Abundance Gradients Research Articles

Metallicity and α-Element Abundance Gradients along the Sagittarius Stream as Seen by APOGEE

Abstract Using 3D positions and kinematics of stars relative to the Sagittarius (Sgr) orbital plane and angular momentum, we identify 166 Sgr stream members observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) that also have Gaia DR2 astrometry. This sample of 63/103 stars in the Sgr trailing/leading arm is combined with an APOGEE sample of 710 members of the Sgr dwarf spheroidal core (385 of them newly presented here) to establish differences of 0.6 dex in median metallicity and 0.1 dex in [α/Fe] between our Sgr core and dynamically older stream samples. Mild chemical gradients are found internally along each arm, but these steepen when anchored by core stars. With a model of Sgr tidal disruption providing estimated dynamical ages (i.e., stripping times) for each stream star, we find a mean metallicity gradient of 0.12 ± 0.03 dex Gyr−1 for stars stripped from Sgr over time. For the first time, an [α/Fe] gradient is also measured within the stream, at 0.02 ± 0.01 dex Gyr−1 using magnesium abundances and at 0.04 ± 0.01 dex Gyr−1 using silicon, which imply that the Sgr progenitor had significant radial abundance gradients. We discuss the magnitude of those inferred gradients and their implication for the nature of the Sgr progenitor within the context of the current family of Milky Way satellite galaxies, and we suggest that more sophisticated Sgr models are needed to properly interpret the growing chemodynamical detail we have on the Sgr system.

Basinal hydrographic and redox controls on selenium enrichment and isotopic composition in Paleozoic black shales

Mass-dependent variations in selenium stable isotope ratios have recently been developed as a paleo-redox proxy. Since the reduction of selenium oxyanions occurs at a relatively high redox potential, this system holds promise for probing conditions relevant to the evolution and diversification of eukaryotic and animal life, which required substantial dissolved oxygen levels. Although several studies have identified selenium isotopic variability during oxygenation events in Earth’s distant past, we still have only a broad understanding of the mechanisms controlling this isotopic variability. This currently limits the robust interpretation of selenium isotope variability to first-order mechanisms driving large-magnitude changes. Here, we explore selenium isotope variability within and among Paleozoic black shales deposited on the North American craton that have been well-studied using a variety of other paleo-environmental proxies. Using this combined dataset, we attempt to unravel the controls on selenium abundance and isotope ratios in organic-rich ancient marine sedimentary rocks. We find that in the Late Pennsylvanian units, an estuarine nutrient trap on the Midcontinent Shelf enabled vigorous selenium recycling, leading to very high concentrations in sediments and enrichment of heavy isotopes in the aqueous selenium reservoir. In contrast, we find that among the Late Devonian units, differences in local basinal hydrography led to a gradient in selenium abundance and isotopic fractionation, with the more restricted basins depleting their selenium reservoirs and causing enrichment of heavy isotopes in the residual aqueous reservoir. In both of these case studies, the additional context provided by complementary paleo-environmental proxies was critical for distinguishing between possible drivers of selenium isotopic variability. When extending such studies to other paleo-environmental settings, we suggest that the continued use of complementary datasets will enable the most robust use of the selenium paleo-redox proxy. Moreover, further development of techniques for high-precision and phase-specific selenium isotope measurements will greatly improve the ability to deduce subtle redox fluctuations with this proxy.

Elemental Abundance Distributions in the (R, Vϕ) Plane with LAMOST DR5 and Gaia DR2

Abstract Since Gaia DR2 was released, many velocity structures in the disk have been revealed, such as large-scale ridge-like patterns in phase space. Both kinematic information and stellar elemental abundances are needed to reveal their evolution history. We have used labels from the APOGEE survey to predict elemental abundances for a huge amount of low-resolution spectra from the Large Sky Area Multi-Object Fibre Spectroscopic Telescope survey. Deep learning with artificial neural networks can automatically draw on physically sensible features in the spectrum for their predictions. Abundances of 12 individual elements, [C/Fe], [N/Fe], [O/Fe], [Mg/Fe], [Al/Fe], [Si/Fe], [S/Fe], [Cl/Fe], [Ca/Fe], [Ti/Fe], [Mn/Fe], and [Ni/Fe], along with basic stellar labels T eff, log g, metallicity ([M/H] and [Fe/H]), and [α/M] for 1,063,386 stars have been estimated. Then, those stars were cross-matched with Gaia DR2 data to obtain kinematic parameters. We present distributions of chemical abundances in the V ϕ versus R coordinate. Our results extend the chemical characterization of the ridges in the (R, V ϕ ) plane to about R = 13 kpc toward the anticenter direction. In addition, radial elemental abundance gradients for disk stars with abs(z) < 0.5 kpc are investigated, and we fitted a line for median abundance values of bins of stars with galactocentric distance between R > 7.84 kpc and R < 15.84 kpc. The radial metallicity gradients for disk stars are, respectively, −0.0475 ± 0.0015 for R ≥ 13.09 kpc and −0.0173 ± 0.0028 for R < 13.09 kpc. Gradients for other elemental abundances are also obtained for disk stars: the [α/M] gradient is 0.0030 ± 0.0002; the [Al/Fe] gradient is 0.0030 ± 0.0002; and the [Mn/Fe] gradient is −0.0078 ± 0.0005.

Metallicity Structure in the Milky Way Disk Revealed by Galactic H ii Regions

Abstract The metallicity structure of the Milky Way disk stems from the chemodynamical evolutionary history of the Galaxy. We use the National Radio Astronomy Observatory Karl G. Jansky Very Large Array to observe ∼8–10 GHz hydrogen radio recombination line and radio-continuum emission toward 82 Galactic H ii regions. We use these data to derive the electron temperatures and metallicities for these nebulae. Since collisionally excited lines from metals (e.g., oxygen, nitrogen) are the dominant cooling mechanism in H ii regions, the nebular metallicity can be inferred from the electron temperature. Including previous single-dish studies, there are now 167 nebulae with radio-determined electron temperature and either parallax or kinematic distance determinations. The interferometric electron temperatures are systematically 10% larger than those found in previous single-dish studies, likely due to incorrect data analysis strategies, optical depth effects, and/or the observation of different gas by the interferometer. By combining the interferometer and single-dish samples, we find an oxygen abundance gradient across the Milky Way disk with a slope of −0.052 ± 0.004 dex kpc−1. We also find significant azimuthal structure in the metallicity distribution. The slope of the oxygen gradient varies by a factor of ∼2 when Galactocentric azimuths near ∼30° are compared with those near ∼100°. This azimuthal structure is consistent with simulations of Galactic chemodynamical evolution influenced by spiral arms.

Atlas of Finnish Bats

This atlas is based on information in museum collections, literature, databases and unpublished data. In the last 150 years, the number of bat species in Finland increased from six to thirteen. Of these, five are common and regularly breeding (Eptesicus nilssonii, Myotis brandtii, Myotis daubentonii, Myotis mystacinus, Plecotus auritus), and eight rare (Eptesicus serotinus, Myotis dasycneme, Myotis nattereri, Nyctalus noctula, Pipistrellus nathusii, Pipistrellus pipistrellus, Pipistrellus pygmaeus, Vespertilio murinus), of which breeding of two (M. nattereri, P. nathusii) have been confirmed. The total number of records in the study is 11 234, of which 9717 are identified to species. The records are from 940 (25%) 10-km2 squares of Finland's land area. Of the records, 89% are new (1993–2014). Of the recorded bat species, only Eptesicus nilssonii occurs in each of the 21 biogeographical provinces. A decreasing south–north gradient in species richness and abundance exists which may be related to research efforts that are clearly higher in the south.

Probing Structure in Cold Gas at z ≲ 1 with Gravitationally Lensed Quasar Sight Lines

Abstract Absorption spectroscopy of gravitationally lensed quasars (GLQs) enables study of spatial variations in the interstellar and/or circumgalactic medium of foreground galaxies. We report observations of four GLQs, each with two images separated by 0.″8–3.″0, that show strong absorbers at redshifts 0.4 < z abs < 1.3 in their spectra, including some at the lens redshift with impact parameters 1.5–6.9 kpc. We measure H i Lyman lines along two sight lines each in five absorbers (10 sight lines in total) using Hubble Space Telescope Space Telescope Imaging Spectrograph, and metal lines using Magellan Echellette or Sloan Digital Sky Survey. Our data have doubled the lens galaxy sample with measurements of H i column densities (N H i ) and metal abundances along multiple sight lines. Our data, combined with the literature, show no strong correlation between absolute values of differences in N H i , N Fe ii , or [Fe/H] and the sight line separations at the absorber redshifts for separations of 0–8 kpc. The estimated abundance gradients show a tentative anticorrelation with abundances at galaxy centers. Some lens galaxies show inverted gradients, possibly suggesting central dilution by mergers or infall of metal-poor gas. [Fe/H] measurements and masses estimated from GLQ astrometry suggest the lens galaxies lie below the total mass–metallicity relation for early-type galaxies as well as measurements for quasar-galaxy pairs and gravitationally lensed galaxies at comparable redshifts. This difference may arise in part from the dust depletion of Fe. Higher resolution measurements of H and metals (especially undepleted elements) for more GLQ absorbers and accurate lens redshifts are needed to confirm these trends.

Carbon, nitrogen and oxygen abundance gradients in M101 and M31

Abstract We present deep spectrophotometry of 18 H ii regions in the nearby massive spiral galaxies M 101 and M 31. We have obtained direct determinations of electron temperature in all the nebulae. We detect the C ii 4267 Å line in several H ii regions, permitting to derive the radial gradient of C/H in both galaxies. We also determine the radial gradients of O/H, N/O, Ne/O, S/O, Cl/O and Ar/O ratios. As in other spiral galaxies, the C/H gradients are steeper than those of O/H producing negative slopes of the C/O gradient. The scatter of the abundances of O with respect to the gradient fittings do not support the presence of significant chemical inhomogeneities across the discs of the galaxies, especially in the case of M101. We find trends in the S/O, Cl/O and Ar/O ratios as a function of O/H in M101 that can be reduced using Te indicators different from the standard ones for calculating some ionic abundances. The distribution of the N/O ratio with respect to O/H is rather flat in M31, similarly to previous findings for the Milky Way. Using the disc effective radius – Re – as a normalization parameter for comparing gradients, we find that the latest estimates of Re for the Milky Way provide an excess of metallicity in apparent contradiction with the mass-metallicity relation; a value about two times larger might solve the problem. Finally, using different abundance ratios diagrams we find that the enrichment timescales of C and N result to be fairly similar despite their different nucleosynthetic origin.

Elevation gradients of lemur abundance emphasise the importance of Madagascar’s lowland rainforest for the conservation of endemic taxa

Abstract Elevation gradients correlate with changes in several environmental conditions and are known to be related to animal abundance. Animals in regions with a naturally limited extent of lowland rainforest are expected to have evolved adaptations to intermediate elevations that provided a stable environment during their evolution. Since the lowland rainforest of Madagascar has a limited extent and suffers from increasing anthropogenic pressure, it is essential to understand how well species tolerate intermediate and high elevations. In this study, we aim to quantify the relationship between lemur abundance and elevation in the eastern rainforest of Madagascar. We correlated abundance data on 26 lemur species (10 genera), including 492 records from 26 studies, with elevation. We analysed the consistency of correlations across species with a meta‐analytical approach. We controlled for species’ body mass, elevational range and median elevation. We then ran generalised linear mixed models to determine whether lemur abundance was related to elevation, body mass, plant productivity and anthropogenic disturbance. Overall, the abundance of lemur species in Malagasy rainforests was negatively correlated with elevation, and species occupying broader elevational ranges showed stronger correlations. Body mass was not related to species’ tolerance of high elevations. Even though several lemur species are able to occupy the entire elevation gradient, the few remaining patches of lowland rainforests host lemur species at greater abundances than other sites. Abundance across species was negatively related to body mass, elevation and seasonality in plant productivity and positively related to plant productivity. Despite the ecological flexibility of many lemur species, the remnant patches of lowland rainforests host the highest levels of lemur abundance and are key to lemur conservation. It is crucial to preserve this priority habitat both for biodiversity conservation and for our understanding of lemur adaptations.

Single-Stranded DNA Viruses in Antarctic Cryoconite Holes.

Antarctic cryoconite holes, or small melt-holes in the surfaces of glaciers, create habitable oases for isolated microbial communities with tightly linked microbial population structures. Viruses may influence the dynamics of polar microbial communities, but the viromes of the Antarctic cryoconite holes have yet to be characterized. We characterize single-stranded DNA (ssDNA) viruses from three cryoconite holes in the Taylor Valley, Antarctica, using metagenomics. Half of the assembled metagenomes cluster with those in the viral family Microviridae (n = 7), and the rest with unclassified circular replication associated protein (Rep)-encoding single-stranded (CRESS) DNA viruses (n = 7). An additional 18 virus-like circular molecules encoding either a Rep, a capsid protein gene, or other unidentified but viral-like open reading frames were identified. The samples from which the genomes were identified show a strong gradient in microbial diversity and abundances, and the number of viral genomes detected in each sample mirror that gradient. Additionally, one of the CRESS genomes assembled here shares ~90% genome-wide pairwise identity with a virus identified from a freshwater pond on the McMurdo Ice Shelf (Antarctica). Otherwise, the similarity of these viruses to those previously identified is relatively low. Together, these patterns are consistent with the presence of a unique regional virome present in fresh water host populations of the McMurdo Dry Valley region.

Galaxy properties as revealed by MaNGA – I. Constraints on IMF and M*/L gradients in ellipticals

ABSTRACT We estimate ages, metallicities, α-element abundance ratios, and stellar initial mass functions (IMFs) of elliptical (E) and S0 galaxies from the MaNGA-DR15 survey. We stack spectra and use a variety of single stellar population synthesis models to interpret the absorption line strengths in these spectra. We quantify how these properties vary across the population, as well as with galactocentric distance. This paper is the first of a series and is based on a sample of pure elliptical galaxies at z ≤ 0.08. We confirm previous work showing that IMFs in Es with the largest luminosity (Lr) and central velocity dispersion (σ0) appear to be increasingly bottom heavy towards their centres. For these galaxies the stellar mass-to-light ratio decreases at most by a factor of 2 from the central regions to Re. In contrast, for lower Lr and σ0 galaxies, the IMF is shallower and M*/Lr in the central regions is similar to the outskirts, although quantitative estimates depend on assumptions about element abundance gradients. Accounting self-consistently for these gradients when estimating both M* and Mdyn brings the two into good agreement: gradients reduce Mdyn by ∼0.2 dex while only slightly increasing the M* inferred using a Kroupa IMF. This is a different resolution of the M*–Mdyn discrepancy than has been followed in the recent literature where M* of massive galaxies is increased by adopting a Salpeter IMF throughout the galaxy while leaving Mdyn unchanged. A companion paper discusses how stellar population differences are even more pronounced if one separates slow from fast rotators.

Abundance/occupancy/patchiness relations in estuarine seagrass macrobenthos

Using >1800 macrobenthic samples from 25 representative intertidal seagrass sites in the Knysna estuarine bay (warm-temperate South Africa), abundance/occupancy relations were examined in the light of Foggo et al.'s (2003) earlier work concerning British estuarine faunas. In addition, the relationship of degree of variation in macrobenthic abundance across unit area, 'patchiness', to this macroecological picture was investigated. Across the group of 30 dominant faunal components as a whole, the most widely and abundantly distributed species had (a) the highest mean abundances and (b) the least spatially-patchy densities in each of Knysna's diverse regions and at all spatial scales from landscape to local; however no significant relationship was observed between mean abundance and degree of patchiness. Individual dominant species displayed considerable variation; some showing positive occupancy/patchiness correlations, and several displaying significant positive and others negative abundance/patchiness ones. As expected along the estuarine gradient, positive and negative correlations of both abundance and occupancy with distance upstream were found in different species (both metrics in a given species always responding in the same manner), although significant upstream relationships with patchiness were found in only two. Local patchiness clearly contributes macroecological information additional to that provided by abundance or occupancy.

Analysis of Paramyxovirus Transcription and Replication by High-Throughput Sequencing.

We have developed a high-throughput sequencing (HTS) workflow for investigating paramyxovirus transcription and replication. We show that sequencing of oligo(dT)-selected polyadenylated mRNAs, without considering the orientation of the RNAs from which they had been generated, cannot accurately be used to analyze the abundance of viral mRNAs because genomic RNA copurifies with the viral mRNAs. The best method is directional sequencing of infected cell RNA that has physically been depleted of ribosomal and mitochondrial RNA followed by bioinformatic steps to differentiate data originating from genomes from viral mRNAs and antigenomes. This approach has the advantage that the abundance of viral mRNA (and antigenomes) and genomes can be analyzed and quantified from the same data. We investigated the kinetics of viral transcription and replication during infection of A549 cells with parainfluenza virus type 2 (PIV2), PIV3, PIV5, or mumps virus and determined the abundances of individual viral mRNAs and readthrough mRNAs. We found that the mRNA abundance gradients differed significantly between all four viruses but that for each virus the pattern remained relatively stable throughout infection. We suggest that rapid degradation of non-poly(A) mRNAs may be primarily responsible for the shape of the mRNA abundance gradient in parainfluenza virus 3, whereas a combination of this factor and disengagement of RNA polymerase at intergenic sequences, particularly those at the NP:P and P:M gene boundaries, may be responsible in the other viruses.IMPORTANCE High-throughput sequencing (HTS) of virus-infected cells can be used to study in great detail the patterns of virus transcription and replication. For paramyxoviruses, and by analogy for all other negative-strand RNA viruses, we show that directional sequencing must be used to distinguish between genomic RNA and mRNA/antigenomic RNA because significant amounts of genomic RNA copurify with poly(A)-selected mRNA. We found that the best method is directional sequencing of total cell RNA, after the physical removal of rRNA (and mitochondrial RNA), because quantitative information on the abundance of both genomic RNA and mRNA/antigenomes can be simultaneously derived. Using this approach, we revealed new details of the kinetics of virus transcription and replication for parainfluenza virus (PIV) type 2, PIV3, PIV5, and mumps virus, as well as on the relative abundance of the individual viral mRNAs.

Stochastic dynamics of wetlandscapes: Ecohydrological implications of shifts in hydro-climatic forcing and landscape configuration

Wetlands are embedded in landscapes in fractal spatial patterns, and are characterized by highly dynamic, interlinked hydrological, biogeochemical, and ecological functions. We propose here a stochastic approach to evaluate and predict the spatiotemporal hydrologic variability of wetlands at landscape scale (100 km2). Stochastic hydro-climatic forcing (daily rainfall and evapotranspiration) and the landscape topographic setting (spatial structure of wetlands within the landscape) are key drivers of wetland eco-hydrologic functionality. The novelty of our approach lies in the quantification of the hydrological dynamics for all wetlands distributed in a given landscape, and in linking stochasticity of hydroclimatic forcing and ecologically meaningful wetland network metrics. We applied the modeling framework to investigate daily hydrologic dynamics in six landscapes across the U.S. that span gradients of hydroclimate and abundance of wetlands. We assess landscape-scale patterns using four key wetland hydrological attributes that have significance in terms of aquatic habitat suitability and dispersal: (1) Abundance (2) Diversity (3) Persistence, and (4) Accessibility. We observe that the hydrologic responses of each of the six landscapes are driven by the interactions between regional stochastic hydro-climatic forcing and landscape topographic setting. Despite differences in these features, similar scaling relations define diversity (area distributions) and accessibility (separation-distance distributions). Persistence of hydrologic regimes, defined by duration of inundation above thresholds, was least in more-arid settings, and higher in humid settings, consistent with intuitive understanding. These results can support assessments of the spatiotemporal variability of ecohydrological attributes in diverse wetlandscapes, including aquatic species dispersal and habitat suitability for unique flora and fauna.

2D chemical evolution model: The impact of Galactic disc asymmetries on azimuthal chemical abundance variations

Context. Galactic disc chemical evolution models generally ignore azimuthal surface density variation that can introduce chemical abundance azimuthal gradients. Recent observations, however, have revealed chemical abundance changes with azimuth in the gas and stellar components of both the Milky Way and external galaxies.Aims. Our aim is to quantify the effects of spiral arm density fluctuations on the azimuthal variations of the oxygen and iron abundances in disc galaxies.Methods.We developed a new 2D Galactic disc chemical evolution model that is capable of following not just radial but also azimuthal inhomogeneities.Results. The density fluctuations resulting from a Milky Way-like N-body disc formation simulation produce azimuthal variations in the oxygen abundance gradients of the order of 0.1 dex. Moreover, the azimuthal variations are more evident in the outer Galactic regions, which is in agreement with the most recent observations in external galaxies. Using a simple analytical model, we show that the largest fluctuations with azimuth result near the spiral structure co-rotation resonance where the relative speed between the spiral and gaseous disc is the slowest.Conclusion. We provide a new 2D chemical evolution model capable of following azimuthal density variations. Density fluctuations extracted from a Milky Way-like dynamical model lead to a scatter in the azimuthal variations of the oxygen abundance gradient, which is in agreement with observations in external galaxies. We interpret the presence of azimuthal scatter at all radii by the presence of multiple spiral modes moving at different pattern speeds, as found in both observations and numerical simulations.

Biotic heterogeneity among scarab beetle communities in an anthropized landscape in the Central Valleys of Oaxaca, Mexico

In the tropics, changes in land use are a major concern given that the transformation of natural ecosystems to degraded environments for human use occurs rapidly and is associated with deforestation, fragmentation, isolation, among other impacts. Scarab beetles are well represented in tropical and warm temperate ecosystems due to their high diversity, numerous ecological functions, sensitivity to habitat disturbances, and responses to anthropized areas, so they are considered as a model group to study the consequences of environmental disturbance in tropical regions. We present an analysis of the scarab beetle diversity in an anthropized landscape with five different land uses in the Central Valleys of the state of Oaxaca, Mexico, with an emphasis on the dissimilarity in species composition. The organisms were collected with dung-, carrion- and fruit-baited traps during five sampling periods. We identified 2766 individuals belonging to 40 species, the majority being coprophagous or copro-necrophagous. The most abundant species was Phanaeus nimrod, with 624 individuals, although the most abundant species changed according to land uses. Beta diversity was high (80% on average), and between four sites dissimilarity was 100%. Species replacement was more important than differences in richness for the total beta diversity calculated with incidence data; and balanced variation in abundance was more important than the abundance gradient in the case of analyses with abundances. The landscape presented high scarab beetle diversity, mainly due to a high species replacement and the balanced variation in abundance, indicating that a high biotic heterogeneity has been maintained, or even enhanced following landscape modification.

The Physical Origin of the Venus Low Atmosphere Chemical Gradient

Abstract Venus shares many similarities with the Earth, but concomitantly, some of its features are extremely original. This is especially true for its atmosphere, where high pressures and temperatures are found at the ground level. In these conditions, carbon dioxide, the main component of Venus’ atmosphere, is a supercritical fluid. The analysis of VeGa-2 probe data has revealed the high instability of the region located in the last few kilometers above the ground level. Recent works have suggested an explanation based on the existence of a vertical gradient of molecular nitrogen abundances, around 5 ppm per meter. Our goal was then to identify which physical processes could lead to the establishment of this intriguing nitrogen gradient, in the deep atmosphere of Venus. Using an appropriate equation of state for the binary mixture CO2–N2 under supercritical conditions, and also molecular dynamics simulations, we have investigated the separation processes of N2 and CO2 in the Venusian context. Our results show that molecular diffusion is strongly inefficient, and potential phase separation is an unlikely mechanism. We have compared the quantity of CO2 required to form the proposed gradient with what could be released by a diffuse degassing from a low volcanic activity. The needed fluxes of CO2 are not so different from what can be measured over some terrestrial volcanic systems, suggesting a similar effect at work on Venus.

Analysis of tropical and temperate elevational gradients in arthropod abundance

Arthropods are a major component of ecosystems, in terms of both their biomass and the variety of functional roles they play. Yet we lack a clear understanding of how arthropod abundance changes along environmental gradients. We compiled published literature on overall arthropod abundances (number of individuals) along elevational gradients and performed a formal meta-analysis on the role of latitude, climatic variables, and interactions with ants in shaping the pattern. Specifically, we asked if patterns of arthropod abundance along different elevational gradients are associated with gradients of seasonality and precipitation and whether ant abundance affects other arthropods. Arthropod abundance peaks at higher elevations at mid-latitudes than low latitudes; hence, the correlation between arthropod abundance and elevation shifts from negative to positive with an increase in latitude. We suggest these patterns reflect a steep elevational gradient in the length of growing season at mid-latitudes, with the short growing season at high elevations in mid-latitudes leading to synchronous emergence and reproduction of arthropods generating a sharp increase in abundance. Precipitation and ant abundance do not have a consistent effect on arthropod abundance along most elevational gradients. However, on gradients with a very dry base and sharp increase in precipitation with elevation, arthropod abundance peaks at higher elevations. Overall, our results suggest that future changes in the length of growing season will impact the elevation at which summer arthropod abundance peaks and the sharpness of the peak, likely affecting diversity and distribution of other taxa that interact with arthropods.

Metallicity gradients in small and nearby spiral galaxies

ABSTRACT Spectra of H ii regions obtained with Gemini/GMOS are used to derive the radial metallicity gradients of four small, low-mass spiral galaxies. The analysis of the outer disc of one of them, NGC 1058, uncovers the characteristic flattening found in similar extended disc galaxies. After combining these data with published long-slit observations of nearby spiral galaxies, no evidence for a dependence of the disc scale length-normalized metallicity gradients with stellar mass is found, down to log (M⋆/M⊙) ∼ 8.5. The abundance gradients derived from these observations are compared to predictions from recent cosmological simulations of galaxy evolution, finding that in several cases the simulations fail to reproduce the mean steepening of the gradients, expressed in dex kpc−1, with decreasing stellar mass for present-day galaxies, or do not extend to sufficiently small stellar masses for a meaningful comparison. The mean steepening of the abundance gradients (in dex kpc−1) with decreasing disc scale length is in qualitative agreement with predictions from the inside-out model of Boissier & Prantzos, although the predicted slopes are systematically steeper than observed. This indicates the necessity of including processes such as outflows and radial mixing in similar models of galactic chemical evolution. Published spatially resolved metallicity and photometric data of dwarf irregular galaxies suggest that significant, but transitory, metallicity gradients can develop for systems that have experienced recent (t < 100 Myr) enhanced star formation in their inner discs.

Helminth endoparasites of black drongo, Dicrurus macrocercus (Passeriformes: Dicruridae) from Chattogram, Bangladesh

The common passerine bird species of Bangladesh, the Black Drongo (Dicrurus macrocercus), was collected between November 2013 and March 2014 from paddy fields around the Chittagong University campus to study endohelminth parasites. In total, four helminth adult parasite species, one trematoda (Eumegacetes triangularis), one cestoda (Notopentorchis sp.), two nematoda (Viguiera dicrurusi and Diplotriaena bargusinica) and single unidentified pleurocercoid larva were evident from the body cavity and digestive tracts of the sampled birds. With prevalence, mean intensity and abundance of 52.94%, 3.33 ± 3.04 and 1.76, respectively. It was apparent that the nematodes were the most prevalent (41.18%) species and the male (60%) hosts were found to be more susceptible to infection with the helminths. The mean intensity and the abundance of male also followed the same trend. However, the maximum (100%) prevalence was in February 2014. Parasitic abundance was found to be dependent with total length (t = 40.19, p = 0.000), body length (t = 31.97, p = 0.000) and body weight (t = 39.88, p = 0.000) of the host. Among the identified helminths E. triangularis, Notopentorchis sp. and Diplotriaena bargusinica are the new records for Bangladesh and from the present host. Stomach contents showed that the bird is affirmed as insectivorous, predominant abundance of insect (97-100%) having an abundance gradient of lepidopteran > odonates > dipteran > coleopterans > hemipterans > hymenopterans. The present study coincided with the winter and spring season of an isolated study area, i.e. Chittagong University campus only. Therefore, a detailed study is expected to have a complete helminth profile and epidemiological aspects of the present host at different ecological areas of the country.
 Bangladesh J. Zool. 47(1): 159-171, 2019

Putting mechanisms in foraging theory: the role of computational mechanisms in optimal decision making

Classic optimization models explain adaptive behaviours in terms of optimal cost–benefit trade-offs. They assume flexibility and make testable predictions about what animals should do in order to maximize their fitness. But they do not usually ask how animals can do it. Since flexible behaviours can be globally optimal but locally suboptimal, the optimality models should directly focus on the underlying mechanisms of behavioural flexibility, such as learning and decision making. In this paper, I use the classic ‘diet’ model of optimal foraging theory (OFT) to investigate the evolution of decision-making mechanisms at both the computational and the algorithmic level. At the computational level, I define benefits (the expected rate of net energy intake) and costs (lost opportunity) and formalize the decision rule. At the algorithmic level, I present two sequential sampling models, which differ in the way information is internally represented and used. The first model represents the prospective items along a one-dimensional scale of values (benefit–cost differences) and it uses a fixed amount of sensory information (and time) to make decisions. The second model represents items in the two-dimensional plane of benefits and costs, and it uses a variable amount of information. I test the models along a gradient of resource abundance. In each environment, I use OFT to classify resources as either profitable or unprofitable and describe the model performance in terms of decision time and accuracy. At high resource density, both models predict foraging choice to be more selective, but less accurate, than at low density, because decisions are strongly biased in the false-positive direction. At low resource density, the two-dimension model performs better than the one-dimension alternative, because it takes less time to make more accurate decisions. These differences, however, disappear when resources are abundant.