1,911 research outputs found

    Migration Strategies Vary in Space, Time, and Among Species in the Smallfish Metacommunity of the Everglades

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    Spatial ecology and movement strategies of aquatic organisms may limit their response to human-caused drying of wetland habitats. We characterized the movement strategies of the most abundant species of fish in the wetlands of the Everglades (USA) to better understand how they cope with annual fluctuations in aquatic habitat size. Over a six-year period, we used a sampling method designed to measure the density, activity levels, and movement direction of small fishes. We estimated changes in displacement speed and directional bias to identify patterns of movement that different fishes use to disperse over the gradient of disturbance in this environment. Movement of fishes ranged from highly active and directed to passive and random, and varied with hydrological condition (water rising, stable, or dropping). Six of the eight species studied displayed directed movement (possibly displaying taxis along environmental gradients) that varied in both speed and directional bias in response to hydrological cues. The remaining two species did not adjust the direction that they moved in response to hydrological cues, but their activity levels increased. Moving with directional bias may improve a fish\u27s chance of early arrival in a newly available habitat or of escaping the risk of desiccation in drying wetlands. Fishes that change activity levels may improve their likelihood of reaching favorable environments by increasing diffusion rates and greater sampling of the environment. Interspecific variation in movement strategies is predicted to play a large role in community structure and may be a primary driver of the dynamics of the Everglades fish metacommunity

    The Evaluation and Followup of Children Referred to Pediatric Endocrinologists for Short Stature

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    Objective. To characterize the pediatric endocrinologists' evaluation and followup of short-statured patients. Study Design. Observational study of 21,548 short-statured children (April 1996 to December 1999). Baseline demographics, laboratory testing, height standard deviation score (SDS), target height, and height relative to target height were analyzed at initial and return visits with the specialist. Patients were scheduled for at least one return visit and no recombinant human growth hormone therapy was administered. Results. Mean patient age was 8.6 years with a mean height SDS of −2.1. Patients were predominantly male (69%), prepubertal (73%), and white (76%). Few screening tests were obtained during initial evaluation. Nearly 40% of children did not return for their second scheduled visit. The follow-up rate was unrelated to demographics or degree of short stature. Conclusions. Low return rates limit specialists' ability to monitor growth or obtain laboratory testing over time. Further studies are needed to determine which tests should be obtained at the initial clinic visit as well as the basis for the low return rate in this group of children

    Critical change in the Fermi surface of iron arsenic superconductors at the onset of superconductivity

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    The phase diagram of a correlated material is the result of a complex interplay between several degrees of freedom, providing a map of the material's behavior. One can understand (and ultimately control) the material's ground state by associating features and regions of the phase diagram, with specific physical events or underlying quantum mechanical properties. The phase diagram of the newly discovered iron arsenic high temperature superconductors is particularly rich and interesting. In the AE(Fe1-xTx)2As2 class (AE being Ca, Sr, Ba, T being transition metals), the simultaneous structural/magnetic phase transition that occurs at elevated temperature in the undoped material, splits and is suppressed by carrier doping, the suppression being complete around optimal doping. A dome of superconductivity exists with apparent equal ease in the orthorhombic / antiferromagnetic (AFM) state as well as in the tetragonal state with no long range magnetic order. The question then is what determines the critical doping at which superconductivity emerges, if the AFM order is fully suppressed only at higher doping values. Here we report evidence from angle resolved photoemission spectroscopy (ARPES) that critical changes in the Fermi surface (FS) occur at the doping level that marks the onset of superconductivity. The presence of the AFM order leads to a reconstruction of the electronic structure, most significantly the appearance of the small hole pockets at the Fermi level. These hole pockets vanish, i. e. undergo a Lifshitz transition, at the onset of superconductivity. Superconductivity and magnetism are competing states in the iron arsenic superconductors. In the presence of the hole pockets superconductivity is fully suppressed, while in their absence the two states can coexist.Comment: Updated version accepted in Nature Physic

    Amblyomma imitator Ticks as Vectors of Rickettsia rickettsii, Mexico

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    Real-time PCR of Amblyomma imitator tick egg masses obtained in Nuevo Leon State, Mexico, identified a Rickettsia species. Sequence analyses of 17-kD common antigen and outer membrane protein A and B gene fragments showed to it to be R. rickettsii, which suggested a potential new vector for this bacterium
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