Limit Species Richness Research Articles

Sandy bottoms have limited species richness but substantially contribute to the regional coastal fish β-diversity: A case study of the Central Mediterranean Sea

Global marine biodiversity loss impairs entire ecosystems and their stability. Robust biodiversity estimates are key to inform policies and management strategies, and need to consider the contribution of diverse habitats, including those for which estimates of biodiversity are scattered or totally absent. This study assessed the fish diversity associated with three main coastal habitats (rocky bottoms, Posidonia oceanica meadows, sandy bottoms), and their role in shaping the overall coastal fish diversity, also in relation to potential environmental and anthropogenic drivers affecting patterns of fish diversity in coastal areas. Using underwater visual census, we sampled 62 sites distributed on the three habitats, for a total of 496 replicates. We assessed the contribution of each habitat to β-diversity, divided into Local Contribution to β-diversity (LCBD), a comparative indicator of the contributions to β-diversity of each habitat, and Species Contribution to β-diversity (SCBD), which measures the relative importance of each species in affecting β-diversity. Finally, we modelled species diversity in relation to potential environmental and anthropogenic drivers. Overall, 72 species were recorded, with the highest species richness observed on rocky bottoms (56 species, 16 unique to this habitat), followed by P. oceanica (38 species, 0 unique) and sandy bottoms (32 species, 14 unique). Sandy bottom assemblages had a significantly higher contribution to LCBD than P. oceanica meadows and rocky bottoms, and two of the five species with the highest contribution to SCBD are exclusively associated with sandy bottoms. Finally, sea surface temperature, sea surface salinity, and habitat were highlighted as significant predictors of species richness. Our findings, aside from highlighting the environmental drivers of coastal fish diversity in the Mediterranean Sea, unravel the potential key role of sandy bottoms in contributing to overall coastal fish diversity and can inform conservation planning.

Taphonomic analysis of microfossil bonebeds from Western Gondwana: a case study from the late Permian Corumbataí Formation (Paraná Basin, Brazil)

ABSTRACT Storm deposits are common in geological records and encompass a variety of fossil assemblages affected by taphonomic processes that can biased fossil preservation. Here, we describe a microfossil bonebed to demonstrate the role played by certain filters in the final composition of storm-generated fossil accumulations. These layers are recurrent in the Paraná Basin and possibly in other localities of Gondwana, outcropping over 1000 km in the Permian units of the basin. Thirty samples were systematically collected from the state of São Paulo and their components were analysed. These samples consisted of clay matrix, coprolites, scales, and teeth of lower actinopterygian (Palaeonisciformes) and Xenacanthiformes fish. The taphonomic signatures demonstrated that the layer formed as a lag deposit in a distal aquatic environment. Two major taphonomic filters were recognised. The first filter selected bioclasts according to size and density, whereas the second filter preserved only the original phosphatic elements. The third and minor filter were related to breakage, which occured during reworking and diagenesis, respectively. All of these processes contributed to the generation of an assemblage with limited composition and species richness. Thus, distal tempestites might not be useful for estimating palaeodiversity, but may still offer some valuable information about intraspecies variation.

Early Cenozoic increases in mammal diversity cannot be explained solely by expansion into larger body sizes

AbstractA prominent hypothesis in the diversification of placental mammals after the Cretaceous–Palaeogene (K/Pg) boundary suggests that the extinction of non‐avian dinosaurs resulted in the ecological release of mammals, which were previously constrained to small body sizes and limited species richness. This ‘dinosaur incumbency hypothesis’ may therefore explain increases in mammalian diversity via expansion into larger body size niches, that were previously occupied by dinosaurs, but does not directly predict increases in other body size classes. To evaluate this, we estimate sampling‐standardized diversity patterns of terrestrial North American fossil mammals within body size classes, during the Cretaceous and Palaeogene. We find strong evidence for post‐extinction diversity increases in all size classes. Increases in the diversity of small‐bodied species (less than 100 g, the common body size class of Cretaceous mammals, and much smaller than the smallest non‐avialan dinosaurs (c. 400 g)) were similar to those of larger species. We propose that small‐bodied mammals had access to greater energetic resources or were able to partition resources more finely after the K/Pg mass extinction. This is likely to be the result of a combination of widespread niche clearing due to the K/Pg mass extinctions, alongside a suite of biotic and abiotic changes that occurred during the Late Cretaceous and across the K/Pg boundary, such as shifting floral composition, and novel key innovations among eutherian mammals.

Predictions and test of multiple climate-species richness hypotheses to explain the spatial distribution of tenebrionid beetles in mountain environments.

Few studies have evaluated how climate is mechanistically related to species richness in mountain environments. We used path analysis to evaluate predictions of several mechanistic hypotheses based on their hypothesized mechanism relating climate with richness of darkling beetles (Coleoptera: Tenebrionidae). We modeled the influence of spatial covariation on climatic variables and tenebrionid richness. Results showed that richness peaks at mid elevations, chiefly influenced by precipitation and temperature, both directly and indirectly through geographic range sizes. The best fitting model explains 84% of the variance of tenebrionid richness. We suggest this pattern is induced by a water-energy balance along the altitudinal gradient. At low elevations, energy availability is high but water deficit may limit species richness; in contrast, at high elevations water availability is high, but energy deficit may limit species richness. These results suggest high susceptibility of the study region to future global climate change.

Is species richness mediated by functional and genetic divergence? A global analysis in birds.

Unravelling why species richness shows such dramatic spatial variation is an ongoing challenge. Common to many theories is that increasing species richness (e.g. with latitude) requires a compensatory trade-off on an axis of species' ecology. Spatial variation in species richness may also affect genetic diversity if large numbers of coexisting, related species result in smaller population sizes.Here, we test whether increasing species richness results in differential occupation of morphospace by the constituent species, or decreases species' genetic diversity. We test for two potential mechanisms of morphological accommodation: denser packing in ecomorphological space, and expansion of the space. We then test whether species differ in their nucleotide diversity depending on allopatry or sympatry with relatives, indicative of potential genetic consequences of coexistence that would reduce genetic diversity in sympatry. We ask these questions in a spatially explicit framework, using a global database of avian functional trait measurements in combination with >120,000 sequences downloaded from GenBank.We find that higher species richness within families is not systematically correlated with either packing in morphological space or overdispersion but, at the Class level, we find a general positive relationship between packing and species richness, but that points sampled in the tropics have comparatively greater packing than temperate ones relative to their species richness. We find limited evidence that geographical co-occurrence with closely related species or tropical distributions decreases nucleotide diversity of nuclear genes; however, this requires further analysis.Our results suggest that avian families can accumulate species regionally with minimal tradeoffs or cost, implying that external biotic factors do not limit species richness. Read the free Plain Language Summary for this article on the Journal blog.

The contribution of forest reserves and managed forests to the diversity of macrofungi of different trophic groups in European mixed coniferous forest ecosystem

We report the first robust estimate of macrofungal assemblages in European mixed coniferous forest to better understand species diversity under two management strategies (managed vs. not managed) and to rate the recovery of forest reserves, transformed from formerly managed stands, to their natural state. Following extensive repeated sampling in three regions of Poland, we found 318 fungal species from different trophic groups. The two forest management strategies showed similar cumulative fungal species richness and values for Shannon, evenness, and Simpson’s dominance indices, suggesting that both management strategies did not generate factors stimulating or limiting species richness and biodiversity. In contrast, site as well as forest management strategy affected the composition of all fungal trophic groups, with site being a stronger influencing factor. Different fungal ecological groups responded differently to environmental drivers, with ectomycorrhizal symbionts and parasites being more tightly linked to tree traits than saprotrophs, which were affected mostly by climate and substrate. Indicator species associated mainly with managed forests comprised predominantly wood-inhabiting fungi. Conservation-relevant species (red-listed in Poland) were noted in both forest reserves and managed forests; however, they predominated in the former. The results indicated a rather slow process of transformation of fungal communities in forest reserves originating from previously managed forests. The major conclusion of this study is that managed forests complement forest reserves in fungal diversity conservation.

Infections and molecular characterization of anisakid nematodes from two species of marine fish northwest Arabian gulf

The present study provides new insight into valuable information on the diverse structure of the anisakid population, discusses the limited species richness, and also discusses the relationship with other closely diversity-related taxa in NCBI databases in the Epinephelus diacanthus and Epinephelus coioides fish. The fishing area consists of various locations in the Arabian Gulf. A total of 69 E.coioides and E. diacanthus were examined, (n= 48) were infected. Larval stages (n=1,119). Isolated larvae were encysted within the mesenteries peritoneum and viscera of fish organs, with a prevalence of 81.25% of infection and 59.459 % in the E. diacanthus and E. coioides respectively. Molecular analysis was carried out on thirty individuals of nematode parasites who have examined the morphology and showed some appearance differences, by amplifying internal transcribed spacers ITS and ITS-1 of nuclear rDNA (rDNA) by PCR using the primer sets NC5/NC2 and SS1/NC13R of DNA products. Evolutionary analyses were conducted in MEGA X. based on the identity percentage in the GenBank database showed that they belong to anisakid nematodes, in particular, they belong to nine distinct taxa within the Hysterothylacium spp. The presence of the same species individuals in one host may be the cause of these genetic variations at the species level, and that's what the current study has recorded. It has been found that there is an overlap in the order of nitrogen bases between the same species, and this occurs through the fertilization process, while the rest is clean or have only a few parasites.

MOLECULAR CHARACTERIZATION OF ANISAKID NEMATODES HYSTEROTHYLACIUM SPECIES FROM JAPANESE THREADFIN BREAM NEMIPTERUS JAPONICUS (BLOCH, 1791) (PERCIFORMES, NEMIPERIDAE) FROM IRAQI MARINE WATER FISH

The present study provides a new insight into valuable information on the diverse structure of the Anisakid population and discusses the limited species richness in the Nemipterus japonicus (Bloch,1791) (Perciformes, Nemiperidae). The fishing area consists of various locations in the Arabian Gulf (29°58 0 33 00 N48°28 0 20 E). A total of 315 marine fish were examined, (n=287) were infected. Larval stages (n= 763) encysted within the mesenteries peritoneum and viscera of fish organs were isolated, with a prevalence of 91.11% of infection and, the intensity was 2.65. Molecular analysis was carried out on thirty individuals who have examined the morphology and showed some appearance differences, by amplifying internal transcribed spacers ITS and ITS-1 of nuclear rDNA (rDNA) by PCR using the primer sets NC5/NC2 and SS1/NC13R of thirteen DNA products. Evolutionary analyses were conducted in MEGA X. based on the identity percentage in the GenBank database showed that they belong to anisakid nematodes, in particular, they belong to eleven distinct taxa within the Hysterothylacium Ward & Magath, 1917 (Rhabditida, Raphidascarididae) and one identified species H. amoyense (Hsü, 1933) Deardorff & Overstreet, 1980. The current study records eleven species that belong to a genus of Hysterothylacium; some of the alignment of sequences polymorphisms reveals new different individuals of larvae species that may be adopted as new species if their adult stage is detected, and N. japonicus fish considered as a new host record. The current study provides some insights on the systematic taxonomy of these parasites, in addition, it supports similar studies that have been published elsewhere.

Species Diversity and Community Assembly of Cladocera in the Sand Ponds of the Ulan Buh Desert, Inner Mongolia of China

In deserts, pond cladocerans suffer harsh conditions like low and erratic rainfall, high evaporation, and highly variable salinity, and they have limited species richness. The limited species can take advantage of ephippia or resting eggs for being dispersed with winds in such habitats. Thus, environmental selection is assumed to play a major role in community assembly, especially at a fine spatial scale. Located in Inner Mongolia, the Ulan Buh desert has plenty of temporary water bodies and a few permanent lakes filled by groundwater. To determine species diversity and the role of environmental selection in community assembly in such a harsh environment, we sampled 37 sand ponds in June 2012. Fourteen species of Cladocera were found in total, including six pelagic species, eight littoral species, and two benthic species. These cladocerans were mainly temperate and cosmopolitan fauna. Our classification and regression tree model showed that conductivity, dissolved oxygen, and pH were the main factors correlated with species richness in the sand ponds. Spatial analysis using a PCNM model demonstrated a broad-scale spatial structure in the cladoceran communities. Conductivity was the most significant environmental variable explaining cladoceran community variation. Two species, Moina cf. brachiata and Ceriodaphnia reticulata occurred commonly, with an overlap at intermediate conductivity. Our results, therefore, support that environmental selection plays a major role in structuring cladoceran communities in deserts.

Ecohydrology of wetland plant communities along an estuarine to tidal river gradient

AbstractAbiotic filters that interact with wetland plant communities along tidal–fluvial gradients are highly dynamic, and understanding their quantitative thresholds and relationships to interspecific competition is important during an era of sea‐level rise and watershed hydrologic change. Yet, landscape‐scale studies of major coastal rivers from the river mouth to the head of tide, such as this study, remain rare. Here, we develop a new predictive framework for estuarine–tidal river research and management using a river‐specific low‐water datum and the wetland inundation indicator SEVg, the growing‐season sum exceedance value of hourly surface‐water depth. The distribution and variability of the wetland species pool (n = 203) on the 234 river kilometer (rkm) lower Columbia River and estuary floodplain are described for the first time. 4,940 quadrats at 50 marshes were surveyed (2005–2016). Throughout the estuarine–tidal river system, SEVg was well suited to describe the wetland inundation regime and its variability based on the combination of longitudinal river position and elevation. SEVg increased significantly landward. Two primary wetland inundation regimes were identified: the seaward‐tidal, usually greatest during the winter months, and landward‐fluvial, greatest during the growing season. Nearest the ocean, salinity is the abiotic factor limiting species richness and non‐native species. Farther upriver, the daily, seasonal, and interannual variability of the wetting and drying cycle encourage disturbance‐tolerant species and non‐natives and limit the number of hydrophytes and total vegetative cover. Hence, the average between‐year similarity of site‐scale areal cover significantly decreased landward. Hierarchical cluster analysis indicated five vegetative groups and five ecohydrologic zones between rkm 0 and 234 were discriminated with 76 significant species–zone associations. All zones had unique indicator species. Species with high indicator values were Carex lyngbyei throughout the estuarine zones, and Eleocharis palustris, Sagittaria latifolia, and the invasive non‐native Phalaris arundinacea in the upper estuarine and lower, middle, and upper tidal river zones (IV > 0.90). Competition from C. lyngbyei nearest the ocean and P. arundinacea in the tidal river was associated with reduced species richness when total cover was >65%. This framework of filters informs the design and prediction of future wetland plant communities on coastal river floodplains.

Endemicity and community composition of marine species along the NW Pacific and the adjacent Arctic Ocean

The NW Pacific lies in the most productive, rich, and diverse region of the World Ocean. The NW Pacific includes several shallow-water oceanic islands and deep-sea basins differing in depths, hydrology and isolation. The adjacent Arctic Ocean includes the northern Bering and southern Chukchi Sea, areas of high productivity characterized by short food chains and shallow depths. Despite the NW Pacifićs high species richness, characterization of species diversity and species composition in this region remains poorly explored and/or unknown. Here we attempt to discover how geographic boundaries and depth shape current community assemblages in this area by limiting species richness and geographic distribution ranges. All geographic distribution records were extracted from Ocean Biogeographic Information System (OBIS) and Global Biodiversity Information Facility (GBIF). Distribution data were then cleaned, resulting in a final dataset including 324,916 distribution records of 17,414 species ranging from 0 to 10,900 m depth. We calculated the rate of endemicity and alpha species richness per ecoregion. We also showed how species composition and community structure varies between the tropical and temperate NW Pacific and adjacent Arctic Ocean when sampling bias is accounted for using rarefaction. After accounting for sampling bias, the Eastern Philippines had the greatest species richness (for both pelagic and benthic species) in the NW Pacific and its adjacent Arctic Ocean, and the lowest species richness was observed in Papua. Despite the high species richness in the Eastern Philippines, the Yellow Sea and Gulf of Tonkin had the highest benthic species endemicity rates (ca. 70%) and Aleutian Islands had the highest pelagic endemicity rate (ca. 45%) among all other ecoregions. In the NW Pacific, Chordata, Arthropoda, and Mollusca contributed more than 50% to the species composition of the community. In the Arctic Ocean, Arthropoda, Annelida, and Mollusca were the dominant taxa comprising ca. 82% of the species community. Two significantly distinguished clusters (North and South clusters) were identified based on species similarity analysis including ecoregions of the (1) polar Arctic Ocean and temperate North NW Pacific, and (2) tropical and sub-tropical NW Pacific. We also calculated marine species richness hotspots (actual and expected ES50) and centers of endemism in the NW Pacific and adjacent Arctic Ocean. These analyses reveal an opportunity for marine conservation planning effort as reporting species richness and endemicity hotspots could be highly effective in pinpointing and preventing biodiversity loss.

Nocturnal noise and habitat homogeneity limit species richness of owls in an urban environment

Habitat loss and fragmentation are listed among the most significant effects of urbanization, which is regarded as an important threat to wildlife. Owls are the top predators in most terrestrial habitats, and their presence is a reliable indicator of ecosystem quality and complexity. However, influence of urbanization on owl communities, anthropogenic noise in particular, has not been investigated so far. The aim of this study was to identify the role of noise and landcover heterogeneity in the species richness of owl assemblage in the urban ecosystem. Owls were surveyed in the city of Kraków (southern Poland) on 65 randomly selected sample plots (1 km2). The area of main landcover types, landcover diversity index, mean size of landcover patch, and nocturnal noise level were defined within the sample plots and correlated with owl species richness. Five owl species were recorded in the study area with forests as the dominant landcover type for Tawny and Ural owls, grasslands for Long-eared and Barn owls, and gardens for Little owls. In total, 52% of sample plots were occupied by at least one species (1–3 species per plot). The number of owl species was positively correlated with landcover diversity index and negatively correlated with nocturnal noise emission. This study demonstrates that species richness of owls in urban areas may be shaped by landcover heterogeneity and limited by noise intensity. This indicates that noise changes top predator assemblage, which in consequence may disturb predator-prey interactions within human-transformed habitats.

Exogenous arbuscular mycorrhizal fungi increase soil organic carbon and change microbial community in poplar rhizosphere

Arbuscular mycorrhizal fungi (AMF) increase soil organic carbon (SOC) deposition via secretion of glomalin-related soil protein (GRSP) and modulation of plant carbon partition. Two exogenous AMF inocula (Rhizophagus irregularis and Glomus versiforme) were applied to the roots of Populus × canadensis seedlings grown in the unsterilized nursery soil. The diversity of fungal and bacterial communities was assessed by the polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) method, while the accumulation of GRSP and SOC content in 22.5 cm-deep soil was measured. The results indicated that two AMF additions increased root colonization frequency as well as poplar biomass, especially root biomass accumulation. Two AMF applications improved the easily extractable-GRSP, total-GRSP, and SOC accumulation in the rhizosphere of poplar seedlings, limited the fungal community, and exerted no influence on the bacterial community. The effect of G. versiforme on GRSP and SOC accumulation was higher than that of R. irregularis. The AMF introduced GRSP, and SOC accumulation was highly correlated the limited fungal species richness.

Assessment of Impacts Caused by Granite Quarries on Vegetation: A Case Study in the Kalutara District

Granite quarrying has become a major business enterprise from which huge economic benefits are gained while neglecting the damage on the environment. In this study we investigated the effect of small and large scale granite quarries (n=2 each) on neighboring individual plants and their communities. We compared foliar anatomy and stomata conductance of Macaranga peltata(MP) and Melastomamalabathricum(MM) sampled (n=6) from the edge and ~700 m away from the quarries. Floristic details of the vegetation were enumerated using three 200 m transects at each site in which three plots (10x10 m2) were set at the edge, 100 m and 200 m away from the quarry. Each plot had three 1x1 m2 sub-plots to determine the herbaceous cover, soil physical properties and seed bank. Ecological dominance of species was determined using the Important Value Index (IVI) and the floral diversity was assessed using standard biodiversity indices. Statistics (t-test) using SPSS (16.0) was applied where appropriate. Wind-blown granite dust was a major problem which affected the foliar anatomy and physiological performance. Stomatal conductance (MP-113.17 & 131.75 mmol.m-2.s-1 and MM- 149.00 & 144.75 mmol.m-2.s-1 in large and small quarries respectively) was lower compared to unaffected leaves (MP-419.67 and MM-432.40 mmol.m-2.s-1). Stomatal size was small in affected leaves (MP-62.87 & 62.81 μm2 and MM-47.87 & 47.05 μm2 in large and small quarries respectively, compared to MP-115.37 and MM-74.27 μm2) but had an increased stomatal density (MP-1502.78, 1419.44 stomata/mm2 and MM-3113.89, 3244.44 stomata/mm2 in large and small quarries respectively, compared to MP-846.67 and MM-875.56 stomata/mm2).IVI indicatedTremaorientalisand Macarangapeltatadominate in the neighbourhood. Diversity indices increased at a distance from the quarry (Shannon Weiner diversity index - 0.601, 0.733, 1.298 in large quarries and 0.655, 1.127, 1.687 in small quarries for edge, 100 m and 200 m respectively) indicating the limited species richness in the immediate neighbourhood. Establishment of invasive alien species Clidemiahirta, Panicum maximum, Dilleniasuffruticosaand Alstoniamacrophyllaat quarry edge also reflected the negative impacts of disturbances to the site. Soil seed banks of weeds exhibited the highest density at the edge and decreased towards the outside. Quarry edge had high soil temperature, low moisture content, low conductivity and acidic pH. Impacts to the vegetation were similar irrespective of the category of granite quarries (small or large) as quarrying activity highly influenced the stomatal functions, and affect establishment of natural plant communities while adding weeds and invasive alien species. Keywords: Quarrying activity, Impacts, Plants, Community

Evidence-Based Decision Making to Underpin the Thresholds in New Zealand's Craft Risk Management Standard: Biofouling on Vessels Arriving to New Zealand

AbstractVessel biofouling is a significant pathway for the introduction of nonindigenous marine species (NIMS). New Zealand is the first nation to regulate the vessel biofouling pathway, with controls scheduled to come into force in May 2018. The Craft Risk Management Standard (CRMS): Biofouling on Vessels Arriving to New Zealand specifies the hull fouling thresholds that vessels must meet; and here, we present the evidence-based decisions that underpin these thresholds.Under the CRMS, a vessel must arrive in New Zealand with a “clean hull,” the thresholds for which are governed by the intended duration of a vessel's stay in New Zealand. For example, long-stay (≥21 days) vessels must meet a more stringent standard of hull cleanliness due to the increased likelihood of release and establishment of NIMS. While setting a clean hull threshold at “slime layer only” can be tractable when vessels operate within the specifications of antifouling coatings, incidental amounts of macrofouling can establish even under the best management practices. Because of such instances, the thresholds within the CRMS were designed to allow for the presence of some macrofouling species, albeit with restrictions to minimize biosecurity risk. These thresholds are intended to limit species richness and to prevent successful reproduction and settlement of the allowed taxonomic groups while considering the practicality and feasibility of implementation.The difficulties of managing biofouling on different areas of the hull are acknowledged within these thresholds. For example, a greater tolerance of macrofouling has been allowed for niche areas due to the difficulties in preventing biofouling on these areas.

Shifts in trait means and variances in North American tree assemblages: species richness patterns are loosely related to the functional space

One of the key hypothesized drivers of gradients in species richness is environmental filtering, where environmental stress limits which species from a larger species pool gain membership in a local community owing to their traits. Whereas most studies focus on small‐scale variation in functional traits along environmental gradient, the effect of large‐scale environmental filtering is less well understood. Furthermore, it has been rarely tested whether the factors that constrain the niche space limit the total number of coexisting species. We assessed the role of environmental filtering in shaping tree assemblages across North America north of Mexico by testing the hypothesis that colder, drier, or seasonal environments (stressful conditions for most plants) constrain tree trait diversity and thereby limit species richness. We assessed geographic patterns in trait filtering and their relationships to species richness pattern using a comprehensive set of tree range maps. We focused on four key plant functional traits reflecting major life history axes (maximum height, specific leaf area, seed mass, and wood density) and four climatic variables (annual mean and seasonality of temperature and precipitation). We tested for significant spatial shifts in trait means and variances using a null model approach. While we found significant shifts in mean species’ trait values at most grid cells, trait variances at most grid cells did not deviate from the null expectation. Measures of environmental harshness (cold, dry, seasonal climates) and lower species richness were weakly associated with a reduction in variance of seed mass and specific leaf area. The pattern in variance of height and wood density was, however, opposite. These findings do not support the hypothesis that more stressful conditions universally limit species and trait diversity in North America. Environmental filtering does, however, structure assemblage composition, by selecting for certain optimum trait values under a given set of conditions.

Reach-Scale Comparison of Habitat and Mollusk Assemblages for Select Sites in the Clinch River with Regional Context

Several hypotheses, including habitat degradation and variation in fluvial geomorphology, have been posed to explain extreme spatial and temporal variation in Clinch River mollusk assemblages. We examined associations between mollusk assemblage metrics (richness, abundance, recruitment) and physical habitat (geomorphology, streambed composition, fish habitat, and riparian condition) at 10 sites selected to represent the range of current assemblage condition in the Clinch River. We compared similar geomorphological units among reaches, employing semi-quantitative and quantitative protocols to characterize mollusk assemblages and a mix of visual assessments and empirical measurements to characterize physical habitat. We found little to no evidence that current assemblage condition was associated with 54 analyzed habitat metrics. When compared to other sites in the Upper Tennessee River Basin (UTRB) that once supported or currently support mollusk assemblages, Clinch River sites were more similar to each other, representing a narrower range of conditions than observed across the larger geographic extent of the UTRB. A post-hoc analysis suggested stream size and average boundary shear stress at bankfull stage may have historically limited species richness in the UTRB (p < 0.001). Associations between mollusk assemblages and physical habitat in the UTRB and Clinch River currently appear obscured by other factors limiting richness, abundance, and recruitment.

Does climate limit species richness by limiting individual species' ranges?

Broad-scale geographical variation in species richness is strongly correlated with climate, yet the mechanisms underlying this correlation are still unclear. We test two broad classes of hypotheses to explain this pattern. Bottom-up hypotheses propose that the environment determines individual species' ranges. Ranges then sum up to yield species richness patterns. Top-down hypotheses propose that the environment limits the number of species that occur in a region, but not which ones. We test these two classes of hypotheses using a natural experiment: seasonal changes in environmental variables and seasonal range shifts of 625 migratory birds in the Americas. We show that richness seasonally tracks the environment. By contrast, individual species' geographical distributions do not. Rather, species occupy different sets of environmental conditions in two seasons. Our results are inconsistent with extant bottom-up hypotheses. Instead, a top-down mechanism appears to constrain the number of species that can occur in a given region.

Ecological limit functions relating fish community response to hydrologic departures of the ecological flow regime in the Tennessee River basin, United States

ABSTRACTEcological limit functions relating streamflow and aquatic ecosystems remain elusive despite decades of research. We investigated functional relationships between species richness and changes in streamflow characteristics at 662 fish sampling sites in the Tennessee River basin. Our approach included the following: (1) a brief summary of relevant literature on functional relations between fish and streamflow, (2) the development of ecological limit functions that describe the strongest discernible relationships between fish species richness and streamflow characteristics, (3) the evaluation of proposed definitions of hydrologic reference conditions, and (4) an investigation of the internal structures of wedge‐shaped distributions underlying ecological limit functions.Twenty‐one ecological limit functions were developed across three ecoregions that relate the species richness of 11 fish groups and departures from hydrologic reference conditions using multivariate and quantile regression methods. Each negatively sloped function is described using up to four streamflow characteristics expressed in terms of cumulative departure from hydrologic reference conditions. Negative slopes indicate increased departure results in decreased species richness.Sites with the highest measured fish species richness generally had near‐reference hydrologic conditions for a given ecoregion. Hydrology did not generally differ between sites with the highest and lowest fish species richness, indicating that other environmental factors likely limit species richness at sites with reference hydrology.Use of ecological limit functions to make decisions regarding proposed hydrologic regime changes, although commonly presented as a management tool, is not as straightforward or informative as often assumed. We contend that statistical evaluation of the internal wedge structure below limit functions may provide a probabilistic understanding of how aquatic ecology is influenced by altered hydrology and may serve as the basis for evaluating the potential effect of proposed hydrologic changes. Published 2013. This article is a U.S. Government work and is in the public domain in the USA. Ecohydrology published by John Wiley &amp; Sons, Ltd.

Biotic interactions limit species richness in an alpine plant community, especially under experimental warming

The determinants of local species richness in plant communities have been the subject of much debate. Is species richness the result of stochastic events such as dispersal processes, or do local environmental filters sort species into communities according to their ecological niches? Recent studies suggest that these two processes simultaneously limit species richness, although their relative importance may vary in space and time. Understanding the limiting factors for species richness is especially important in light of the ongoing global warming, as new species establish in resident plant communities as a result of climate‐driven migration. We examined the relative importance of dispersal and environmental filtering during seedling recruitment and plant establishment in an alpine plant community subjected to seed addition and long‐term experimental warming. Seed addition increased species richness during the seedling recruitment stage, but this initial increase was cancelled out by a corresponding decrease in species richness during plant establishment, suggesting that environmental filters limit local species richness in the long term. While initial recruitment success of the sown species was related to both abiotic and biotic factors, long‐term establishment was controlled mainly by biotic factors, indicating an increase in the relative importance of biotic interactions once plants have germinated in a microhabitat with favourable abiotic conditions. The relative importance of biotic interactions also seemed to increase with experimental warming, suggesting that increased competition within the resident vegetation may decrease community invasibility as the climate warms.