. However, fish assemblages showed a relatively low association with all water quality variables in the study; this might be explained by the high abundance of omnivores and air breathers which are able to tolerate a variety of environmental conditions. These patterns were also confirmed by the mantel test as well as the other CCAs carried out to investigate the simultaneous effects of environmental variables. These findings indicate that diatoms are more powerful indicators in accessing ecological stream/river quality and have potential for application in routine monitoring programs in tropical streams.     

Changes in fish assemblages in catchments in north‐eastern Spain: biodiversity,conservation status and introduced species

1. North‐eastern Spain is a hot spot for the introduction of alien fish species, and its native fish fauna is one of the most endangered worldwide. We used an extensive data set from 2002 to 2003 and historical information from the area to characterize fish diversity and establish conservation priorities in river catchments. 2. Diversity indices were used to characterize fish diversity at the basin scale. An index of conservation status was applied for each species, which considers the occurrence, abundance and endemicity of each taxon. We used indirect ordination methods to test the relationship among basin features and to identify those variables most correlated with each other. To identify physical, biotic and environmental characteristics that seem to make a basin particularly susceptible to invasion, we performed a step‐wise multiple regression to examine the relationship between the number of native, translocated and introduced fish species (including the original native species richness of each basin), and landscape variables. 3. Over a period of approximately 50 years, the mean range size of native fish species has decreased by 60%. The greatest decline occurred in Gasterosteus gymnurus, Anguilla anguilla and Salaria fluviatilis, for which species over 75% of the original distribution area has been lost. The species with the highest conservation index were Gasterosteus gymnurus and Salaria fluviatilis. 4. Basin area and the catchment type explained 70% of variation in native species richness, whereas the number of dams and basin area accounted for more than 80% of variation in the number of introduced species. 5. The original native species richness and the number of introduced species at basin scale were not related, and thus there was no evidence of “biotic resistance” to invasion. The restoration of natural hydrologic processes and the development of specific management tools to protect native species, such as the prioritization of areas for fish conservation and the eradication of local populations of exotic species, are required to restore native fish fauna in these catchments.     

Development of a preliminary index of biotic integrity (IBI) based on fish assemblages to assess ecosystem condition in the lakes of central Mexico

The lakes of central Mexico have great cultural, economic, and biological value, but they are being degraded at an accelerating rate. We employed historical data on fish communities from 19 of these lakes and case studies of community responses to environmental degradation from four of the best-studied, Xochimilco, Cuitzeo, Chapala, and Pátzcuaro, to construct a preliminary index of biotic integrity (IBI). This IBI was designed to be an easily applied method for assessing lake ecosystem health and evaluating restoration efforts. The IBI had 10 metrics: number of total native species, number of common native species, number of native Goodeidae species, number of native Chirostoma species, number of native sensitive species, percent of biomass as tolerant species, percent of biomass as exotic species, percent of biomass as native carnivorous species, maximum standard length of native species, and percent of exotic invertebrate parasite species on or in native fishes. Initial applications of the index showed promise, accurately ranking the relative degradation of the four case-study lakes. Further tests of the index are warranted, and more data are needed to standardize sampling procedures, improve species classifications, and refine metric scoring criteria.     

Benthic macroinvertebrate communities of intermittent streams in the middle reaches of the Guadiana Basin (Portugal)

The Guadiana River has an irregular hydrological regime, with severe droughts and floods, but little is known about how aquatic fauna respond to these natural events. Macroinvertebrate data and environmental information were collected at seven sites from three tributaries in the middle reaches of the Guadiana River, approximately every 3 months from April 1995 to April 1997. Despite considerable annual variation in discharge (related to duration of flood and drought periods), the number of macroinvertebrates found was consistently high. Diptera represented the major proportion of the benthic fauna (73.2%) followed by Ephemeroptera (10.3%), Coleoptera (4.1%) and Trichoptera (3.1%). Canonical correspondence analysis (CCA) was used to evaluate the relationships between taxa density and habitat variables. Generally, Plecoptera and Ephemeroptera were found in the upstream sampling sites. Wider and deeper sites were associated with the presence of Diptera and were least diverse. High values for both the Shannon-Wiener diversity index and the average score per taxon were usually found at upstream sites where Ephemeroptera and Plecoptera predominated. The data suggest that macroinvertebrates have a great capacity to recover rapidly from severe drought periods, both in terms of taxonomic diversity and number of individuals.     

Fish Communities of the Sacramento River Basin: Implications for Conservation of Native Fishes in the Central Valley, California

The associations of resident fish communities with environmental variables and stream condition were evaluated at representative sites within the Sacramento River Basin, California between 1996 and 1998 using multivariate ordination techniques and by calculating six fish community metrics. In addition, the results of the current study were compared with recent studies in the San Joaquin River drainage to provide a wider perspective of the condition of resident fish communities in the Central Valley of California as a whole. Within the Sacramento drainage, species distributions were correlated with elevational and substrate size gradients; however, the elevation of a sampling site was correlated with a suite of water-quality and habitat variables that are indicative of land use effects on physio-chemical stream parameters. Four fish community metrics – percentage of native fish, percentage of intolerant fish, number of tolerant species, and percentage of fish with external anomalies – were responsive to environmental quality. Comparisons between the current study and recent studies in the San Joaquin River drainage suggested that differences in water-management practices may have significant effects on native species fish community structure. Additionally, the results of the current study suggest that index of biotic integrity-type indices can be developed for the Sacramento River Basin and possibly the entire Central Valley, California. The protection of native fish communities in the Central Valley and other arid environments continues to be a conflict between human needs for water resources and the requirements of aquatic ecosystems; preservation of these ecosystems will require innovative management strategies.     

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.     

Environmental rather than spatial factors structure bacterioplankton communities in shallow lakes along a > 6000 km latitudinal gradient in South America

Metacommunity studies on lake bacterioplankton indicate the importance of environmental factors in structuring communities. Yet most of these studies cover relatively small spatial scales. We assessed the relative importance of environmental and spatial factors in shaping bacterioplankton communities across a > 6000 km latitudinal range, studying 48 shallow lowland lakes in the tropical, tropicali (isothermal subzone of the tropics) and tundra climate regions of South America using denaturing gradient gel electrophoresis. Bacterioplankton community composition (BCC) differed significantly across regions. Although a large fraction of the variation in BCC remained unexplained, the results supported a consistent significant contribution of local environmental variables and to a lesser extent spatial variables, irrespective of spatial scale. Upon correction for space, mainly biotic environmental factors significantly explained the variation in BCC. The abundance of pelagic cladocerans remained particularly significant, suggesting grazer effects on bacterioplankton communities in the studied lakes. These results confirm that bacterioplankton communities are predominantly structured by environmental factors, even over a large‐scale latitudinal gradient (6026 km), and stress the importance of including biotic variables in studies that aim to understand patterns in BCC.     

Contrasting patterns and mechanisms of spatial turnover for native and exotic freshwater fish in Europe

Aim We compare the distribution patterns of native and exotic freshwater fish in Europe, and test whether the same mechanisms (environmental filtering and/or dispersal limitation) govern patterns of decrease in similarity of native and exotic species composition over geographical distance (spatial species turnover). Locations Major river basins of Europe. Methods Data related to geography, habitat diversity, regional climate and species composition of native and exotic freshwater fish were collated for 26 major European river basins. We explored the degree of nestedness in native and exotic species composition, and quantified compositional similarity between river basins according to the beta‐sim (independent of richness gradient) and Jaccard (dependent of richness gradient) indices of similarity. Multiple regression on distance matrices and variation‐partitioning approaches were used to quantify the relative roles of environmental filtering and dispersal limitation in shaping patterns of decreasing compositional similarity over geographical distance. Results Native and exotic species exhibited significant nested patterns of species composition, indicating that differences in fish species composition between river basins are primarily the result of species loss, rather than species replacement. Both native and exotic compositional similarity decreased significantly with increasing geographical distance between river basins. However, gradual changes in species composition with geographical distance were found only for exotic species. In addition, exotic species displayed a higher rate of similarity decay (higher species turnover rate) with geographical distance, compared with native species. Lastly, the majority of explained variation in exotic compositional similarity was uniquely related to geography, whereas native compositional similarity was either uniquely explained by geography or jointly explained by environment and geography. Main conclusions Our study suggests that large‐scale patterns of spatial turnover for exotic freshwater fish in Europe are generated by human‐mediated dispersal limitation, whereas patterns of spatial turnover for native fish result from both dispersal limitation relative to historical events (isolation by mountain ranges, glacial history) and environmental filtering.     

Vegetation-environment relationships of old-growth spruce forest vegetation in Østmarka Nature Reserve, SE Norway, and comparison of three ordination methods

The relationship between vegetation and environmental variables has been studied in 100 sample plots, each 0.25 m², in old-growth spruce forest at Høgkollen, ØOstmarka Nature Reserve, SE Norway. Each sample plot was supplied with measurements of 13 environmental and 5 biotic variables. Parallel application of three ordination techniques, PCA, DCA and LNMDS, resulted in different sample plot configurations. PCA performed poorest due to strong influence of outliers and circumstantial evidence indicated better performance of LNMDS than DCA. Statistical analyses of the relationships between vegetation and ecological data revealed a parallel gradient in soil moisture (decreasing) and canopy closure (increasing) as the most important for differentiation of the vegetation. Species number and field layer cover decreased, while bottom layer cover increased, due to increasing cover of Dicranum majus , with decreasing soil moisture and increasing canopy closure. Constrained canonical correspondence analysis (CCA) was used to partition the variation of the species-sample plot matrix into spatial, environmental and unexplained variation, and combinations. The fraction of unexplained variation was high (80.9 %), most likely due to small sample plot size and short gradient lengths. Most of the explained variation was attributable to environmental factors alone (54.5%). Only 6.3% was shared between environmental and spatial variation, which indicated minor importance of broad-scale and geographically structured environmental variation. Strictly spatial variation constituted 39.3%. However, the spatially structured environmental variation was low, so the causes of spatial variation were likely not to be found among the measured environmental variables.     

Environmental factors determining invasibility of urban waters for exotic macroinvertebrates

Aim Urbanization usually leads to biotic homogenization with a decrease in native species and increase in exotic species. We investigated whether local environmental factors in urban water bodies, such as water quality, habitat structure and biotic interactions, influenced the invasion of these systems by exotic macroinvertebrate species. Location Urban surface water systems in lowlands of the Rhine‐Meuse delta. Methods Presence and abundance of native and exotic macroinvertebrate species were compared between different urban water types and related to environmental variables with multivariate analysis and spearman’s correlations. Moreover, co‐existence of related native and exotic species was studied. Results In total nine exotic species were found in the following taxa: Tricladida (1), Crustacea (5), Bivalvia (1) and Gastropoda (2). Taxonomically related native and exotic crustacean species did not seem to be influenced by competition in nutrient‐rich urban waters; most species showed high abundances. Nevertheless, two exotic crustacean species were much more abundant in waters where other crustacean species were absent, possibly filling empty niches. Native species richness and abundance was positively related to environmental heterogeneity in the form of submerged vegetation. The occurrence and abundance of most exotic species were positively related to several eutrophication indicators, such as nitrate, sludge layer and lemnid vegetation. Main conclusions Exotic species in urban waters were mostly detritivorous or omnivorous and therefore dependent on leaf breakdown. In nutrient‐rich water systems, where food availability was high, exotic crustacean species co‐existed with native crustacean species, while in nutrient‐poor, richly vegetated systems, native Asellidae dominated exotic Asellidae. In the turbid water bodies with very little vegetation, native species richness was low and two exotic crustacean species were relatively abundant in these water systems. Invasibility of urban water systems could be reduced by stimulating the development of submerged and nymphaeid vegetation and decreasing nutrient levels.     

Environmental variables interact across spatial scales to structure trichopteran assemblages in Ouachita Mountain rivers

An important goal in aquatic ecology is to determine the interacting variables that regulate community structure; however, complex biotic and abiotic interactions coupled with the significance of scale have confounded the interpretation of community data. We evaluated stream and riparian habitat features in southeastern Oklahoma, USA at a range of spatial scales from local, in-stream variables to large-scale, regional characteristics to address the following questions: (1) How much variation in trichopteran community composition can be attributed to local, regional, and spatial variables? and (2) What environmental variables are most important in determining trichopteran community structure? We collected data on caddisfly community structure, local and regional environmental variables, and spatial location on the landscape from 25 sites in four rivers. We analyzed these data using canonical correspondence analysis (CCA) and variation partitioning. Our analysis explained approximately 60% of the variation in caddisfly community composition. We found that local and regional environmental variables were near equal in importance in governing caddisfly communities, with each accounting for approximately a quarter of the explained variation. Although pure spatial variables were less important, the amount of variation shared among spatial variables and local and regional variables was substantial, indicating that biogeographic history is also key to understanding caddisfly distributions. We also found a strong influence of human landuse (i.e., percent of land in agriculture, distance to roads) on caddisfly community composition. Our study indicated that communities are influenced by factors across scales, and that bioassessments should focus on not only local habitat conditions, but also incorporate larger-scale factors.