Average niche breadths of species in lake macrophyte communities respond to ecological gradients variably in four regions on two continents

Different species’ niche breadths in relation to ecological gradients are infrequently examined within the same study and, moreover, species niche breadths have rarely been averaged to account for variation in entire ecological communities. We investigated how average environmental niche breadths (climate, water quality and climate–water quality niches) in aquatic macrophyte communities are related to ecological gradients (latitude, longitude, altitude, species richness and lake area) among four distinct regions (Finland, Sweden and US states of Minnesota and Wisconsin) on two continents. We found that correlations between the three different measures of average niche breadths and ecological gradients varied considerably among the study regions, with average climate and average water quality niche breadth models often showing opposite trends. However, consistent patterns were also found, such as widening of average climate niche breadths and narrowing of average water quality niche breadths of aquatic macrophytes along increasing latitudinal and altitudinal gradients. This result suggests that macrophyte species are generalists in relation to temperature variations at higher latitudes and altitudes, whereas species in southern, lowland lakes are more specialised. In contrast, aquatic macrophytes growing in more southern nutrient-rich lakes were generalists in relation to water quality, while specialist species are adapted to low-productivity conditions and are found in highland lakes. Our results emphasise that species niche breadths should not be studied using only coarse-scale data of species distributions and corresponding environmental conditions, but that investigations on different kinds of niche breadths (e.g., climate vs. local niches) also require finer resolution data at broad spatial extents.     

Modeling wetland plant metrics to improve the performance of vegetation-based indices of biological integrity

The objective of this study was to determine if the accuracy and precision of wetland plant indices of biological integrity (IBIs) could be improved through the use of modeling techniques. To do this, we developed a modeled vegetation IBI (MVIBI) based on metrics previously used to develop vegetation indices of biological integrity (VIBIs) for Ohio wetlands (e.g. % invasive grass, % sensitive species, shrub richness). We selected 82 emergent, forested, and shrub-dominated reference sites distributed across the State of Ohio and built Random Forest models to predict plant metric scores at reference wetlands from naturally occurring environmental features related to climate, hydrology, geology, soils, and landscape position. The models explained between 14 and 52% of the variance in the scores of 21 metrics indicating that variation in wetland plant assemblages was significantly associated with naturally occurring environmental gradients. We used principal component analysis to identify ten groups of statistically independent metrics and selected one metric from each group that discriminated most strongly between reference and most degraded sites based on t-scores. Two axes did not contain discriminating metrics so we used eight metrics in the MVIBI. Analysis of variance of reference site MVIBI scores indicated that we could use one distribution of reference site scores to assess multiple wetland types, thus eliminating the need to separately designate wetland types. We used the MVIBI to assess 170 test sites and compared the accuracy, precision, responsiveness, and sensitivity of the MVIBI to those of the original VIBIs. The MVIBI was up to twice as accurate and precise as the original VIBIs, indicating that modeling can be used to improve the performance of vegetation-based IBIs. The use of model-based IBIs for wetland plants should reduce assessment errors associated with natural variation in plant metrics and should increase confidence in wetland assessments.     

Macrophyte communities in unimpacted European streams: variability in assemblage patterns, abundance and diversity

Macrophytes are an important component of aquatic ecosystems and are used widely within the Water Framework Directive (WFD) to establish ecological quality. In the present paper we investigated macrophyte community structure, i.e., composition, richness and diversity measures in 60 unimpacted stream and river sites throughout Europe. The objectives were to describe assemblage patterns in different types of streams and to assess the variability in various structural and ecological metrics within these types to provide a basis for an evaluation of their suitability in ecological quality assessment. Macrophyte assemblage patterns varied considerably among the main stream types. Moving from small-sized, shallow mountain streams to medium-sized, lowland streams there was a clear transition in species richness, diversity and community structure. There was especially a shift from a predominance of species-poor mosses and communities dominated by liverwort in the small-sized, shallow mountain streams to more species-rich communities dominated by vascular plants in the medium-sized, lowland streams. The macrophyte communities responded to most of the features underlying the typological framework defined in WFD. The present interpretation of the WFD typology may not, however, be adequate for an evaluation of stream quality based on macrophytes. First and most important, by using this typology we may overlook an important community type, which is characteristic of small-sized, relatively steep-gradient streams that are an intermediate type between the small-sized, shallow mountain streams and the medium-sized, lowland streams. Second, the variability in most of the calculated metrics was slightly higher when using the pre-defined typology. The consistency of these results should be investigated by analysing a larger number of sites. Particularly the need of re-defining the typology to improve the ability to detect impacts on streams and rivers from macrophyte assemblage patterns should be investigated. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

A fish index of biotic integrity for South Dakota's Northern Glaciated Plains Ecoregion

Ecosystem goods and services in streams are impaired when their biotic communities are degraded by anthropogenic stressors. An index of biotic integrity (IBI) translates community structure into a standardized ecoregion-specific stream health score. Documenting stream health is especially important in the Northern Glaciated Plains (NGP) Ecoregion, which is undergoing rapid landscape alterations through increased agriculture production. Our objectives were to develop a fish IBI and validate candidate reference sites for NGP wadeable perennial streams. Fish were sampled from 54 sites (consisting of reference sites, known-condition least and most disturbed sites, and random sites) during summers 2006–2011. Candidate metrics were sorted into nine metric classes based on attributes of fish assemblage form and function. Metric values were screened using metric range, signal-to-noise ratios, responsiveness to disturbance, and redundancy tests until each metric class contained only those metrics most responsive to anthropogenic stressors. The final IBI consisted of six metrics that were reflective of prairie stream fish assemblages, and differentiated between known-condition least and most disturbed sites. The mean reference sampling site IBI scores were found to be similar to both least and most disturbed sites (Mann–Whitney U-test; P < 0.05). Twelve reference site scores were below the NGP's median (69), whereas the other 11 sites were above the median and were representative of least disturbed conditions. We now have developed a standardized bioassessment tool for evaluating stream health, as well as a baseline for long-term monitoring in a dynamic ecoregion.     

Macrophyte communities of European streams with altered physical habitat

The impact of altering hydro-morphology on three macrophyte community types was investigated at 107 European stream sites. Sites were surveyed using standard macrophyte and habitat survey techniques (Mean Trophic Rank Methodology and River Habitat Survey respectively). Principal Components Analysis shows the macrophyte community of upland streams live in a more structurally diverse physical habitat than lowland communities. Variables representing the homogeneity and diversity of the physical environment were used to successfully separate un-impacted from impacted sites, e.g. homogeneity of depth and substrate increased with decreasing quality class for lowland sites (ANOVA p < 0.05). Macrophyte attribute groups and structural metrics such as species richness were successfully linked to hydro-morphological variables indicative of impact. Most links were specific to each macrophyte community type, e.g., the attribute group liverworts, mosses and lichens decreased in abundance with increasing homogeneity of depth and decreasing substrate size at lowland sites but not at upland sites. Elodea canadensis, Sparganium emersum and Potamogeton crispus were indicative of impacted lowland sites. Many of the indicator species are also known to be tolerant to other forms of impact. The potential for a macrophyte tool indicative of hydro-morphological impact is discussed. It is concluded one could be constructed by combining indicator species and metrics such as species richness and evenness.     

The search for reference conditions for stream vegetation in northern Europe

1. The European Water Framework Directive provides a framework for improving the ecological quality of stream ecosystems, with deviation from reference used as a measure of ecological status.
2. Here we examine the possibility of using less impacted stream sites from Latvia, Lithuania and Poland to establish a Danish reference network for macrophyte assemblages, and as a guiding image for identification of possible references sites within Denmark. Both approaches were evaluated using historical Danish records.
3. Four different macrophyte assemblages were identified for mid-sized streams in the Central and Eastern Lowland ecoregions. Macrophyte assemblages could not be delineated using physical stream site characteristics; however a gradual change in assemblage composition was attributed to differences in alkalinity and human impact.
4. Assemblages of contemporary vegetation in Denmark were quite similar to those found in Polish, Latvian and Lithuanian streams (26–35%). However, more importantly, from species-based predictions we noted higher similarity, particularly with Latvian and Lithuanian streams, before intensive land use commenced in Denmark ( c. 1900). These results show that stream sites from these three countries can be considered in a Danish reference network.
5. Two of the four macrophyte assemblages comprised species such as Fontinalis antipyretica, Myriophyllum spicatum , Nuphar lutea , Potamogeton alpinus and P. perfoliatus that have a very scattered occurrence in the contemporary vegetation in Denmark. These groups were closely associated with the predictions from historic records, thereby lending support the conjecture that these assemblages could be part of the guiding image for the identification of potential reference sites within Denmark.     

Performance of a new phytoplankton composition metric along a eutrophication gradient in Nordic lakes

A new phytoplankton metric is presented, which is developed from a large dataset of Norwegian lakes (>2,000 samples from >400 lakes). In contrast to previous metrics, this index is not built on selected ‘indicative’ taxa, but uses all available taxonomic information at genus and species level. Taxa optima with respect to lake trophic status (derived from total phosphorus concentrations) are used to calculate a phytoplankton trophic index (TI) for each sample. Analysis of the TI shows that phytoplankton communities exhibit highly non-linear responses to eutrophication in Norwegian lakes. Reference lakes are characterized by very similar TIs despite having considerable variation in total phosphorus and chlorophyll a concentrations. TI exhibits a non-linear distribution along the eutrophication gradient which separates unimpacted from impacted sites in the study area. We further show that TI exhibits smaller seasonal variations than chlorophyll a, making it a more reliable indicator for lake monitoring. Implications for its applicability within the WFD are discussed.     

Assessment of ecological status in U.K. rivers using diatoms

1. The European Union's Water Framework Directive requires all water bodies to achieve ‘good ecological status’ by 2015 and this paper describes a rationale for defining ‘good ecological status’ based on diatoms, a significant component of the biological quality element ‘macrophyte and phytobenthos’. 2. A database of benthic diatom samples collected over the past 20 years was assembled. New sampling, specifically for this project, was undertaken during 2004 to supplement these data. In total 1051 samples were included in the database with matching environmental data. 3. ‘Reference sites’, relatively unimpacted by human activity, were selected from this database by a series of screening steps and these sites were used to develop a site‐specific reference typology. 4. Environmental variables not related to the pressure gradient were used to predict the ‘expected’ Trophic Diatom Index (TDI) values at each site. Site‐specific TDI predictions were used to generate ecological quality ratios (EQRs) ranging from ≥1, where the diatom assemblage showed no impact, to (theoretically) 0, when the diatom assemblage was indicative of major anthropogenic activities. 5. The boundary between ‘high’ and ‘good’ status was defined as the 25th percentile of EQRs of all reference sites. The boundary between ‘good’ and ‘moderate’ status was set at the point at which nutrient‐sensitive and nutrient‐tolerant taxa were present in equal relative abundance. An ecological rationale for this threshold is outlined in the paper.     

Application of a direct toxicity assessment approach to assess the hazard of potential pesticide exposure at selected sites on the Crocodile and Magalies rivers,South Africa

The potentially hazardous effects of agricultural pesticide usage in the Crocodile (west) Marico catchment were evaluated using the Danio rerio and Daphnia pulex lethality, Selenastrum capricornutum growth inhibition and the Ames mutagenicity plate incorporation assays. Hazard assessment categories are proposed to standardise the output of the different toxicity assessments. Sites were categorised according to the toxicity hazard indicated and were related to the gradients of agricultural land-use intensity at each site. Intensive agricultural sites showed the highest effects to all tested biota. Receiving water at urban sites associated with increased nutrients and lowest pesticide usage showed few adverse effects, while the relatively unimpacted site indicated no hazard to any organism, and only a slight stimulation to algal growth. Weighted hazard scores indicated that the unimpacted sites were least hazardous, falling within a B category, the urban sites were moderately hazardous (C category), and the agricultural sites (D category) had the highest potential impacts on aquatic organisms. This study demonstrated the usefulness of using the hazard assessment approach and the role it could play in assessing site-specific potential toxicity hazards of river water impacted by agrochemicals. It can be used together with other assessment methods, such as biological indices, in a tiered approach.     

Indicators of ecological change: comparison of the early response of four organism groups to stress gradients

A central goal in monitoring and assessment programs is to detect change early before costly or irreversible damage occurs. To design robust early-warning monitoring programs requires knowledge of indicator response to stress as well as the uncertainty associated with the indicator(s) selected. Using a dataset consisting of four organism groups (fish, macrophytes, benthic diatoms and macroinvertebrates) and catchment, riparian and in-stream physico-chemical variables from 77 mountain and 85 lowland streams we determined the relationships between indicator response and complex environmental gradients. The upper (>75th percentile) and lower (<25th percentiles) tails of principal component (PC) gradients were used to study the early response of the four organism groups to stress. An organism/metric was considered as an early warning indicator if the response to the short gradients was more robust (higher R² values, steeper slope and lower error) than the null model (organism response to the full PC gradient). For mountain streams, both fish and macrophyte CA scores were shown to exhibit an early warning response to the upper tail of the 1st PC gradient when compared to the null model. Five of the eight metrics showed better response to the upper tail of the 2nd PC gradient compared to the null model, while only one metric (macrophyte CA scores) showed improvement when compared to the lower tail of the 2nd PC gradient. For lowland streams all four organism-groups showed better response (CA scores) to the upper tail of the PC gradient when compared to the null model. Only one metric (fish CA scores) regressed against the lower tail of the 2nd PC gradient was found to be more robust than the PC2 null model. These findings indicate that the nonlinear relationships of organism/metric response to stress can be used to select potentially robust early warning indicators for monitoring and assessment.     

An assessment of macrophyte community metrics in the determination of the ecological condition and total phosphorus concentration of Mediterranean ponds

The macrophyte vegetation of 51 ponds situated in the Duero river basin on the Northern Iberian Plateau was studied with the aim of selecting metrics that responded clearly to perturbation and that should be included in a multimetric index for assessing the ecological condition (expression of the quality of the structure and functioning of aquatic ecosystems associated with surface waters) of Mediterranean ponds. Furthermore, the specific response of metrics to the total phosphorus (TP) concentration was also investigated because of agricultural activities, which usually increase the concentration of nutrients in aquatic systems, are one of the principal types of impairment affecting ponds in the Duero basin.A total of 19 metrics representing several aspects of the structure of macrophyte communities (cover, richness, diversity) were selected as potential metrics.The strong overlap which was observed for macrophyte metrics between classes of ecological conditions (bad, poor, moderate, good, best available) and TP concentrations (>600, 600-300, 300-100, 100-50, <50 μg L⁻¹ TP) constitutes a major problem in defining and separating these classes. For this reason, Mann-Whitney U-test and discrimination efficiency were carried out to determine which of the measures best discriminated between slightly impaired sites (ponds in good or best available condition) and perturbed ponds (ponds in moderate, poor or bad condition). Most of the metrics showed significant differences (p < 0.05) between slightly impaired and perturbed ponds. However, only total cover and hydrophyte richness, which had the highest discrimination efficiency (>78%), showed no inter-quartile overlap (25th-75th percentile) between the two ecological classes. For these reasons, both total cover and hydrophyte richness were selected for the implementation of a multimetric index able to discriminate between slightly impaired and degraded Mediterranean flatland ponds.Currently, eutrophication is considered the main pressure on lakes. However, macrophyte metrics were far less sensitive to TP concentration than to ecological condition changes. This shows that other pressures are also of considerable importance. Thus, the definition of the ecological condition of lakes using macrophytes should not be based only on responses to eutrophication pressure, as has been done in some European countries.     

Developing a Benthic Index of Biological Integrity and Some Relationships to Environmental Factors in the Subtropical Xiangxi River,China

The aim of this study is to develop a benthic index of biotic integrity (B‐IBI) to help understand how the increasing anthropogenic pressure may impact the subtropical Xiangxi River in China. Benthic macroinvertebrate and environmental surveys were conducted at 77 sites in early summer 2004. Each collection site was categorized as reference or impaired based on physical, chemical, biological, and land‐use information. Six non‐redundant metrics from 35 metrics were used to differentiate between reference and impaired sites. We selected six metrics for the final IBI. The scoring criteria of each metric were normalized based on the quadrisection and 0–10 scaling systems. Both scaling methods were used to assess the aquatic health of each site in the Xiangxi River watershed. The results showed that most sites were in fairly poor condition. Furthermore, we identified the relationship between B‐IBI metrics, water‐quality, and land‐use variables with a principal component analysis. A composite of nutrients and land‐use intensity explained most variances. These results suggest that the B‐IBI may be a suitable method for assessing river conditions within the subtropical Xiangxi River in central China (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A multimetric diatom index to assess the ecological status of coastal Galician rivers (NW Spain)

There are many rivers in northwest Spain as a consequence of the mountainous landscape and the granitic geology subjected to Atlantic influences. Water and epilithic diatoms samples were collected at 72 sites in Galicia flowing into the Atlantic Ocean and Cantabrian Sea in summer 2002–2003 and spring 2004. These sites included minimally disturbed sites, defined as reference sites, and impacted sites which were influenced by different human pressures. We used the diatom assemblages to calculate diatom indices using the Omnidia software, but we also developed new metrics based on the similarity of species composition in reference sites. The response of the metrics was tested in relation to physicochemical variables. We developed a diatom multimetric index (MDIAT) as a combination of metric values. The sensitivity of the MDIAT to organic and nutrient stressors supports the use of this index to classify the ecological status of Galician rivers. The MDIAT showed higher correlations with some variables and nutrients than the individual metrics. According to the MDIAT, 69% of the sites sampled in Galician coastal rivers achieve good ecological status. The MDIAT has been developed specifically for Galician granitic rivers (NW Spain), and has been intercalibrated at the European level in the Central Baltic Rivers GIG. Our study validates the application of this multimetric index to evaluate the water quality in coastal Galician rivers.     

Development of a diatom index of biotic integrity for acid mine drainage impacted streams

Acid mine drainage (AMD) resulting from extensive coal mining throughout Appalachia since the 1800s has caused a legacy of severe acid and dissolved metal loads to thousands of stream miles, which has critically impacted aquatic life and ecological attributes. Relationships of diatoms and macroinvertebrates with AMD have been established, but no index specifically designed to quantify AMD impacts using diatoms has been created, nor have the response of multiple organism groups been compared for their utility as indices assessing AMD severity.For the purpose of developing an effective assessment and management strategy for AMD impacted streams, this study created and tested a multi-metric AMD-diatom index of biotic integrity (AMD-DIBI) and compared its response to AMD severity with an already established multi-metric macroinvertebrate community index (ICI). In 2006, 41 sites in southeast Ohio were sampled that represented an AMD impact gradient and non-AMD impacted reference sites. Metrics comprising the AMD-DIBI were selected based on their responsiveness to AMD and nutrient impacts. In the following year, the AMD-DIBI and its metrics were tested on a validation dataset consisting of 18 sites in an AMD impacted watershed. Results indicated a significant correlation between AMD-DIBI and ICI scores, and both indices and all metrics were strongly correlated with water chemistry variables indicative of AMD pollution (P < 0.05). Stepwise multiple regression selected alkalinity and conductivity as most influential to AMD-DIBI (adjusted r² = 0.70) and ICI scores (adjusted r² = 0.66). Narrative classes (e.g., Poor, Fair, Good, and Excellent) defined by index scores provided effective classifications of AMD severity. When tested on the watershed scale, AMD-DIBI and its metrics very successfully quantified AMD gradients and coal mining impacts as indicated by canonical correspondence analysis. This newly developed AMD-DIBI will be very useful for assessing impairment, sensitivity, and recovery of diatom communities in streams damaged or threatened by coal mining activities. In addition, because the AMD-DIBI was very responsive to a gradient of AMD pollution, it could be used in future studies measuring the long-term status of streams and effectiveness of various remediation methods. This study highlights the responsive power of diatom-based metrics.     

Comparative study of diatoms and macroinvertebrates as indicators of severe water pollution: Case study of the Kebena and Akaki rivers in Addis Ababa,Ethiopia

We assessed the relative performance of diatoms and macroinvertebrates to measure municipal and industrial impacts on the ecological integrity of the three major rivers flowing through Addis Ababa. Both community metric and multivariate statistical techniques were used to analyze the environmental variables and species data along the pollution gradient. This study in the Addis Ababa urban area revealed that three biologically highly stressed rivers are being impacted primarily by physical habitat degradation and both point and nonpoint pollution. The macroinvertebrate composition was liable to severe physical habitat and chemical water quality degradation. Consequently, macroinvertebrates were less diverse and not found at all at the most polluted sites with very low dissolved oxygen levels. Based on community metrics and multivariate analysis results, diatoms more reliably indicated a gradient of pollution than macroinvertebrates. However, both organism groups equally discriminated the two relatively unimpacted upstream sites from all other impacted sites. As diatoms are immobile and ubiquitous (i.e., at least a few can be found under almost any condition), they are good indicators of pollution levels among heavily impacted sites where macroinvertebrates are completely absent or less diverse. Therefore, diatoms are the powerful bioindicators for monitoring urban-impacted and seriously stressed rivers and to examine pollution gradients and impacts of specific pollution sources.     

Using macroinvertebrates to assess ecological integrity of streams remediated for acid mine drainage

Studies assessing stream restoration of acid mine drainage (AMD) usually use community metrics that emphasize only a few attributes of ecological integrity. We used a variety of measures to examine recovery of macroinvertebrate communities in a watershed receiving passive treatment for AMD. We measured density, richness, functional diversity (FD), taxonomic diversity (Δ, a measure of taxonomic relatedness), and community taxonomic composition at 4 stream sites over an 11‐year period: 1 reference not affected by AMD, 2 downstream of treatment systems, and 1 AMD‐impacted site with no treatment. Our objectives were to determine if communities have responded to remediation, and examine whether FD and Δ provided additional insights into ecology integrity. Richness, FD, and Δ at the reference site were significantly higher than for other sites, but did not consistently differentiate treated and untreated sites. Additional information provided by FD and Δ on attributes related to functional redundancy and taxonomic relatedness was self‐evident given the simplicity of nonreference communities. Ordination of community composition incorporating knowledge of taxa autecologies indicated that treated sites were slightly less impacted than the untreated site, and trend analysis indicated slight improvement at 1 treated site. However, communities below treatment remained taxonomically distinct, with much lower densities than reference condition. Macroinvertebrate communities showed less recovery than was determined for benthic diatom communities at the same sites in a previous study. Several attributes of ecological integrity for different types of stream organisms should be incorporated into bioassessment of AMD restoration because they can vary in their response.     

Macrophytes as indicators of stream condition in the wet tropics region,Northern Queensland,Australia

This study investigates the use of aquatic macrophytes as indicators of stream condition in catchments with varied land use and levels of riparian disturbance in the Wet Tropics region of North Queensland (Australia), a region of global significance in terms of faunal and floral diversity. In a paired catchment design spatial variations in macrophyte assemblage structure were characterised using multivariate and univariate techniques. Seven metrics were trialled: total macrophyte cover, species richness, % alien taxa, % native taxa, % submerged taxa, % emergent taxa and % Poaceae. Forty-four macrophyte taxa were recorded from the study area. Poaceae, Cyperaceae and mosses were the most frequently recorded taxa. Upper catchment areas in all tributaries surveyed were dominated by mosses and Cladopus queenslandicus (Domin) C.D.K. Cook (Podestemaceae). This assemblage occurred in areas with intact riparian canopy cover and good overall riparian condition. Macrophyte assemblages in lower catchment areas were distributed along gradients of riparian disturbance. Simultaneous autoregression model coefficients indicated that riparian condition had a negative influence on macrophyte cover, species richness and the proportions of alien taxa, emergent taxa and Poaceae present at sites in the Wet Tropics. Macrophyte metrics were not strongly influenced by the types of land use or water quality. These findings suggest that a riparian condition assessment would provide an adequate first assessment of the state of aquatic macrophyte assemblages in Wet Tropics streams.     

A Framework for Evaluating Heterogeneity and Landscape-Level Impacts of Non-native Aquatic Species

Non-native species are a major component of global environmental change, and aquatic systems are especially vulnerable to non-native species impacts. Much of the research on aquatic non-native species impact has occurred at the local or site level. In reality, non-native species impacts play out across multiple spatial scales on heterogeneous landscapes. How can we ‘scale up’ our understanding of site-level impacts to the broader landscape scale? To address this disconnect, we synthesize our current understanding of key components of landscape-scale non-native species impacts: geographic range, abundance, and local impacts. Most aquatic non-native species have small ranges, while a few have large ranges. However, aquatic non-native species are often far from saturated on landscapes, and occurrence records are often woefully incomplete. Aquatic non-native species are often at low abundances where they are present, reaching high abundance in a small number of locations. Finally, local-scale impact can be estimated from abundance, but this requires knowledge of the abundance–impact relationship. Considering these multiple components enables understanding of non-native species impacts at broader spatial scales. Although the landscape-level impacts of aquatic non-native species may be high, the spatial distribution of site-level impacts is uneven, and highly impacted sites may be relatively uncommon. This heterogeneity in impacts provides an opportunity to optimize and prioritize non-native species management and prevention efforts.