Best Predictor Of Species Richness Research Articles

Effects of Environmental Changes on Gerromorpha (Heteroptera: Hemiptera) Communities from Amazonian Streams

The presence of environmental conditions that are suitable for the development of agricultural and cattle-raising activities has favored the expansion of land use in the Brazilian Amazon. This expansion changes the environmental conditions of aquatic ecosystems by altering the structure of the streambeds and the physicochemical conditions of the water. Therefore, this manuscript aimed to assess the effect of changes in environmental conditions along a gradient of different land uses on the richness and species composition of Gerromorpha (Heteroptera: Hemiptera) communities in streams from the Brazilian Amazon. We sampled Gerromorpha communities and environmental variables (physical structure of the habitat and physicochemical characteristics of the water) in 45 streams in the municipality of Paragominas in the state of Pará, Brazil. We then performed variation partitioning analyses to assess the relative contribution of habitat structure and water physicochemical variables on the richness and species composition. The Habitat Integrity Index (HII), which measures the habitat physical structure, was the best predictor of species richness. The HII and physicochemical variables were jointly the best predictors of species composition. Our results show that Gerromorpha communities respond to gradients of environmental anthropization in the Amazon region, which reinforces the importance of habitat integrity and water quality for the maintenance of Gerromorpha communities.

Biodiversity hotspot revisited: reptile and amphibian assemblages of the Uluguru Mountain Forest Reserves, south-eastern Tanzania

ABSTRACT Although the knowledge about the Tanzanian reptiles and amphibians has drastically increased in recent years, the knowledge on the amphibian and reptile fauna of the remaining forest reserves of the Uluguru Mountains (UMs) is only partially known. Amphibians and reptiles were studied in 17 forest reserves and sampled using diurnal and nocturnal time-constrained visual encounter surveys, acoustic surveys and opportunistic searches. A total of 51 quadrats of 25 × 25 m were randomly established on the forest floor, totalling 459 person-hours of sampling effort. Overall, 54 herpetofauna species (27 amphibian and 27 reptile species) were recorded from the area, including two anurans and seven reptile species, which are new records for the region. The estimated Chao1 species richness for the study region was 51 for amphibians and 76 for reptiles. Amphibian and reptile species composition did not differ significantly across the forest reserves, but similarities in species composition of different levels were evident across the reserves. For both communities, α-diversity and relative abundance did not vary significantly across the reserves. Based on multiple regression analyses, leaf-litter anuran species richness was positively influenced by percentage canopy cover, whereas abundance was negatively influenced by total fragment size. Partial correlations showed that variation in leaf-litter anuran abundance was explained by relative humidity, percentage canopy cover and soil moisture, while controlling for altitude. Percentage canopy cover was also the best predictor of species richness of leaf-litter anurans. Overall, these findings highlight the importance of protecting the UMs forest reserves for herpetofauna conservation.

Life‐form diversity across temperate deciduous forests of Western Eurasia: A different story in the understory

AbstractAimTo analyse the biogeographic patterns of Temperate Deciduous Forests (TDFs) in Western Eurasia based on different life‐forms and forests layers and explore their relationships with the current climate, Last Glacial Maximum (LGM) climate and topography.LocationWestern Eurasia.TaxonVascular plants.MethodsWe delimited nine regions encompassing the variability of TDFs in Western Eurasia and collected 1000 vegetation plots from each. We deconstructed the plant communities into three layers, tree, shrub and floor. We used (i) generalized linear mixed models (GLMM) to analyse the influence of current climate, historical climate and topography on species richness by accounting for regional effects and (ii) redundancy analysis (RDA) with variance partitioning to describe the variation in life forms along abiotic gradients. The three forest layers were analysed jointly and separately.ResultsThe Balkans, Alps and Carpathians appeared to be the richest in plant species, whereas the British Isles and the Hyrcanian region were the poorest. Annual temperature range and annual mean temperature were the best predictors of species richness for the whole dataset and for the shrub layer. The tree layer richness was mainly explained by the annual temperature range and by elevation, whereas the forest floor richness was more related to the annual temperature range and the annual mean temperature differences between the LGM and current climate. The current climate was the main predictor of the composition of the whole community, the tree layer and the floor layer, while the shrub layer was also influenced by historical climate.Main conclusionsOur overview of the diversity of temperate deciduous forests in Western Eurasia demonstrates different patterns and drivers across life‐forms and forest layers. While the diversity of trees is mainly linked to current climatic conditions, the shrub layer is also driven by postglacial‐glacial climatic stability, suggesting a different origin from forest trees.

Wooded areas promote species richness in urban parks

Urban green spaces can have an important role in biodiversity conservation. However, they are not often a focus of biodiversity studies, although their global area is raising. We investigated the impact of habitat characteristics of urban green spaces on an ecologically important group of insects—ants (Hymenoptera: Formicidae). We tested three questions: 1) does ant species richness positively correlate with park size and the extent of wooded areas? 2) Is ground temperature the best predictor of ant activity? 3) Will communities be dominated by thermophilic, dry-tolerant, generalist species? Using pitfall traps, hand collecting, and baiting we sampled 7595 ants belonging to 30 species, across eight localities in urban protected areas of the city of Zagreb, Croatia. Parks with larger wooded areas had high species richness, but park size was not a good predictor of species richness. Ant activity was largely influenced by temperature. Five out of eight localities had similar ant community composition. The remaining three localities each had unique ant communities with no overlap with any other locality. Overall, in addition to typical urban species, we recorded species of conservation interest. Thus, small urban green spaces have the capacity to support and conserve diverse ant communities, but their habitat characteristics need to be considered in urban planning.

Bottom-up and top-down processes influence contemporary patterns of mammal species richness in Australia's monsoonal tropics

In recent decades severe mammal declines have occurred in the vast and uncleared tropical savannas of northern Australia. Mounting evidence suggests that feral cats (Felis catus), large feral herbivores and increased frequency of high-severity fires, are all contributing to declines; however, the respective influence of each threat remains unclear. There is an urgent need to quantify the relative impacts of both ‘bottom-up’ (i.e. the depletion of resources and habitat simplification from contemporary disturbance regimes) and ‘top-down’ (i.e. increased predation pressure) factors on small mammal populations to inform where, and how, remedial conservation efforts should be targeted in northern Australia. We conducted an extensive survey of mammals across ca. 370,000 km2 of monsoonal northern Australia using both camera-trapping and live-trapping methods. From multispecies occupancy models, we found that feral herbivore abundance, dingo abundance and feral cat occupancy were the best predictors of species richness of small (<5500 g) mammals, with species richness declining as the respective pressures increased. Our results highlight that underlying productivity has seemingly enhanced the capacity of native mammals to withstand pressures, as species that have severely declined are now more likely to occur in areas with greater productivity. We suggest that a disturbance-driven reduction in habitat complexity and productivity has significantly disrupted bottom-up processes in northern Australia, subsequently increasing top-down pressure from predation, causing severe declines across much of the native mammal assemblage. Without maintaining, enhancing or recovering habitat condition in this region, long-term conservation and recovery of mammal diversity is unlikely.

Amphibian community structure along elevation gradients in eastern Nepal Himalaya

BackgroundSpecies richness and composition pattern of amphibians along elevation gradients in eastern Nepal Himalaya are rarely investigated. This is a first ever study in the Himalayan elevation gradient, the world’s highest mountain range and are highly sensitive to the effects of recent global changes. The aim of the present study was to assess amphibian community structure along elevation gradients and identify the potential drivers that regulate community structures. Amphibian assemblages were sampled within 3 months in both 2014 and 2015 (from May to July) using nocturnal time constrained and acoustic aids visual encounter surveys. In total, 79 transects between 78 and 4200 m asl were sampled within 2 years field work. A combination of polynomial regression, generalized linear models, hierarchical partitioning and canonical correspondence analysis were used to determine the effects of elevation and environmental variables on species richness, abundance, and composition of amphibian communities.ResultsSpecies richness and abundance declined linearly with increasing elevation, which did not support the Mid-Domain Model. Among all the environmental variables, elevation, surface area and humidity were the best predictors of species richness, abundance and composition of amphibians. The majority of amphibian species had narrow elevation ranges. There was no significant correlation between species range size and elevation gradients. However, body size significantly increased along elevation gradients, indicating that Bergmann’s rule is valid for amphibians in eastern Nepal Himalaya.ConclusionsThis study indicates that eastern Nepal Himalaya is a hotspot in amphibian diversity, and it should be served as a baseline for management and conservation activities.

Dissecting bird diversity in the Pantepui area of endemism, northern South America

The Pantepui region harbors one of the most distinctive endemic montane bird faunas in the Neotropics, and is located across the tepuis of northern South America. We made an extensive literature review to understand large-scale distribution patterns of birds in the Pantepui. The core avifauna of the tepuis is composed of 138 bird species, most forest-dwelling insectivores, of which 43 are endemic. In addition, 167 subspecies of birds are endemic to these mountains. The number of endemic species peaks between 1251 and 2000 m, whereas the diversity of non-endemic species is highest between 751 and 1000 m. A high proportion (28%) of endemic taxa (species plus subspecies) is restricted to a single mountain. Tepuis are distributed along a longitudinal gradient, and bird species compositions of mountains located in the eastern and western portion of this gradient are clearly differentiated. Maximum altitude of the mountains and their distance from the Andes are the best predictors of species richness and composition across the Pantepui. Also, bird species composition at site level exhibits significant nested patterns which are affected by maximum altitude, isolation and geographic position of the mountains. The unique bird assemblage of the Pantepui area of endemism is structured through a combination of the spatial distribution of the mountains, habitat diversity, and complex historical events.

Energy determines broad pattern of plant distribution in Western Himalaya

Several factors describe the broad pattern of diversity in plant species distribution. We explore these determinants of species richness in Western Himalayas using high‐resolution species data available for the area to energy, water, physiography and anthropogenic disturbance. The floral data involves 1279 species from 1178 spatial locations and 738 sample plots of a national database. We evaluated their correlation with 8‐environmental variables, selected on the basis of correlation coefficients and principal component loadings, using both linear (structural equation model) and nonlinear (generalised additive model) techniques. There were 645 genera and 176 families including 815 herbs, 213 shrubs, 190 trees, and 61 lianas. The nonlinear model explained the maximum deviance of 67.4% and showed the dominant contribution of climate on species richness with a 59% share. Energy variables (potential evapotranspiration and temperature seasonality) explained the deviance better than did water variables (aridity index and precipitation of the driest quarter). Temperature seasonality had the maximum impact on the species richness. The structural equation model confirmed the results of the nonlinear model but less efficiently. The mutual influences of the climatic variables were found to affect the predictions of the model significantly. To our knowledge, the 67.4% deviance found in the species richness pattern is one of the highest values reported in mountain studies. Broadly, climate described by water–energy dynamics provides the best explanation for the species richness pattern. Both modeling approaches supported the same conclusion that energy is the best predictor of species richness. The dry and cold conditions of the region account for the dominant contribution of energy on species richness.

Non-random lizard extinctions in land-bridge Amazonian forest islands after 28 years of isolation

Major hydropower infrastructure has become a leading driver of biodiversity loss in the lowland tropics. Terrestrial species typically become stranded in post-isolation land-bridge islands within hydroelectric reservoirs. Understanding the resulting extinction dynamics of insular communities is critical to inform, if not to avert, the ongoing blitzkrieg of dam development. Here we assess the effects of forest patch and landscape metrics on diurnal lizard species richness and composition within the Balbina Hydroelectric Dam and surrounding areas in the Central Brazilian Amazon. This 28-yr-old dam created a reservoir of ~4438km2, comprising 3546 islands. We sampled 25 of these islands (0.83–1466ha) and five mainland continuous forest sites, one of which placed along stream banks. We further related morpho-ecological traits and the geographic distribution of lizard species to the spatial metrics of islands where they occurred. Using 100L-pitfall traps operated over 5447 trap-days, we recorded 1123 lizards from 17 taxa, two of which exclusively found along stream banks within continuous forest. Island area was the best predictor of species richness and composition. Small islands (≤2ha) harboured fewer than a third of all species typically observed in larger islands and continuous forest (≥8 species), and only islands ≥100ha retained nearly complete lizard faunas. Lizard assemblages inhabiting small, isolated islands consisted almost exclusively of an oversimplified set of widely distributed, large-bodied, habitat generalist, heliophile species associated with open areas and forest edges, and that feed on a wide spectrum of prey sizes. These wholesale changes in lizard community structure were characterized by severe losses in functional traits, and may profoundly affect ecosystem functioning.

Fish composition and species richness in eastern South American coastal lagoons: additional support for the freshwater ecoregions of the world.

The relationships between fish composition, connectivity and morphometry of 103 lagoons in nine freshwater ecoregions (FEOW) between 2·83° S and 37·64° S were evaluated in order to detect possible congruence between the gradient of species richness and similarities of assemblage composition. Most lagoons included in the study were <2 km(2) , with a maximum of 3975 km(2) in surface area. Combined surface area of all lagoons included in the study was 5411 km(2) . Number of species varied locally from one to 76. A multiple regression revealed that latitude, attributes of morphometry and connectivity, and sampling effort explained a large amount of variability in species richness. Lagoon area was a good predictor of species richness except in low latitude ecoregions, where lagoons are typically small-sized and not affected by marine immigrants, and where non-native fish species accounted for a significant portion of species richness. Relationships between species and area in small-sized lagoons (<2 km(2) ) is highly similar to the expected number in each ecoregion, with systems located between 18·27° S and 30·15° S attaining higher levels of species richness. Similarities in species composition within the primary, secondary and peripheral or marine divisions revealed strong continental biogeographic patterns only for species less tolerant or intolerant to salinity. Further support for the FEOW scheme in the eastern border of South America is therefore provided, and now includes ecotonal systems inhabited simultaneously by freshwater and marine species of fishes.

Environmental drivers of aquatic macrophyte assemblages in ponds along an altitudinal gradient

The aim of this study was to explore the environmental drivers of the aquatic macrophyte assemblage in a large, heterogeneous Spanish region covering a wide altitudinal range. We hypothesized that physicochemical variables affecting assemblages would differ depending on altitude. The study was conducted in 46 plateau ponds and 21 mountain ponds. Our results revealed a shift in hydrophyte assemblage composition and structure along an altitude and water chemistry gradient. However, altitude was not a good predictor of species richness. Conductivity and nutrient concentrations were higher in plateau ponds than in mountain ponds and binary logistic regression showed that conductivity was the best variable for differentiating between both pond types. Canonical correspondence analysis indicated that conductivity was the main factor responsible for the species distribution in both pond types. Generalized linear models showed that in plateau ponds, total phosphorus and mean depth were the strongest predictors of submerged macrophyte coverage, and no model could be created for richness. In the mountain ponds, conductivity and pond area explained coverage of submerged plants, while richness was related to pond area. Our results corroborated the hypothesis to be tested, and the conclusions obtained may be of relevance for making decisions on conservation and restoration.

Scale-specific responses of saproxylic beetles: combining dead wood surveys with data from satellite imagery

We investigate relevant scales and cost-efficient methods for measurements of habitat for saproxylic, aspen-associated beetles, in two boreal forest landscapes in south-eastern Norway. We sampled saproxylic beetles with window traps on fresh aspen dead wood and conducted field surveys of dead wood in the surroundings (1, 2 and 3 km radius). In addition, we used maps derived from satellite imagery to extract data on forest age and volume for the same surroundings. We found that species richness of saproxylic beetles was related to dead wood volume estimated by field survey within 1 or 2 km radius from the trapping points. However, the map-derived variable describing area of forest with high levels of deciduous wood volume was the overall best predictor of species richness. The scale of response to this variable differed; species richness of all saproxylic beetles was best predicted by estimates for the 2 km radius whereas richness of species more strongly associated with aspen was best predicted by estimates for the 3 km radius. This might indicate that aspen-associated beetles disperse over larger distances than many other beetles; possibly an adaptation to the scattered distribution and ephemeral nature of aspen dead wood in boreal forests. Our results show that substrate variables must be measured at the scale that is ecologically relevant for the study organism to elicit a response. We also show that remote sensing data -often easy to obtain for large scales—can be used to complement studies of scale-specific responses and thus improve conservation measures for saproxylic beetle species.

Decreased competitive interactions drive a reverse species richness latitudinal gradient in subarctic forests.

The tendency for species richness to decrease toward the poles is one of the best-characterized patterns in biogeography. The mechanisms behind this pattern have received much attention, yet very few studies have investigated very high-latitude communities. Here, using data from 134 permanent sample plots from 60 degrees to 68 degrees N, we show that boreal forest plant communities in northwestern Canada increase in richness toward the poles, despite a strong increase in climatic harshness. We hypothesized three possible explanations for this pattern: (1) historical biogeography, (2) reduced competition for light at high latitudes (biotic interactions), and (3) changes in soil characteristics with latitude. We used multidimensional scaling to investigate the community composition at each site and found no clustering of communities by latitude, suggesting that historical biogeography was not constraining site diversity. We then investigated the mechanisms behind this gradient using both abiotic (climate and soil) and biotic (tree stand characteristics) variables in a multiple factor analysis. We found that the best predictor of species richness is an environmental gradient that describes an inverse relationship between temperature and tree-stand density, suggesting that reduced competition for light due to reduced tree growth at low temperatures at higher latitudes allows greater species richness. This study shows that low energy availability and climatic harshness may not be limiting species richness toward the poles, rather, abiotic effects act instead on the strength of biotic interactions.

Integrating life history traits and forest structure to evaluate the vulnerability of rainforest birds along gradients of deforestation and fragmentation in eastern Australia

The management of rainforest ecosystems for multi-species conservation must assess the impacts of habitat degradation at different spatial scales as well as determine which species are vulnerable to landscape change. Our research objectives were to (1) evaluate the effects of life history traits on the patch occupancy and vulnerability of rainforest birds, (2) determine the relative effects of stand, landscape and patch structure on species richness, and (3) evaluate the relative contributions of deforestation and fragmentation to species richness. We collected presence–absence data for 29 bird species in 46 rainforest patches in South East Queensland, Australia. We used a multi-species occupancy model that accounted for incomplete detection to evaluate hypotheses for occupancy and species richness. Avian occupancy was strongly influenced by life history traits, including population density, foraging behavior, dispersal strategy, clutch size, body mass and diet. Sedentary species with low population sizes, low clutch sizes, large body sizes and insectivorous diets, such as the ground-dwelling Albert’s Lyrebird (Menura alberti) and canopy-dwelling Paradise Riflebird (Ptiloris paradiseus), were the most vulnerable to landscape change. After accounting for life history, the positive effect of stand basal area was the best predictor of species richness followed by the positive effects of rainforest and dry eucalypt forest in the landscape, with less support for the positive effect of patch size and negative effect of patch isolation. Deforestation was more detrimental to the bird community than rainforest fragmentation. Conservation actions that retain high stand basal area, promote unmodified landscape mosaics and maintain moderate patch sizes near large rainforest tracts are expected to be effective strategies for managing the rainforest bird community. Landscape conservation strategies to minimize habitat loss are expected to be more effective than managing the configuration of rainforest patches.

Lake connectivity and fish species richness in southern Brazilian coastal lakes

Connectivity is a key factor in metacom- munity ecology, because it influences dispersal and colonization rates. However, it has received less attention in aquatic than in terrestrial ecology research. We investigated whether connectivity is a good predictor of species richness in functional fish communities (freshwater, FS; estuarine, ES and estu- arine-freshwater, EFS) from 31 coastal lakes in southern Brazil. We used a model selection approach, including lake area and distance from the ocean as additional predictors of species richness and two connectivity metrics: primary connectivity (CP) and estuarine connectivity (CE), which measure connec- tivity to neighboring lakes and system-wide connec- tivity, respectively. Both metrics estimate functional connectivity and were calculated on habitat-based cost distances. Connectivity was more important for pre- dicting richness of functional communities than for total richness, particularly CE, which was distinctively related to each functional fish community richness (directly related to ES and EFS, and inversely related to FS; CP was related only to ES). Remarkably, connectivity was more important than area for predicting ES and EFS richness. These results add support to dispersal limitation as an important mech- anism influencing fish communities. We suggest that incorporating environmental filters (habitat type) to quantify connectivity is useful for accessing the patterns of species richness.

Birds and small urban parks: a study in a high plateau city

The goal of the present study was to assess which variables influence the distribution of birds in urban parks. Twenty parks were studied in a city on the Spanish high plateau during the breeding season, and relationships among park characteristics, bird richness, and individual bird presences were established. The park area and tree cover accounted for 73.1% of the variation in bird richness. Although area size was the best predictor of species richness (65% of the variation), some species did not seem to be related to this variable but rather to others, such as vegetation and the degree of park isolation with respect to other areas. Contrary to what is normally expected, the distance from our parks to the river did not influence their bird species richness.

Terrestrial gastropod diversity in an alpine region: disentangling effects of elevation, area, geometric constraints, habitat type and land‐use intensity

Elevational gradients have proven to be useful to examine key factors shaping species diversity patterns. This study examines the effects of elevation, area, geometric constraints, habitat type, environmental factors and land‐use intensity on terrestrial gastropod diversity patterns in Val Müstair, an alpine region influenced by different types of agricultural land use in the eastern Alps, Switzerland. Gastropods were sampled using a standardized method in 180 sites spanning an elevational range from 1215 to 2770 m and covering 11 different habitat types. A total of 11 102 specimens representing 70 species were recorded. Observed species richness, statistically estimated true richness (Chao) and geographically interpolated observed richness were used as measures of local species richness. The comparison of three alternative models (environmental, geometric constraints and gastropod abundance models) revealed that the environmental model explained most of the variation in all measures of local diversity. The best model combining the predictors of all three models showed that elevation, soil pH and habitat type affected all measures of local species richness. Similar analyses conducted at the level of 150‐m elevational bands showed that elevation was again the best predictor of species richness, while the area of the elevational band did not have any influence. However, in one out of the two measures of band species richness, the best model indicated that geometric constraints may also contribute to the observed pattern. At both spatial scales, all measures of species richness decreased with increasing elevation. An analysis of species‐specific life‐history traits showed that adult shell size of land snails decreased with increasing elevation. Most species with large shells were confined to lower elevations. The results indicate that environmental factors might be most important in shaping the observed patterns.

Southeast Asian primate communities: the effects of ecology and Pleistocene refuges on species richness

We examined historical and ecological factors affecting current primate biodiversity in Southeast Asia. In Africa, Madagascar and South America, but not Southeast Asia, primate species richness is positively associated with average rainfall and distance from the equator (latitude). We predicted that Southeast Asia's non-conformance may be due to the effect of dispersed Pleistocene refuges (locations of constricted tropical forests during glacial maxima which today are at least 305 m in altitude). Based on 45 forested sites (13 on large islands; 32 on the mainland) of at least 100 km(2) to minimize recent human impact, we determined correlations between extant primate species richness and rainfall, latitude and supplementary ecological variables, while controlling for refuges and islands. We found that refuge sites had significantly higher primate species richness than non-refuges (t = -2.76, P < 0.05), and distance from the nearest Pleistocene refuge was negatively correlated with species richness for non-refuge sites (r = -0.51, P < 0.05). There was no difference in species richness between sites on large islands and the mainland (t = -1.4, P = 0.16). The expected positive relationship between rainfall and species richness was not found (r = 0.17, P = 0.28). As predicted, primate species richness was negatively correlated with latitude (r = -0.39, P < 0.05) and positively correlated with mean temperature (r = 0.45, P < 0.05). General linear models indicated that a site's latitude (F1,38 = 6.18, P < 0.05) and Pleistocene refuge classification (F1,42 = 5.96, P < 0.05) were the best predictors of species richness. Both ecological and historical factors contribute to present day primate species richness in Southeast Asia, making its biodiversity less of an outlier than previously believed.

Woody Species Regeneration in Atlantic Forest Restoration Sites Depends on Surrounding Landscape

Both passive and active restoration are limited by diaspore arrival from neighboring sources. Surrounding landscape is thus important for restoration success in fragmented landscapes, where reforestation, for long periods, may be limited to the planted species pool. We sampled woody species regeneration in 17 reforestation sites in Southern Brazil to investigate the effects of distance to seed sources and amount of remaining nearby Atlantic Forest habitat. The abundance and species richness of regenerating plants were explained by multiple regressions performed using plantation age and species richness, distance to the nearest patch, and surrounding habitat. Distance to the nearest forest remnant, through both the matrix in a straight line and riparian vegetation, was the best predictor of species richness and abundance of regenerating plants. Riparian corridors doubled the distance at which forest remnants influenced restoration sites. However, the area of forest remnants in the site neighborhood did not influence regeneration diversity, suggesting that the quality of both the seed source (including the status of seed dispersing fauna) and reforestation environment for the establishment of forest species should be investigated.

Biogeography of pelagic bacterioplankton across an antagonistic temperature–salinity gradient in the Red Sea

The Red Sea is a unique marine ecosystem with contrasting gradients of temperature and salinity along its north-to-south axis. It is an extremely oligotrophic environment that is characterized by perpetual year-round water column stratification, high annual solar irradiation, and negligible riverine and precipitation inputs. In this study, we investigated whether the contemporary environmental conditions shape community assemblages by pyrosequencing 16S rRNA genes of bacteria in surface water samples collected from the northeastern half of this water body. A combined total of 1855 operational taxonomic units (OTUs) were recovered from the 'small-cell' and 'large-cell' fractions. Here, a few major OTUs affiliated with Cyanobacteria and Proteobacteria accounted for ∼93% of all sequences, whereas a tail of 'rare' OTUs represented most of the diversity. OTUs allied to Surface 1a/b SAR11 clades and Prochlorococcus related to the high-light-adapted (HL2) ecotype were the most widespread and predominant sequence types. Interestingly, the frequency of taxa that are typically found in the upper mesopelagic zone was significantly elevated in the northern transects compared with those in the central, presumably as a direct effect of deep convective mixing in the Gulf of Aqaba and water exchange with the northern Red Sea. Although temperature was the best predictor of species richness across all major lineages, both spatial and environmental distances correlated strongly with phylogenetic distances. Our results suggest that the bacterial diversity of the Red Sea is as high as in other tropical seas and provide evidence for fundamental differences in the biogeography of pelagic communities between the northern and central regions.