The roles of environment and space in shaping stream diatom communities

Diatoms are widely used in stream quality assessment due to their response to the local environment. Diatoms are also influenced by many large-scale processes and so the diatom communities of boreal streams incorporate a strong spatial component at a regional level. What is not properly known yet is whether the variation in diatom communities between regions is larger than the variation in measured environmental variables. We studied the roles of environment and space in accounting for variability in stream diatom communities across four regions in Finland. According to canonical correspondence analysis, geographical coordinates, nutrient concentrations (total N and P), and water conductivity were the most important factors affecting variation in diatom community composition. Of physical factors, depth and current velocity were also significant. According to Mantel tests, both environmental and geographical distances were related to dissimilarity in diatom community composition. Analysis of Similarities indicated that the regional differences in diatom community composition were larger than the regional differences in environmental variables. We also found many indicator species confined to certain regions. Our results suggest that the four study regions differ in their diatom species composition more than in their environmental features and that diatoms are structured not only by the local environment but also by large-scale processes, possibly related to history, climate and dispersal. These results imply that, while diatom species composition reflects well the environmental differences between regions, future bioassessments would benefit from regional stratification. Otherwise, relationships with environmental variables may be masked by trans-regional differences in species pools caused by the large-scale processes.     

Determinants of Benthic Diatom Community Structurein Boreal Streams: the Role of Environmental and Spatial Factors at Different Scales

Benthic diatoms were sampled at 197 sites in Finnish boreal streams. The diatom distribution patterns were related to environmental and spatial factors at three spatial scales using Canonical Correspondence Analysis (CCA). The results emphasized the predominance of chemical‐constituent concentration and ion composition on structuring benthic diatom communities in running waters. Within a river system, physical factors had notable influence on diatom community structure reflecting mainly only minor changes in water chemistry within a watershed. Partial CCA revealed that pure spatial component explained ca. 20% of explainable variation in diatom data at each of the three scales. Environmental factors captured 53–78% of explained variation in species data at ecoregion and river system level. According to Procrustean Randomization Test, spatial coordinates of the study sites and patterns in diatom community structure were strongly concordant (m² = 0.862, p = 0.001) across the largest spatial scale. Similarly, at smaller spatial scales in southern and central Finland, congruence was significant. These data support the view that diatom communities exhibit a rather strong spatial component especially at largest, national scale.     

Beta diversity of stream diatoms at two hierarchical spatial scales: implications for biomonitoring

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SPATIAL PATTERNS AND ECOLOGICAL DETERMINANTS OF BENTHIC ALGAL ASSEMBLAGES IN MID-ATLANTIC STREAMS, USA

We attempted to identify spatial patterns and determinants for benthic algal assemblages in Mid-Atlantic streams. Periphyton, water chemistry, stream physical habitat, riparian conditions, and land cover/use in watersheds were characterized at 89 randomly selected stream sites in the Mid-Atlantic region. Cluster analysis (TWINSPAN) partitioned all sites into six groups on the basis of diatom species composition. Stepwise discriminant function analysis indicated that these diatom groups can be best separated by watershed land cover/use (percentage forest cover), water temperature, and riparian conditions (riparian agricultural activities). However, the diatom-based stream classification did not correspond to Omernik's ecoregional classification. Algal biomass measured as chl a can be related to nutrients in habitats where other factors do not constrain accumulation. A regression tree model indicated that chl a concentrations in the Mid-Atlantic streams can be best predicted by conductivity, stream slope, total phosphorus, total nitrogen, and riparian canopy coverage. Our data suggest that broad spatial patterns of benthic diatom assemblages can be predicted both by coarse-scale factors, such as land cover/use in watersheds, and by site-specific factors, such as riparian conditions. However, algal biomass measured as chl a was less predictable using a simple regression approach. The regression tree model was effective for showing that ecological determinants of chl a were hierarchical in the Mid-Atlantic streams.     

Testing for temporal coherence across spatial extents: the roles of climate and local factors in regulating stream macroinvertebrate community dynamics

Temporal coherence or spatial synchrony refers to the tendency of population, community or ecosystem dynamics to behave similarly among locations through time as a result of spatially‐correlated environmental stochasticity (Moran effect), dispersal or trophic interactions. While terrestrial studies have treated synchrony mainly as a population‐level concept, the majority of freshwater studies have focused on community‐level patterns, particularly in lake planktonic communities. We used spatially and temporally hierarchical data on benthic stream invertebrates across six years, with three seasonal samples a year, in 11 boreal streams to assess temporal coherence at three spatial extents: 1) among regions (watersheds), 2) among streams within a region, and 3) among riffles within a stream, using the average of correlation coefficients for stream/riffle pairs across years. Our results revealed the primacy of strongly synchronized climatic factors (precipitation, air temperature) in inducing temporal coherence of macroinvertebrate assemblages across geographically distinct sites (i.e. Moran effect). Coherence tended to decrease with increasing spatial extent, but positive coherence was detected for most biological variables even at the largest extent (about 350 km). The generally high level of coherence reflected the strong seasonality of boreal freshwater communities. A hydrologically exceptional year enhanced the synchrony of biological variables, particularly total macroinvertebrate abundance. Regionally low precipitation in that year led to a substantial decrease in benthic densities across a broad spatial extent, followed by a rapid post‐drought recovery. Coherence at the among‐riffle (within‐stream) extent was lower than expected, implying that local‐scale habitat filters determine community dynamics at smaller spatial extents. Thus, temporal coherence of stream benthic communities appears to be controlled by partly different processes at different spatial scales.     

The Influence of Environmental,Biotic and Spatial Factors on Diatom Metacommunity Structure in Swedish Headwater Streams

Stream assemblages are structured by a combination of local (environmental filtering and biotic interactions) and regional factors (e.g., dispersal related processes). The relative importance of environmental and spatial (i.e., regional) factors structuring stream assemblages has been frequently assessed in previous large-scale studies, but biotic predictors (potentially reflecting local biotic interactions) have rarely been included. Diatoms may be useful for studying the effect of trophic interactions on community structure since: (1) a majority of experimental studies shows significant grazing effects on diatom species composition, and (2) assemblages can be divided into guilds that have different susceptibility to grazing. We used a dataset from boreal headwater streams in south-central Sweden (covering a spatial extent of ∼14000 km²), which included information about diatom taxonomic composition, abundance of invertebrate grazers (biotic factor), environmental (physicochemical) and spatial factors (obtained through spatial eigenfunction analyses). We assessed the relative importance of environmental, biotic, and spatial factors structuring diatom assemblages, and performed separate analyses on different diatom guilds. Our results showed that the diatom assemblages were mainly structured by environmental factors. However, unique spatial and biological gradients, specific to different guilds and unrelated to each other, were also evident. We conclude that biological predictors, in combination with environmental and spatial variables, can reveal a more complete picture of the local vs. regional control of species assemblages in lotic environments. Biotic factors should therefore not be overlooked in applied research since they can capture additional local control and therefore increase accuracy and performance of predictive models. The inclusion of biotic predictors did, however, not significantly influence the unique fraction explained by spatial factors, which suggests low bias in previous assessments of unique regional control of stream assemblages.     

Invertebrate Metacommunity Structure and Dynamics in an Andean Glacial Stream Network Facing Climate Change

Under the ongoing climate change, understanding the mechanisms structuring the spatial distribution of aquatic species in glacial stream networks is of critical importance to predict the response of aquatic biodiversity in the face of glacier melting. In this study, we propose to use metacommunity theory as a conceptual framework to better understand how river network structure influences the spatial organization of aquatic communities in glacierized catchments. At 51 stream sites in an Andean glacierized catchment (Ecuador), we sampled benthic macroinvertebrates, measured physico-chemical and food resource conditions, and calculated geographical, altitudinal and glaciality distances among all sites. Using partial redundancy analysis, we partitioned community variation to evaluate the relative strength of environmental conditions (e.g., glaciality, food resource) vs. spatial processes (e.g., overland, watercourse, and downstream directional dispersal) in organizing the aquatic metacommunity. Results revealed that both environmental and spatial variables significantly explained community variation among sites. Among all environmental variables, the glacial influence component best explained community variation. Overland spatial variables based on geographical and altitudinal distances significantly affected community variation. Watercourse spatial variables based on glaciality distances had a unique significant effect on community variation. Within alpine catchment, glacial meltwater affects macroinvertebrate metacommunity structure in many ways. Indeed, the harsh environmental conditions characterizing glacial influence not only constitute the primary environmental filter but also, limit water-borne macroinvertebrate dispersal. Therefore, glacier runoff acts as an aquatic dispersal barrier, isolating species in headwater streams, and preventing non-adapted species to colonize throughout the entire stream network. Under a scenario of glacier runoff decrease, we expect a reduction in both environmental filtering and dispersal limitation, inducing a taxonomic homogenization of the aquatic fauna in glacierized catchments as well as the extinction of specialized species in headwater groundwater and glacier-fed streams, and consequently an irreversible reduction in regional diversity.     

Different roles of environmental variables and spatial factors in structuring stream benthic diatom and macroinvertebrate in Yangtze River Delta,China

Exploring the relative contribution of spatial factors and environmental variables in shaping communities is of widespread interest in biodiversity conservation and environmental management. Stream communities are hierarchically regulated by environmental variables over multiple spatial scales, and the reaction of different organisms to stressors are still equivocal. We sampled both macroinvertebrates and diatom at 80 sites and additional 10 sites for macroinvertebrates, field measured and laboratory analyzed environmental variables, from the tributaries of Qiantang River, Yangtze River Delta China in 2011. We used PCNM (principal coordinates of neighbor matrices) to generate spatial predictors. We applied redundancy analysis and variation partitioning procedures to identify key spatial and environmental factors, and to quantify their relative roles in shaping diatom and macroinvertebrate assemblages. Our results demonstrated the role of spatial and environmental variables differed in shaping benthic diatom and macroinvertebrate. Diatom assemblage variations were better explained by spatial factors, however macroinvertebrate assemblage variations were better explained by environmental variables. In terms of environmental variables, catchment scale variables (e.g., land use estimators, land use diversity) played the primary role in determining the patterns of both diatom and macroinvertebrate assemblages, whereas the influence of reach-scale variables (e.g., pH, substrates, and nutrients) appeared less. However, nutrients were the stronger factors influencing benthic diatom, whereas physical habitat (e.g., substrates) played more important role than water chemistry in structuring macroinvertebrates. Our results provided more evidence to the incorporation of spatial factors interpreting spatial patterns of stream organisms, and highlighted the useful of multiple organisms and environmental variables at different spatial scales in diagnosing mechanism of stream degradation and in building a sound stream conditions monitoring program for Yangtze River Delta.     

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

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

Assembly rules and community models for unicellular organisms: patterns in diatoms of boreal streams

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 non‐random distribution patterns previously found for higher plants and metazoans.     

How does water scarcity affect spatial and temporal patterns of diatom community assemblages in Mediterranean streams?

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Ecological uniqueness of macroinvertebrate communities in high-latitude streams is a consequence of deterministic environmental filtering processes

Variation in biological communities is a consequence of stochastic and deterministic factors. Examining the relative importance of these factors helps to understand variation in the whole biodiversity in a region. We examined the roles of stochastic and deterministic factors in structuring macroinvertebrate communities in high-latitude streams across two seasons. We predicted that if communities are the result of deterministic environmental filtering processes, the communities should show strong association with environmental variables, as taxa would be selected according to stream environmental conditions. However, if communities are driven by stochastic factors, they should show strong association with spatial variables, as the distribution of taxa in communities would be driven by spatially related dispersal factors. We studied these predictions by calculating the degree of uniqueness of the streams in terms of their taxonomic and functional community compositions and by modelling the resulting index values using spatial and environmental variables. Our results supported the first prediction where the communities are more influenced by the environmental filtering processes, although indications of the effect of spatial processes in structuring the communities were present especially in autumn. High-latitude stream communities also seem to be sensitive to environmental changes, as even small changes in environment were enough to affect the ecological uniqueness of the streams. These findings highlight the vulnerability of northern streams in the face of the climate change. To maintain biodiversity in high-latitude catchments, it would be important to protect varying habitat conditions, which are the main forces affecting the ecological uniqueness of the streams.     

Environmental degradation results in contrasting changes in the assembly processes of stream bacterial and fungal communities

Environmental degradation may have strong effects on community assembly processes. We examined the assembly of bacterial and fungal communities in anthropogenically altered and near‐pristine streams. Using pyrosequencing of bacterial and fungal DNA from decomposed alder Alnus incana leaves, we specifically examined if environmental degradation deterministically decreases or increases the compositional turnover of bacterial and fungal communities. Our results showed that near‐pristine streams and anthropogenically altered streams supported distinct fungal and bacterial communities. The mechanisms assembling these communities were different in near‐pristine and altered environments. Environmental disturbance homogenized bacterial communities, whereas fungal communities were more dissimilar in disturbed sites than in near‐pristine sites. Compositional variation of both bacteria and fungi was related to water chemistry variables in disturbed sites, further implying the influence of environmental degradation on community assembly. Bacterial and fungal communities in near‐pristine streams were weakly controlled by environmental factors, suggesting that the relative importance of niche‐based versus neutral processes in assembling microbial communities may strongly depend on the spatial scale and local environmental context. Our results thus suggest that environmental degradation may strongly affect the composition and β‐diversity of stream microbial communities colonizing leaf litter, and that the direction of the change can be different between bacteria and fungi. A better understanding of the environmental tolerances of microbes and the mechanisms assembling microbial communities in natural environmental settings is needed to predict how environmental alteration is likely to affect microbial communities.     

Spatial scale and ecological relationships between the macroinvertebrate communities of stony habitats of streams and lakes

1. Comparative studies of distinct, but not ecologically isolated, systems such as lakes and streams may improve our understanding of the importance of ecological linkages in aquatic ecosystems. 2. In this study we compared the macroinvertebrate benthos of stony habitats in Swedish lakes and streams. Community composition was used to evaluate zoogeographic patterns and functional feeding guilds were used to identify mechanisms potentially affecting such patterns. 3. Stream communities were generally more diverse and species‐rich and had a higher proportion of grazers, shredders and passive‐filter feeders than lakes. Lake communities had a higher proportion of predators and collector‐gatherers. Of the 10 most common taxa, only Leptophlebia mayflies, clams (Sphaeriidae) and the isopod Asellus aquaticus were recorded in both lakes and streams. 4. Among‐site variance in macroinvertebrate communities accounted for by regional‐scale variables was low (6.4% for lakes and 10.1% for streams), compared with that by local‐scale variables (21% for lakes and 37.6% for streams). For lakes, the among‐site variance in macroinvertebrate communities was best explained by habitat‐scale characteristics followed by ecosystem, riparian, catchment, geographic position and ecoregion. For streams, the variance in macroinvertebrate communities was best explained by ecosystem characteristics followed by habitat, catchment, riparian, ecoregion and geographic position. 5. Conspicuous differences in spatial pattern were revealed between lakes and streams. For lakes, the most unequivocal differences in community composition and function occurred at the transition zone between the mixed forests in the south and the boreal coniferous forests in the north. Surprisingly, streams did not respond as strongly to profound landscape‐level differences in climate and vegetation cover. 6. The spatial differences noted between macroinvertebrate communities of lakes and streams may be because of differences in retention of detrital matter. Our findings imply that detrital inputs are qualitatively similar, but that the retention and processing of coarse particulate organic matter was presumably higher in lake littoral regions than in stream riffle habitats. 7. Although our findings support the conjecture that species distribution is determined fundamentally by conditions prevailing at the local‐scale, regional factors such as land use/type and the role of history were important and seemingly act as strong determinants of large‐scale patterns in biodiversity.     

Ecological filters and variability in stream macroinvertebrate communities: do taxonomic and functional structure follow the same path?

We examined the community-environment relationships of lotic macroinvertebrates in near-pristine headwater streams, and the correlation between patterns in taxonomic and functional structure at two regional extents. The across-ecoregion scale comprised five ecoregions spanning all of Finland, while the within-ecoregion scale comprised of north boreal and middle boreal ecoregions. We expected that taxonomic structure should exhibit stronger relationships than functional structure to spatial gradients, while the reverse should be true for local environmental factors. We found some support for this notion, because spatial variables were marginally more important for taxonomic than functional structure. Furthermore, within the two ecoregions, local environmental variables were slightly more important for functional than taxonomic structure. Geographical location (i.e. spatial variables) was more influential at the across-ecoregions extent than within the two ecoregions. Largely the same local environmental variables accounted for variation in both taxonomic and functional structure, including water pH, nutrients, colour, and stream size. Similar responses to local environmental features likely contributed to the significant correlation between patterns in taxonomic and functional structure. It has been suggested that functional traits may be rather insensitive to natural variation, yet our functional categorization showed distinct variation along local environmental, catchment, and spatial gradients across near-pristine streams. Thus, natural ecological filters should be accounted for prior to the examination of the effects of anthropogenic filters on stream macroinvertebrate community structure.     

Broad‐scale environmental response and niche conservatism in lacustrine diatom communities

Aim (1) To resolve theoretical debates regarding the role of environment versus dispersal limitation, the conservatism of niches across distances and the prevalence of environmental specialists in diatom communities. (2) To provide guidance on the use of diatom communities and other microbial analogues to analyse ecological response to environmental change. Location Eight hundred and ninety‐one lakes in five regional datasets from north‐western Europe and four regional datasets from north‐eastern North America. Methods Lacustrine diatom communities were analysed at three scales: inter‐continental, intra‐continental and regional. Nested partial redundancy analyses (RDAs) were used to determine spatial versus environmental components of community variation. Weighted‐averaging (WA) regression and calibration, as well as logistic and quadratic regressions, were used to detect niche conservatism and the prevalence of environmental specialists. Results Community patterns indicate that dispersal limitation acts predominantly at the inter‐continental scale, while at the regional (less than c. 1,000,000 km²) scale, a single environmental variable (pH) explains more than five times the community variation as spatial (dispersal‐related) variables. In addition, pH niche components appear to be conserved at the inter‐continental scale, and environmental specialization does not impose relative rarity, as specialists apparently readily disperse to suitable environments. Main conclusions Analysis at multiple scales is clearly important in determining the influences of community variation. For diatom communities, dispersal limitation acts most strongly at the broadest scales, giving way to environment at the scales considered by most analyses. The availability of a wide variety of propagules with consistent niches across regions indicates that diatom communities reflect the succession of taxa according to local environmental conditions, rather than disequilibrium with the environment or adaptation of local populations. While multi‐scale analyses must be undertaken for other groups to resolve debates over community drivers and determine appropriate scales for prediction, for diatoms (and probably other microbial communities), responses to environmental change can be inferred using analogue datasets from large geographic areas.     

Are biological classifications of headwater streams concordant across multiple taxonomic groups?

1. Studies assessing human impacts on freshwater ecosystems are typically based on a single taxonomic group, often macroinvertebrates or fish. Unfortunately, the degree to which such macroinvertebrate or fish‐based surveys can be generalised across other taxonomic groups remains largely unknown. A prerequisite for useful generalisations is that different taxonomic groups exhibit concordant patterns of community structure across sites. 2. We examined the concordance among fish, benthic macroinvertebrates and bryophytes in 32 streams in a boreal catchment in Finland. Our goal was to test how consistently different taxonomic groups classify stream sites; for example, can site groupings based on macroinvertebrates be used as a surrogate for bryophyte or fish assemblage classification? 3. Our results show that community classifications in headwater streams are not concordant across taxonomic groups, at least not at the within‐river system scale. The lack of concordance reflected the fact that all three groups responded to different environmental factors. Macroinvertebrate community structure was mainly correlated with stream size and pH, whereas bryophytes were related to water colour, nutrient content and in‐stream habitat variability. Fish community structure was best described by stream depth, substrate size and water oxygen concentration. 4. Our results suggest that great care should be taken when typologies based on benthic macroinvertebrates, or any other taxonomic group, are extrapolated to other groups in creating typologies of lotic environments.     

Control of stream insect assemblages: roles of spatial configuration and local environmental factors

Abstract 1. Current views in ecology emphasise that community structure is the sum of multiple processes, with imprints of both regional and local drivers. However, the degree to which stream insect assemblages are structured by spatial configuration (complying with the dispersal‐based neutral hypothesis) and local environmental features (complying with the niche‐based species sorting hypothesis) has not been rigorously examined based on surveys in multiple years. 2. Stream sites in a boreal drainage system were surveyed during three consecutive years and the relative contribution of spatial configuration and local environmental variables to aquatic insect assemblage structure (characterised by both abundance and presence–absence data) was assessed. Separate analyses were conducted for mayflies (Ephemeroptera), stoneflies (Plecoptera), caddisflies (Trichoptera), and non‐biting midges (Diptera: Chironomidae) in each year. 3. There were no relationships between the spatial location and local environmental features of streams in Mantel tests, facilitating exploration of their independent effects on assemblage structure. The study found virtually no effects of spatial location on stream insect assemblages across the study drainage system, as evidenced by Mantel tests and canonical correspondence analyses (CCA). The environmental variables were also rather weakly associated with assemblage structure, with the total amount of explained variation ranging from 9.8% to 31.7% in the CCAs. There were no appreciable differences in the amount of environment‐related explained variation in assemblage structure between mayflies, stoneflies, caddisflies, and midges, but some between‐year differences were noticeable in most insect groups. The environmental variables that were significantly related to assemblage structure exhibited some between‐group and between‐year variability. In general, patterns shown by abundance and presence–absence data were highly similar. 4. It appears that stream insect assemblages comply with the niche‐based species sorting hypothesis in the context of metacommunity ecology. In contrast, the absence of spatial structuring suggests that stream insect assemblages do not comply with the neutral hypothesis, being not strongly dispersal limited at the within‐drainage basin scale.     

Environmental and spatial controls of taxonomic versus trait composition of stream biota

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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.