Seasonal variability of macroinvertebrate assemblages in two regions of South Africa: implications for aquatic bioassessment

Seasonal variability of macroinvertebrate assemblages was examined in two regions of South Africa: Western Cape and Mpumalanga. Sampling was undertaken at reference sites using the rapid bioassessment method SASS4 (South African Scoring System, Version 4). This study examined the influence of sampling season on the occurrence of taxa, macroinvertebrate assemblages and SASS scores. In the Western Cape a few taxa were more common at certain times of the year, and multivariate analysis of macroinvertebrate assemblages showed that assemblages grouped by season. The number of taxa and the Average Score per Taxon (ASPT) were significantly different among seasons in the Western Cape, with fewer taxa recorded in winter compared to summer, and significantly higher ASPT values recorded in winter and spring than summer and autumn. Whereas more taxa were recorded in autumn than in spring, a higher proportion of sensitive and high-scoring taxa were recorded in spring. In Mpumalanga macroinvertebrate assemblages did not group by season, and differences in SASS scores were not significant, although more taxa were recorded in winter than in spring or autumn, and ASPTs were slightly higher in winter than in spring. Results are discussed in relation to ecological reference conditions and the interpretation of bioassessment data.     

The influence of biotope availability on macroinvertebrate assemblages in South African rivers: implications for aquatic bioassessment

1. Macroinvertebrate biotope preferences and the influence of differences in the availability of biotopes on individual taxa, macroinvertebrate assemblages and a biotic index, the South African Scoring System (SASS), were investigated in two regions of South Africa. 2. Among biotope differences in individual taxa and macroinvertebrate assemblages resulted in differences in SASS scores, with differences in assemblages being greater among biotopes than between sites. 3. All three metrics studied (SASS score, number of taxa and average score per taxon, ASPT), differed significantly among biotopes, with highest scores consistently recorded in the stones biotope, while lowest SASS scores and fewest taxa were recorded in the sand biotope. 4. SASS score and number of taxa were positively, while ASPT was negatively correlated with number of biotopes sampled. 5. The observed biotope differences highlight the importance of sampling and comparing data from sites separately for each biotope.     

Relationship of fish and macroinvertebrate assemblages to environmental factors: implications for community concordance

Community concordance describes similarity in distributions and abundances of organisms from different taxonomic groups across a region of interest, with highly concordant communities assumed to respond similarly to major environmental gradients, including anthropogenic stressors. While few studies have explicitly tested for concordance among stream-dwelling organisms, it frequently is assumed that both macroinvertebrates and fish respond in concert to environmental factors, an assumption that has implications for their management. We investigated concordance among fish and macroinvertebrates from tributaries of two catchments in southeastern Michigan having varied landscape characteristics. Classifications of fish and macroinvertebrate assemblages resulted in groups distinguished by differences in taxonomic characteristics, functional traits, and stressor tolerance of their respective dominant taxa. Biological groups were associated with principal landscape gradients of the study region, which ranged from forests and wetlands on coarse surficial geology to agricultural lands on finer, more impervious surficial geology. Measures of stream habitat indicated more stable stream flows and greater heterogeneity of conditions at site groups with catchments comprising forests and wetlands on the coarsest geology, but did not distinguish well among remaining site groups, suggesting that habitat degradation may not be the driving mechanism leading to differences in groups. Despite broadly similar interpretations of relationships of site groups with landscape characteristics for both fish and macroinvertebrates, examination of site representation within groups indicated weak community concordance. Our results suggest that explicit responses of fish and macroinvertebrates to landscape factors vary, due to potential differences in their susceptibility to controls or to differences in the scale at which landscape factors influence these organisms. Handling editor: Robert Bailey     

Functional biodiversity of macroinvertebrate assemblages along major ecological gradients of boreal headwater streams

1. Biodiversity–environment relationships are increasingly well‐understood in the context of species richness and species composition, whereas other aspects of biodiversity, including variability in functional diversity (FD), have received rather little rigorous attention. For streams, most studies to date have examined either taxonomic assemblage patterns or have experimentally addressed the importance of species richness for ecosystem functioning. 2. I examined the relationships of the functional biodiversity of stream macroinvertebrates to major environmental and spatial gradients across 111 boreal headwater streams in Finland. Functional biodiversity encompassed functional richness (FR – the number of functional groups derived from a combination of functional feeding groups and habit trait groups), FD – the number of functional groups and division of individuals among these groups, and functional evenness (FE – the division of individuals among functional groups). Furthermore, functional structure (FS) comprised the composition and abundance of functional groups at each site. 3. FR increased with increasing pH, with additional variation related to moss cover, total nitrogen, water colour and substratum particle size. FD similarly increased with increasing pH and decreased with increasing canopy cover. FE decreased with increasing canopy cover and water colour. Significant variation in FS was attributable to pH, stream width, moss cover, substratum particle size, nitrogen, water colour with the dominant pattern in FS being related to the increase of shredder‐sprawlers and the decrease of scraper‐swimmers in acidic conditions. 4. In regression analysis and redundancy analysis, variation in functional biodiversity was not only related to local environmental factors, but a considerable proportion of variability was also attributable to spatial patterning of environmental variables and pure spatial gradients. For FR, 23.4% was related to pure environmental effects, 15.0% to shared environmental and spatial effects and 8.0% to spatial trends. For FD, 13.8% was attributable to environmental effects, 15.2% to shared environmental and spatial effects and 5% to spatial trends. For FE, 9.0% was related to environmental variables, 12.7% to shared effects of environmental and spatial variables and 4.5% to spatial variables. For FS, 13.5% was related to environmental effects, 16.9% to shared environmental and spatial effects and 15.4% to spatial trends. 5. Given that functional biodiversity should portray variability in ecosystem functioning, one might expect to find functionally rather differing ecosystems at the opposite ends of major environmental gradients (e.g. acidity, stream size). However, the degree to which variation in the functional biodiversity of stream macroinvertebrates truly portrays variability in ecosystem functioning is difficult to judge because species traits, such as feeding roles and habit traits, are themselves strongly affected by the habitat template. 6. If functional characteristics show strong responses to natural environmental gradients, they also are likely to do so to anthropogenic environmental changes, including changes in habitat structure, organic inputs and acidifying elements. However, given the considerable degree of spatial structure in functional biodiversity, one should not expect that only the local environment and anthropogenic changes therein are responsible for this variability. Rather, the spatial context, as well as natural variability along environmental gradients, should also be explicitly considered in applied research.     

Chemical soil factors influencing plant assemblages along copper-cobalt gradients: implications for conservation and restoration

Aims

Define the chemical factors structuring plant communities of three copper-cobalt outcrops (Tenke-Fungurume, Katangan Copperbelt, D. R. Congo) presenting extreme gradients.

Methods

To discriminate plant communities, 172 vegetation records of all species percentage cover were classified based on NMDS and the Calinski criterion. Soil samples were analyzed for 13 chemical factors and means compared among communities with ANOVA. Partial canonical correspondence analysis (pCCA) was used to determine amount of variation explained individually by each factor and site effect.

Results

Seven communities were identified. Six of the studied communities were related to distinct sites. Site effect (6.0 % of global inertia) was identified as the most important factor related to plant communities’ variation followed by Cu (5.5 %), pH (3.6 %) and Co (3.5 %). Unique contribution of site effect (3.8 %) was higher than that of Cu (1.1 %) and Co (1.0 %).

Conclusions

In restoration, not only Cu and Co contents will be important to maintain vegetation diversity, attention should also be given to co-varying factors potentially limiting toxicity of metals: pH, organic matter, Ca and Mn. Physical parameters were also identified as important in the creation of adequate conditions for diverse communities. Further studies should focus on the effect of physical parameters and geology.     

Aquatic macroinvertebrate assemblages in mitigated and natural wetlands

Many wetlands have been constructed in West Virginia as mitigation for a variety of human disturbances, but no comprehensive evaluation on their success has been conducted. Macroinvertebrates are extremely valuable components of functioning wetland ecosystems. As such, benthic and water column invertebrate communities were chosen as surrogates for wetland function in the evaluation of 11 mitigation and 4 reference wetlands in West Virginia. Mitigation wetlands ranged in age from 4 to 21 years old. Overall familial richness, diversity, density and biomass were similar between mitigation and reference wetlands (p > 0.05). Within open water habitats, total benthic invertebrate density was higher in reference wetlands, but mass of common taxa from water column samples was higher in mitigation wetlands (p < 0.05) Planorbidae density from benthic samples in emergent habitats was higher in reference than mitigated wetlands. Benthic Oligochaeta density was higher across open water habitats in mitigation wetlands. All other benthic taxa were similar between wetland types. Among the most common water column orders, Isopoda density was higher in reference wetlands, but Physidae density was higher in mitigation wetlands. Within mitigation wetlands, emergent areas contained higher richness and diversity than open areas. These data indicate that mitigation and reference wetlands generally support similar invertebrate assemblages, especially among benthic populations. The few observed differences are likely attributable to differences in vegetative community composition and structure. Mitigation wetlands currently support abundant and productive invertebrate communities, and as such, provide quality habitat for wetland dependent wildlife species, especially waterbirds and anurans.     

Replicability of physicochemistry and macroinvertebrate assemblages in stream mesocosms: implications for experimental research

1. The replicability of mesocosms is assessed in a case study of artificial streams (4 m²) flanking a lowland chalk river in southern U.K. Among‐mesocosm comparisons of physicochemistry and macroinvertebrate assemblage composition (in drift and benthos) were made during a 2‐year survey of 12 outdoor once‐through linear channels. 2. Mesocosm physicochemistry was highly replicable, with statistically significant congruence in temporal variation across the mesocosm series, low spatial variation and no significant blocking or positional effects arising from mesocosm layout. Parallel physicochemistry was attributed to the outdoor stream‐side location and short water residence times. 3. Mesocosms were colonized by 127 macroinvertebrate taxa from 15 taxonomic orders. Both benthic and drifting assemblages were comparable among mesocosms, with no significant effect of mesocosm layout. Small differences in taxonomic composition were, however, evident among particular mesocosms, with higher (e.g. Tinodes, Limnius and Elmis) or lower (e.g. Pisidium and Valvata) abundances observed for a minority (5%) of taxa. We conclude that large (4 m²) outdoor flow – through mesocosms can be replicable when located near to the source system and allowed to establish naturally.     

Influence of the colonizing substrate on diatom assemblages and implications for bioassessment: a mesocosm experiment

Although diatoms are important bioindicators of ecological quality, their ecological traits are still not well understood. A major issue is that of substrate preferences, which may result in differences in production, and assemblage structure and composition, and which should therefore be taken into account for ecological quality assessment studies. Thus, in this work, the periphyton grown on sand and ceramic tiles in indoor controlled channels were compared to understand whether substrate differences lead to differences in: periphyton production (chlorophyll-a), chlorophyll-b and c concentrations, diatom assemblages (diversity-Shannon-Wiener, cell density, taxonomic composition, trait proportions), and ecological quality assessments (IPS-‘Indice de Polluosensibilité Spécifique’). A combined inoculum of periphyton from four Portuguese streams was introduced to the running channels (six sand and six tile) and left to colonize for 35 days. Epilithic (tiles) and epipsammic (sand) assemblages were sampled at days 14 and 35. We verified that there were no differences in chlorophyll-a concentration over time and between substrates. On both sampling occasions, the epipsammic assemblages had higher concentration of chlorophyll-c and diatom density but without significant differences over time in each substrate. The taxonomic composition was different between substrates and over time. However, these differences were not reflected in ecological quality assessment. The diversity was also similar between substrates in both sampling occasions, but it was higher at day 14. Mobile and stalked species were more abundant over the entire study and differed significantly between substrates, with the epipsammic assemblages presenting higher abundances of both traits. We concluded that the colonizing substrate influences diatom assemblages but not the ecological quality assessment.     

Spatial patterns of fish communities along two estuarine gradients in southern Florida

In tropical and subtropical estuaries, gradients of primary productivity and salinity are generally invoked to explain patterns in community structure and standing crops of fishes. We documented spatial and temporal patterns in fish community structure and standing crops along salinity and nutrient gradients in two subtropical drainages of Everglades National Park, USA. The Shark River drains into the Gulf of Mexico and experiences diurnal tides carrying relatively nutrient enriched waters, while Taylor River is more hydrologically isolated by the oligohaline Florida Bay and experiences no discernable lunar tides. We hypothesized that the more nutrient enriched system would support higher standing crops of fishes in its mangrove zone. We collected 50 species of fish from January 2000 to April 2004 at six sampling sites spanning fresh to brackish salinities in both the Shark and Taylor River drainages. Contrary to expectations, we observed lower standing crops and density of fishes in the more nutrient rich tidal mangrove forest of the Shark River than in the less nutrient rich mangrove habitats bordering the Taylor River. Tidal mangrove habitats in the Shark River were dominated by salt-tolerant fish and displayed lower species richness than mangrove communities in the Taylor River, which included more freshwater taxa and yielded relatively higher richness. These differences were maintained even after controlling for salinity at the time of sampling. Small-scale topographic relief differs between these two systems, possibly created by tidal action in the Shark River. We propose that this difference in topography limits movement of fishes from upstream marshes into the fringing mangrove forest in the Shark River system, but not the Taylor River system. Understanding the influence of habitat structure, including connectivity, on aquatic communities is important to anticipate effects of construction and operational alternatives associated with restoration of the Everglades ecosystem.     

Development of a regional macroinvertebrate index for large river bioassessment

Large river bioassessment protocols lag far behind those of wadeable streams and often rely on fish assemblages of individual rivers. We developed a regional macroinvertebrate index and assessed relative condition of six large river tributaries to the upper Mississippi and Ohio rivers, Midwest USA. In 2004 and 2005, benthic macroinvertebrates, water chemistry, and habitat data were collected from randomly selected sites on each of the St. Croix, Wisconsin, Minnesota, Scioto, Wabash, and Illinois rivers. We first identified the human disturbance gradient using principal components analysis (PCA) of abiotic data. From the PCA, least disturbed sites showed strong separation from stressed sites along a gradient contrasting high water clarity, canopy cover, habitat scores, and plant-based substrates at one end and higher conductivity and nutrient concentrations at the other. Evaluation of 97 benthic metrics identified those with good range, responsiveness, and relative scope of impairment, as well as redundancies with other metrics. The final index was composed of Diptera taxa richness, EPT taxa richness, Coleoptera taxa richness, percent oligochaete and leech taxa, percent collector-filterer individuals, predator taxa richness, percent burrower taxa, tolerant taxa richness, and percent facultative individuals. Each of the selected metrics was scored using upper and lower thresholds based on all sites, and averaging across the nine metric scores, we obtained the Non-wadeable Macroinvertebrate Assemblage Condition Index (NMACI). The NMACI showed a strong response to disturbance using a validation data set and was highly correlated with non-metric multidimensional scaling (NMDS) ordination axes of benthic taxa. The cumulative distribution function of index scores for each river showed qualitative differences in condition among rivers. NMACI scores were highest for the federally protected St. Croix River and lowest for the Illinois River. Other rivers were intermediate and generally reflected the mixture of land use types within individual basins. Use of regional reference sites, though setting a high level of expectation, provides a valuable frame of reference for the potential of large river benthic communities that will aid management and restoration efforts.     

Using stressor gradients to determine reference expectations for great river fish assemblages

We describe a simple empirical modeling approach for determining least-disturbed conditions for the great rivers of the Upper Mississippi River basin: Missouri, Upper Mississippi, and Ohio Rivers. We used multivariate analysis to identify reference strata (reaches for which a single reference expectation was appropriate) on each river. Strata included the Upper Missouri, Lower Missouri, impounded Upper Mississippi, unimpounded Upper Mississippi, and the Ohio River. We created a multimetric stressor gradient for each stratum using a suite of site- and landscape-scale metrics. Site-scale metrics included water chemistry, aquatic and riparian habitat, and human disturbance metrics. Landscape-scale metrics included land use, land cover, and proximity to human disturbance. The gradient was scaled from 0 (least stressed) to 1 (most stressed). Multimetric indices of condition based on fish assemblages for the Lower Missouri and Upper Mississippi River were responsive to stressor gradients based on 18–24 abiotic stressor metrics. Ohio River fish assemblages were responsive to a hand-picked three-metric gradient. We used the y-intercept of quantile regression to predict the fish index value for a stressor gradient value of 0 (the fish index value at a site with the lowest mean stressor gradient score in the reference stratum) which we designated as least-disturbed condition for the fish index for that stratum. We trisected the difference between predicted least-disturbed condition (ceiling value) and a floor value set at the 5th percentile of the sample to create thresholds for three condition classes: least-disturbed, intermediate, and most-disturbed. Based on the derived condition class thresholds for the fish index, 10% (by length) of the Lower Missouri was in least-disturbed condition, compared to 14% of the Ohio River and 19% of the impounded Upper Mississippi River. The index of condition exhibited longitudinal variation that was associated with the location of major urban areas along each river. We conclude that empirical modeling based on an abiotic stressor gradient can provide an alternative approach for deriving internal reference expectations for great rivers with few, if any, minimally disturbed sites.     

Structure of fish assemblages along spatial gradients in a deep subtropical reservoir (Itaipu Reservoir,Brazil-Paraguay border)

Synopsis We assessed the structure of the fish assemblages in the Itaipu Reservoir (Paraná River, Brazil-Paraguay) along a longitudinal or river-dam gradient (composed of riverine, transitional and lacustrine zones) and transversal or upstream–downstream gradients of the tributaries (composed of lotic and lentic stretches of tributaries and reservoir shores). We sampled stations distributed along the environmental gradients quarterly during two years. We caught mostly piscivorous, detritivorous and insectivorous fishes. The structure of the whole fish assemblage both at the regional spatial scale and at the zones of each gradient was significantly non-random according to null models and ordination analysis. When the assemblage was broken into groups, the piscivores and detritivores as well as the predators and prey showed significant non-random structure whereas the insectivores, omnivores, herbivores, benthophages and mud-eaters showed random structure. The first groups are those that had more species and the last mentioned are those with fewer species. The tributaries varied more in composition among themselves than the reservoir shores, showing a high heterogeneity in the transversal gradient. The most widespread species was the introduced piscivore Plagioscion squamosissimus. The loricariids were most abundant in the lotic and lentic stretches of the tributaries. We also observed temporal variability in species composition, mainly in the reservoir shores possibly due to the influence of the upstream floodplain on the migratory fish that periodically use the reservoir as feeding habitat. In the tributaries, the temporal changes were less predictable. A combination of habitat preferences and interactions among species, especially predator–prey relations, might be responsible for the observed structure patterns.     

Biotests with phytoplankton assemblages. Growth limitation along temporal and spatial gradients

The development of eight different species (populations) along temporal and vertical gradients in several lakes was studied. Many populations had an exponential growth phase and a decline phase. The growth rate was often high during the exponential phase. Some species, e.g. Oscillatoria spp. and Synedra cf. acus, often also had a long stationary phase. The growth rate and the sinking rate of these populations were often very low. Laboratory batch experiments with dilute phytoplankton populations were carried out to estimate the degree of growth limitation (L) for different populations sample from different lakes during the three growth phases. L was always low and often zero for populations initially in the exponential phase and always high for populations initially in the decline phase. The biotests also gave results that can help to explain the vertical distribution of Oscillatoria or Asterionella in three lakes investigated. The results indicate that the growth rates and the development of the populations were dependent on the external chemical and physical conditions. The transition between the different growth phases seemed often to be dependent on the external nutrient conditions. P, N, Si and Fe were probably the most growth-limiting nutrients. The growth rate of some diatoms was probably limited directly or indirectly at high pH. Laboratory biotests with natural populations may give valuable information on the growth-properties of different populations in the lakes. The biotests should, however, be carried out in combination with chemical and physical measurements and quantitative determinations of population densities.     

Shifts in taxonomical and guild composition of littoral diatom assemblages along environmental gradients

Diatoms are commonly and frequently used as water quality indicators, but only a few studies have been done to evaluate the importance of littoral, contemporary diatoms as bioindicators. This study aims to determine the main predictors of diatom community composition from 73 Swedish lakes. Canonical correspondence analysis (CCA) revealed pH, phosphate, nitrite/nitrate levels, longitude and percentage of forest in the catchment to be the most important factors of 51 environmental variables for structuring diatom assemblages. Cluster analysis separated the lakes into three groups based on the diatom community composition. Lakes belonging to these groups were characterised as: (1) acidic, nutrient-poor; (2) circumneutral, nutrient-poor and (3) alkaline, nutrient-rich, according to the results of a discriminant function analysis and dominant diatom taxa revealed by similarity percentage analysis. Ecological guilds according to growth morphology and the ability of nitrogen-fixation were assigned to all diatom taxa. All three lake groups exhibited a distinct guild composition. Nitrogen-fixing diatoms were found in nutrient-rich lakes, only. Our results indicate that taxonomical composition of littoral diatom assemblages can be applied in the assessment of nutrient and acidity status of Swedish lakes. Differences in distribution of the ecological guilds were connected to several environmental factors such as nutrients, light and grazing; their application in assessment of trophic status of lakes is therefore precarious.     

Spatial variability in macroinvertebrate assemblages along and among neighbouring equatorial glacier‐fed streams

1. During the past two decades, understanding of the structure and function of glacier‐fed stream ecosystems at temperate latitudes has increased substantially. In contrast, information on their tropical counterparts is very limited. We studied three neighbouring glacier‐fed streams in the tropical Andes of Ecuador. Our main goals were (i) to determine overall longitudinal patterns in density, taxon richness and the composition of macroinvertebrate assemblages and driving factors in equatorial glacial streams and (ii) to examine variability among replicate streams in faunal metrics and assemblages, and stream‐specific effects of supposed environmental key factors. 2. We measured four geographical and 17 environmental factors and collected five Surber samples (500 cm²) of macroinvertebrates at each of nine sites, three sites along three streams. The streams were located 1–5 km apart. In each stream, the three sites were placed at comparable distances from the glacier and were grouped as ‘upper’ (50–200 m), ‘middle’ (1.5 km) and ‘lower’ sites (3.5–5.6 km). 3. In total, 2200 individuals (64% chironomids) were collected and 47 taxa (30 dipterans, 18 of these Chironomidae) identified. Density ranged from 176 to 372 ind. m^?2, and the number of taxa ranged from 2 to 6 at the upper sites and 868–3044 ind. m^?2 and 21–27 taxa at the lower sites. Density, number of taxa, rarefied richness and axis‐1 coordinates from a MDS ordination increased logarithmically with distance from the glacier. These faunal metrics were equally related to altitude and glacier per cent of catchment and correlated with maximum conductivity, mean temperature, mean daily maximum temperature and a channel stability index. As expected, the mean difference in distance decay in similarity was higher at the upper (47% km^?1) than at the lower reaches (20% km^?1) of the streams. 4. The number of taxa varied among sites within the upper and middle groups, but not among the lower sites. In contrast, but in accordance with our expectation, assemblage composition did not differ among upper sites but did so at middle and lower sites, following a supposed decrease in environmental harshness along the streams. Relationships between faunal metrics and the four environmental variables mean temperature, the stability index, chlorophyll a and coarse particulate organic matter also varied among the three streams. Generalised linear model analyses revealed that temperature interacted with stream on macroinvertebrate density, while chlorophyll a had a significant effect on the number of taxa in interaction with stream and stability. 5. The basic predictions of the Milner et al. (2001a) , model regarding longitudinal faunal patterns and temperature and stability as main driving factors were met by our three replicate equatorial glacial streams. Qualitative departures from the model were mainly because of zoogeographical differences. We demonstrated that variability in assemblages between comparable sites in closely situated streams was considerable, and the effect of key environmental factors varied among streams and interacted with other factors. Quantifying spatial variation in benthic assemblages may help us foresee possible consequences for biodiversity as a result of glacial retreat.     

Historical processes enhance patterns of diversity along latitudinal gradients

One of the more vexing issues in ecology is how historical processes affect contemporary patterns of biodiversity. Accordingly, few models have been presented. Two corollary models (centre of origin, time-for-speciation) can be used to make quantitative predictions characterizing the tropical niche conservatism hypothesis and describe diversification as diffusion and subsequent cladogenesis of species away from the place of origin of a higher taxon in the tropics. Predictions derived from such models are: (i) species richness declines toward the periphery of the range of a higher taxon; (ii) taxa are more derived toward the periphery than the centre; (iii) ages of taxa are lower toward the periphery than the centre; and (iv) ages and measures of derivedness are less variable toward the periphery of the range of a higher taxon. I tested these predictions to better understand the formation of one of the most ubiquitous patterns of biodiversity-the latitudinal gradient in species richness. Results indicate well-supported predictions for New World leaf-nosed bats and that diversification has had strong influences on latitudinal gradients of species richness. A better understanding of how evolutionary diversification of taxa contributes to formation of patterns of species richness along environmental gradients is necessary to fully understand spatial variation in biodiversity.     

Competition and evolution along environmental gradients: patterns, boundaries and sympatric divergence

The present study examined how competitive interactions and environmental conditions generate species boundaries and determine species distributions. A spatially explicit, quantitative genetic, two-species competition model was used to manipulate the strengths of competition, gene flow and local adaptation along environmental gradients. This allowed us to assess the long-term persistence of each species and whether the ranges they inhabited had boundaries in space or were unlimited. We found that a species boundary arises along less steep environmental gradients when the strength of stabilizing selection and diversifying selection are similar. We also found that a species boundary may arise along shallow environmental gradients if interspecific competition is more intense than intraspecific, which relaxes previous requirements for steep gradients for generating range limits. We determined an analytical form for the critical environmental gradient as a function of ecological and genetic parameters at which a species boundary is expected to arise by competition. Results suggest an alternative to resource competition as an explanation for phenotypic divergence between sympatric competitors. Competitors sharing a trait that is under stabilizing selection along an environmental gradient may segregate spatially and evolve in different regions, with phenotypic sympatric divergence reflecting the resulting clines. Along various types of environmental gradients, variation in stabilizing selection intensities could lead to contrasting patterns in the distribution of species. For stabilizing selection strengths in accord with field data estimates, this study predicts that the level of sympatric character divergence would be limited along environmental gradients.