The roles of abiotic factors, dispersal, and species interactions in structuring stream assemblages of black flies (Diptera: Simuliidae)

ABSTRACT: BACKGROUND: The patterns and drivers of species assemblages represent the core of community ecology.We focus on the assemblages of a single family of ubiquitous lotic insects, the Simuliidae(black flies), of which the larvae play a critical role in resource turnover in steams. We useMantel tests and null models to tease out the potential influence of abiotic stream conditions,species interactions, and dispersal on the assemblage patterns of larval black flies over twospatial scales (within and across ecoregions) and two seasons (spring and summer). RESULTS: When stream sites were considered across ecoregions in the spring, stream conditions anddispersal were correlated significantly with species similarity; however, within ecoregions inthe spring, dispersal was important only in the Piedmont and Sandhills and abiotic factorsonly in the Mountains. In contrast, results of the summer analyses within and acrossecoregions were congruent; assemblage similarity was significantly correlated with streamconditions both across and within ecoregions. Null models suggested that patterns of speciessegregation in the spring were consistent with a community structured by competition,whereas patterns in the summer were consistent with species assemblages influenced byabiotic factors. CONCLUSIONS: Species composition of black flies at streams sites is correlated with dispersal factors andstream conditions, but results vary over spatial and temporal scales. Communities of blackflies can be viewed within a metacommunity context; local assemblages are consistent withspecies sorting and mass effects. Given that black flies have a terrestrial stage, with femalesdeciding where to place the eggs, a full understanding of the processes that determine local aquatic assemblages will require integration of the dynamics of the aquatic immature stagesand the terrestrial adults.     

Concordance among stream assemblages and spatial autocorrelation along a fragmented gradient

Patterns of spatial autocorrelation of biota and distributional similarity (concordance) between assemblages of different organism groups have important implications in both theoretical ecology and biodiversity conservation. Here we report environmental gradients and spatial distribution patterns of taxonomic composition among stream fish, benthic macroinvertebrate, and diatom assemblages along a fragmented stream in south‐western France. We quantified spatial patterns of lotic assemblage structure along this stream, and we tested for concordance in distribution patterns among the three taxonomic groups. Our results showed that both environmental characteristics and stream assemblages were spatially autocorrelated. For stream fish and diatom assemblages, these patterns reflected assemblage changes along the longitudinal stream gradient, whereas environmental variables and benthic macroinvertebrates exhibited a more patchy spatial pattern. Cross‐taxa concordance was significant between stream fish and diatoms, and between stream fish and benthic macroinvertebrates. The assemblage concordance between stream fish and diatoms could be attributed to similar responses along the longitudinal gradient, whereas those between stream fish and benthic macroinvertebrates may result from biotic interactions. Based on potential dispersal capacities of taxa, our results validated the hypotheses that weakly dispersing taxa exhibit greater concordance than highly dispersing ones and that dispersal capacities affect how taxonomic groups respond to their local environment. Both diatoms and highly dispersing stream fish were affected by stream fragmentation (i.e. the number of dams between sites), while the effect of fragmentation was not significant for invertebrates that fly well in their adult stage, thus emphasizing the importance of the way of dispersal. These results suggest that addressing the effects of dispersal capacity on stream assemblage patterns is crucial to identifying mechanisms behind patterns and to better understanding the determinants of stream biodiversity.     

Similarity in parasite communities of the teleost fish Pinguipes brasilianus in the southwestern Atlantic: Infracommunities as a tool to detect geographical patterns

Patterns of distance decay in similarity among communities of the fish Pinguipes brasilianus (Teleostei: Pinguipedidae) from five areas in the southwestern Atlantic were investigated to determine whether the rate of decay varied depending on the community level or the parasite guild analyzed (ectoparasites, adult endoparasites and larval endoparasites). Similarities in species composition were computed at both the component community and infracommunity levels. Similarity indices were calculated between all possible pairs of assemblages from different zones. Infracommunity similarity values between and within host populations were averaged. Significance of linear regressions for similarity values against distance was assessed using randomization tests. Different patterns were observed for each guild, and similarity among infracommunities within host populations varied accordingly. Decay in similarity over distance was recorded for most communities. The slopes differed significantly between infracommunities and component communities in all cases, and stronger decay was always observed for infracommunities. Different geographical patterns in parasite communities were a consequence of variability in parasite availability in the different regions, modulated by oceanographic conditions, as well as variation among species in terms of host specificity and life-cycles strategies. Infracommunities showed a stronger effect of distance than component communities, probably due to the influence of short term and local variability of oceanographic conditions.     

Variation in life cycle affects the distance decay of similarity among bluegill sunfish parasite communities

Rates of community similarity decay were examined among parasite communities of the bluegill sunfish from isolated populations located throughout the midwestern and southeastern United States. Rates of decay were compared for assemblages composed of all species, autogenic species only, and allogenic species only. Rates of similarity decay were calculated by regressing a matrix of intercommunity distance against a matrix of intercommunity Sorenson similarity for each group of species. Significance was determined with the use of a Mantel test using 1,000 permutations. Significant rates of decay were found for all groups; however, allogenic similarity decayed according to an exponential function, whereas autogenic similarity decayed according to a linear function. Overall, the results suggest that autogenic-allogenic status is an important factor determining the rate at which similarity decays, but illustrate the need for more insight regarding the factors that affect parasite dispersal ability.     

Patterns in the species composition of fish assemblages among Wisconsin streams

Synopsis To better understand patterns of fish assemblage composition in Wisconsin streams in relation to major environmental gradients, I carried out multivariate direct gradient analysis (canonical correspondence analysis) of two large independent datasets on fish species abundance in Wisconsin streams. Analysis of the two datasets yielded similar results, suggesting that observed patterns and relationships were real. Stream sites were distributed along fish species-environment gradients, but segregation into distinct stream temperature and geographic groups was also evident. The strongest gradient in both datasets was related to summer water temperature patterns, and encompassed a transition from small, coldwater streams dominated by salmonids, cottids, certain cyprinids, and few other species, to both small and large, warmwater streams dominated by a high diversity of different cyprinids, catostomids, ictalurids, centrarchids, and percids. A second gradient in both datasets was complex but largely geographic. Within it, sites from each of the four ecoregions that occupy Wisconsin formed fairly discrete groups. The strongest differences were between sites in the two southern Wisconsin ecoregions, the Driftless Area and the Southeastern Wisconsin Till Plains, that were dominated by certain cyprinids, ictalurids, and centrarchids, and sites in the two northern Wisconsin ecoregions, the North Central Hardwood Forests and the Northern Lakes and Forests, that were dominated by a different set of cyprinids and ictalurids, plus some petromyzontids, salmonids, catostomids, and percids. Sites from the Driftless Area that were mostly higher-gradient (steep stream slope) and had many riffle-dwelling species could also be distinguished from sites in the Southeastern Wisconsin Till Plains that were mostly lower-gradient and had many pool-dwelling species. The patterns of fish assemblage composition among sites and the associated fish species-environment relationships that were revealed by the analyses provided a framework for developing an ecologically meaningful hierarchical classification of Wisconsin stream sites based on stream thermal regime, ecoregion, stream size, and stream gradient.     

Biogeography,ecoregions, and geomorphology affect fish species composition in streams of eastern Oklahoma,USA

Stream fish assemblages are structured by biogeographical, physical and biological factors acting on different spatial scales. We determined how physical factors, geomorphology and stream habitat, influenced fish species composition (presence–absence) in eastern Oklahoma, USA relative to the ecoregion and biogeographic effects previously reported. We sampled fish assemblages and surveyed geomorphology and habitat at 107 stream sites in the Boston Mountains, Ouachita Mountains, and Ozark Highlands ecoregions in eastern Oklahoma. Partial canonical correspondence analyses (pCCAs) and variance partitioning showed that patterns of endemism related to drainage basins and ecoregions explained important variation in fish species composition in all streams, but stream size and local channel morphology explained more variation overall. Stream size effects were most important in explaining variability in fish species composition in both northeastern and southeastern Oklahoma streams. Local channel morphology and substrate characteristics were secondarily important. Variables typically considered important as fish habitat (aquatic vegetation, etc.) had little effect on fish species composition.     

Fish ecoregions of Kansas: stream fish assemblage patterns and associated environmental correlates

Synopsis Principal components analysis was performed on fish presence/absence data for 39 common fish species from 410 stream sites in Kansas. The analysis confirmed ten ecologically meaningful fish assemblages, based on species associations. Factor scores based on these assemblages were then clustered into six geographic areas or fish ecoregions. Canonical discriminant analysis identified environmental variables that distinguished the derived fish ecoregions. Mean annual runoff, mean annual growing season, and discharge appear most important. Mean width, mean depth, chloride concentration, water temperature, substrate type, gradient, and percent of pool habitat were less important. Correspondence exists between these fish ecoregions and the patterns of physiographic regions, river basins, geology, soil, and potential natural vegetation in Kansas. The multivariate statistical approach used to classify fish ecoregions should have considerable potential value for fish assessment and management purposes in areas other than the state of Kansas.     

Predicting aquatic vertebrate assemblages from environmental variables at three multistate geographic extents of the western USA

Variables for predicting assemblage differences change as the geographic extent of studies change, hindering development of useful predictive models where study data are limited, or where the chief predictive variables available are fish zones, river size, physiographic regions, ecoregions, hydrologic units, and river basins. In addition, some studies have shown that site-scale predictor metrics have accounted for more of the variation in fish assemblage response metrics than catchment-scale metrics and other studies have shown the reverse. We used cluster analysis on a 780-site database to determine 12–15 aquatic vertebrate clusters at three geographic extents (all 12 conterminous western U.S. states, all western mountain ecoregions, Pacific Northwest mountain ecoregions). Next, we determined predictor variables for those assemblage clusters through use of stepwise discriminant function analysis. Site longitude, site latitude, and catchment dam count were the most significant predictors at the three geographic extents. Site-scale variables represented most of the significant predictors for all three geographic extents, but explained only slightly more aquatic vertebrate assemblage variance than catchment or pure spatial variables. Catchment- and site-scale classification variables accounted for less than half the mean within-cluster similarity demonstrated by the aquatic vertebrate assemblage clusters. We conclude that (a) the large geographic extent of the analysis did not result in catchment-scale predictor variables being more important than site-scale predictors, (b) both catchment- and site-scale variables are important predictors, and (c) existing river basin and ecoregion classifications are useful but insufficient predictors of aquatic vertebrate assemblages.     

Response of three lotic assemblages to riparian and catchment‐scale land use: implications for designing catchment monitoring programmes

1. Although many studies have focussed on the effects of catchment land use on lotic systems, the importance of broad (catchment) and fine (segment/reach) scale effects on stream assemblages remain poorly understood. 2. Nine biological metrics for macrophytes (498 sites), benthic macroinvertebrates (491) and fish (478) of lowland and mountain streams in four ecoregions of France and Germany were related to catchment and riparian buffer land use using partial Redundancy Analysis and Boosted Regression Trees (BRTs). 3. Lotic fauna was better correlated (mean max., r = 0.450) than flora (r = 0.277) to both scales of land use: the strongest correlations were noted for mountain streams. BRTs revealed strong non‐linear relationships between mountain assemblage metrics and land use. Correlations increased with increasing buffer lengths, suggesting the importance of near‐stream land use on biotic assemblages. 4. Several metrics changed markedly between 10–20% (mountain ecoregions) and 40–45% (lowland) of arable land use, irrespective of the buffer size. At mountain sites with >10% catchment arable land use, metric values differed between sites with <30% and sites with >30% forest in the near‐stream riparian area. 5. These findings support the role of riparian land use in catchment management; however, differences between mountain and lowland ecoregions support the need for ecoregion‐specific management.     

Influence of physiographic and climatic factors on spatial patterns of acorn production in Maryland and Virginia,USA

Aim The aims of this study were to identify the effects physiographic differences have on the spatial synchrony of acorn production within red (Quercus rubra) and white (Quercus alba) oak subgenus groups, to identify climatic factors associated with acorn production patterns, and to assess if and how these relationships vary across a distinct physiographic boundary. Location Maryland and Virginia in the eastern United States. Methods Using data from 36 survey sites in Virginia (1973–2000) and Maryland (1977–2000), we described the spatial synchrony of annual acorn production based on Mantel tests and modified correlograms at three spatial scales: (1) the entire study area, (2) within two ecological regions (mountain and Piedmont), and (3) between these ecoregions. Using climate data obtained from the National Climate Data Centre, we described the relationship between climate and acorn production within each ecological region using linear regression. Results Spatial synchrony in the white oak group acorn production was strong among sites closest together, and declined with distance. The average April temperature during the year of acorn drop was positively related to acorn crop size within the mountain and Piedmont ecoregions. Spatial synchrony in the red oak group was strong among sites closest together and declined with distance in the mountain region. Synchrony was weaker in the Piedmont than in the mountain ecoregion and declined slightly with increasing distance. Between the mountain and Piedmont ecoregions, synchrony was not detected among the closest sites, but was detected with increasing strength as the distance between site pairs increased. In the Piedmont ecoregion, the number of April freeze events and total July precipitation, both 2 years prior to acorn drop, were negatively related to red oak acorn production. In the mountains, average July temperature 2 years prior to acorn drop was positively related to acorn production. Main conclusions Physiographic differences affected the spatial acorn production synchrony in the red oak but not in the white oak subgenus group. Climatic factors related to annual acorn production variability differed between subgenus groups and physiographic regions. The physiographic differences between the mountain and Piedmont ecoregions may define spatial patterns of acorn production synchrony in some oak species and mediate climatic influences on acorn production.     

Comparative Environmental Assessment in the Studies of Benthic Diatom,Macroinvertebrate, and Fish Communities

Community response to environmental gradients operating at hierarchical scales was assessed in studies of benthic diatoms, macroinvertebrates and fish from 44 stream sites in the New York City watershed. Hierarchical cluster analysis (TWINSPAN) of diatoms and fish partitioned the study sites into four groups, i.e., acid streams, reservoir outlets and wetland streams, large eutrophic streams, and small eutrophic streams; macroinvertebrate TWINSPAN distinguished an additional group of silty eutrophic streams. The correspondence among the three assemblage TWINSPAN groupings was moderate, ranging from 51 to 57%. The similarity across the four major group types was the highest among large eutrophic stream and acid stream assemblages, and the lowest among small eutrophic stream assemblages. Stepwise discriminant function analysis revealed that environmental factors discriminated most effectively the diatom grouping and least effectively the fish grouping. The best environmental predictors for diatom and macroinvertebrate grouping were conductance and percent surface water, while population density was most powerful in separating the fish groups. Carbaryl was the only pesticide that correlated with macroinvertebrate grouping. Partial redundancy analyses suggested a differential dependence of freshwater communities on the scale of the environmental factors to which they respond. The role of small‐scale habitat and habitatland cover/land use interaction steadily increased across the diatom, macroinvertebrate, and fish assemblages, whereas the effect of large‐scale land cover/land use declined.     

Distance decay of similarity in freshwater communities: do macro‐ and microorganisms follow the same rules?

Aim An intensively debated issue in macroecology is whether unicellular organisms show biogeographic patterns different from those of macroorganisms. One aspect of this debate addresses beta diversity, that is, do microbial organisms exhibit distance‐decay patterns similar to those of macroorganisms? And if so, is the decay of community similarity caused by spatially limited dispersal or by niche‐related factors? We studied the community similarity of stream diatoms, macroinvertebrates and bryophytes across the same set of sites in relation to environmental and geographic distance. Location A geographical gradient of c. 1100 km in Finland. Methods We first identified the subset of environmental variables that produced the highest correlation with community similarities for each taxonomic group. Based on these variables, we used partial Mantel tests to separate the independent influences of environmental and geographical distance for distance decay of community similarity, separately for diatoms, bryophytes and macroinvertebrates. Finally, macroinvertebrates were divided into three groups based on their different dispersal categories and a partial Mantel test was used to assess whether each of these groups were differently affected by environmental versus geographic distance, i.e. is dispersal a key factor in tests of niche versus neutral models. Results The level of environmental control was by far the strongest for diatoms; however, all groups were controlled more by environmental factors than by limited dispersal. Macroinvertebrate species with low dispersal ability were significantly related to geographic distance, while more effective dispersers showed no relationship to geography but were instead strongly related to environmental distance. Main conclusions Our results suggest that patterns between macro‐ and microorganisms are not fundamentally different, but the level of environmental control varies according to dispersal ability. The relative importance of niche versus dispersal processes is not simply a function of organism size but other traits (e.g. life‐history type, dispersal capacity) may obscure this relationship.     

The decay of similarity with geographical distance in parasite communities of vertebrate hosts

Aim The rate at which similarity in species composition decays with increasing distance was investigated among communities of parasitic helminths in different populations of the same host species. Rates of distance decay in similarity of parasite communities were compared between populations of fish and mammal hosts, which differ with respect to their vagility and potential to disperse parasite species over large distances. Location Data on helminth communities were compiled for several populations of three mammalian host species (Ondatra zibethicus, Procyon lotor and Canis latrans) and three fish host species (Perca flavescens, Catostomus commersoni and Esox lucius) from continental North America. Methods Distances between localities and similarity in the composition of helminth communities, the latter computed using the Jaccard index, were calculated for all possible pairs of host populations within each host species. Similarity values were then regressed against distance to see if they decayed at exponential rates, as reported for plant communities; the significance of the regressions was assessed using randomization tests. Results The number of hosts examined per population did not correlate with the number of helminth species found per population, and thus sampling effort is unlikely to have confounded the results. In four (two mammals and two fish) of the six host species, similarity in helminth communities decayed exponentially with distance. When the log of similarity is plotted against untransformed distance, the slopes obtained for the two fish species are lower than those obtained for the two mammalian host species. Main conclusions Similarity in the composition of parasite communities appears to decay exponentially with increasing distance in some host species, but not in all host species. The rate of decay is not necessarily associated with the vagility of the host. Although distance decay of similarity is generally occurring, it seems that other ecological processes, related either to the host or its habitat, can obscure it.     

The influence of substrate quality and stream size on wood decomposition dynamics

Woody materials decayed more rapidly in a first order stream than in larger streams in eastern Quebec, Canada. The rate of annual mass loss (k) was highest (k=1.20) for alder wood chips in a first order stream and lowest (k=0.04) for black spruce wood chips in a ninth order stream. Decay rates for woody materials in a first order stream were inversely related to their initial lignin to nitrogen ratios. In larger streams, decay rates of woody materials were inversely related to their initial lignin concentrations. A number of quantifiable relationships were found to exist between the initial lignin and nitrogen contents of woody materials and the nitrogen dynamics of decaying wood.     

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.     

Scale, time, space, and predictability: species distributions of preimaginal black flies (Diptera: Simuliidae)

This study examines species-specific distributions of preimaginal black flies (Diptera: Simuliidae) over two large spatial scales (within and across ecoregions) and two seasons (spring and summer). The study area in the western two-thirds of South Carolina, United States, was divided into three ecoregions (Mountains, Piedmont, Sandhills). The mean correct classification of species distributions among stream sites was 81.3%. Predictability of species distributions varied with spatial scale, location, and time. Species occurrence was not independent of ecoregion; distributions of 12 of the 13 species examined at this scale were predictable on the basis of changes in water column (temperature, percentage dissolved oxygen) and channel characters (velocity, streambed-particle size). However, with the exception of the Mountains during the summer, predicting species distributions within ecoregions was far less successful than predicting distributions across these regions; predictability was particularly poor in the Sandhills. Seasonal effects on predictability were most pronounced in the Mountains. As stream sites became more homogeneous, predictive capability declined. Species-level identifications, aided by cytogenetic techniques, were of paramount importance in recognizing the association between species distributions and stream conditions, emphasizing that considerable ecological information is lost when species are not considered. This study emphasizes the critical role of taxonomic resolution, linked with spatial and temporal scale of observation, in elucidating patterns of species assemblages. It also demonstrates that predictability of species distributions at a fixed scale can vary with geographic location and time. Received: 2 May 1997 / Accepted: 22 September 1997     

Host ontogeny and the temporal decay of similarity in parasite communities of marine fish

Geographical distances between host populations are key determinants of how many parasite species they share. In principle, decay in similarity should also occur with increasing distance along any other dimension that characterizes some form of separation between communities. Here, we apply the biogeographical concept of distance decay in similarity to ontogenetic changes in the metazoan parasite communities of three species of marine fish from the Atlantic coast of South America. Using differences in body length between all possible pairs of size classes as measures of ontogenetic distances, we find that, using an index of similarity (Bray-Curtis) that takes into account the abundance of each parasite species, the similarity in parasite communities showed a very clear decay pattern; using an index (Jaccard) based on presence/absence of species only, we obtained slightly weaker but nevertheless similar patterns. As we predicted, the slope of the decay relationship was significantly steeper in the fish Cynoscion guatucupa, which goes through clear ontogenetic changes in diet and therefore in exposure to parasites, than in the other species, Engraulis anchoita and Micropogonias furnieri, which maintain a roughly similar diet throughout their lives. In addition, we found that for any given ontogenetic distance, i.e. for a given length difference between two size classes, the similarity in parasite communities was almost always higher if they were adult size classes, and almost always lower if they were juvenile size classes. This, combined with comparisons among individual fish within size classes, shows that parasite communities in juvenile fish are variable and subject to stochastic effects. We propose the distance decay approach as a rigorous and quantitative method to measure rates of community change as a function of host age, and for comparisons across host species to elucidate the role of host ecology in the development of parasite assemblages.     

Bank erosion hazard index as an indicator of near-bank aquatic habitat and community structure in a southeastern Piedmont stream

Streams in the southeastern Piedmont region display varying degrees of bank erosion, which in turn can influence multiple physical and biological processes. In efforts to assess and predict bank erosion, the multi-metric tool Bank Erosion Hazard Index (BEHI) is often calculated. As designed, the BEHI takes into consideration many physical phenomena and scores stream banks categorically from very low to extreme bank erosion hazard; however the power of this index to directly predict in-stream biological conditions is unknown. The predictive ability of BEHI was evaluated on near-bank aquatic biota and organic matter (OM) retention in an AL Piedmont stream with a range of bank conditions. Fifteen banks were characterized using the BEHI and subsequently, macroinvertebrates, crayfishes, fishes and organic matter associated with each bank were quantified. Results suggest increased macroinvertebrate richness and OM retention with low BEHI score. Multivariate analyses suggest functional similarity in macroinvertebrate assemblages associated with low BEHI banks and greater variation in assemblages as BEHI increased. Crayfish were more abundant at banks with high BEHI, a pattern attributed to increases in a generalist species in more-degraded habitats. Fish response showed no discernible pattern in relationship to BEHI score. These results suggest that BEHI may have applications as an informative ecological evaluation tool integrating physical and biological conditions in streams.     

Water quality,substrate, and distribution of macroinvertebrates in the Patuxent River,Maryland

The distribution of benthic macroinvertebrates in the Patuxent River, Maryland, was analyzed for the period between June 1975 and February 1976. Numerical diversity was analyzed using data collected from seven sampling stations along the nontidal portion of the Patuxent River. Both natural and artificial substrates were examined. Dissolved oxygen, temperature, soil texture, and amount of organic matter were also measured to determine their correlation with the invertebrate distribution measurements. A 2-way ANOVA revealed significant interaction between site and collecting period in the diversity measurements of the natural substrates. This site by collecting period interaction was not observed in the collections made from the artificial substrates. Dissolved oxygen and temperature did not change significantly with distance along the river. Soil texture and organic matter varied significantly with distance. The distribution of benthic macroinvertebrates was more closely correlated to substrate than to water quality. this finding contradicts previous work on this portion of the Patuxent River.     

A comparative measure of biodiversity based on species composition

In conservation planning, species richness and species endemism are the most often used metrics for describing the biodiversity importance of areas. However, when it comes to prioritizing regions for conservation actions these measures alone are insufficient because they do not reveal how similar or different the actual composition of species may be from one area to another. For comparative analysis an additional useful metric would be one that indicates the degree to which the species assemblage in one area is also represented in—or is distinct from—species assemblages of other areas. Here we describe a method for quantifying the compositional representativeness of species assemblages among geographic regions. The method generates asymmetric pairwise similarity coefficients that are then used to calculate separate measures for the representativeness and the distinctiveness of species assemblages in the regions being compared. We demonstrate the method by comparing fish communities among freshwater ecoregions of the Mississippi Basin, and then among smaller hydrological units within two individual freshwater ecoregions. At both scales of analysis, our measures of representativeness and distinctiveness reveal patterns of fish species composition that differ from patterns of species richness. This information can enhance conservation planning processes by ensuring that priority-setting explicitly consider the most representative and distinctive species assemblages.