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Single Blocks (single + block)
Selected AbstractsMetapopulation Extinction Risk under Spatially Autocorrelated DisturbanceCONSERVATION BIOLOGY, Issue 2 2005A. S. KALLIMANIS patrón espacial de perturbación; simulaciones espacialmente explícitas; SLOSS; umbral de extinción Abstract:,Recent extinction models generally show that spatial aggregation of habitat reduces overall extinction risk because sites emptied by local extinction are more rapidly recolonized. We extended such an investigation to include spatial structure in the disturbance regime. A spatially explicit metapopulation model was developed with a wide range of dispersal distances. The degree of aggregation of both habitat and disturbance pattern could be varied from a random distribution, through the intermediate case of a fractal distribution, all the way to complete aggregation (single block). Increasing spatial aggregation of disturbance generally increased extinction risk. The relative risk faced by populations in different landscapes varied greatly, depending on the disturbance regime. With random disturbance, the spatial aggregation of habitat reduced extinction risk, as in earlier studies. Where disturbance was spatially autocorrelated, however, this advantage was eliminated or reversed because populations in aggregated habitats are at risk of mass extinction from coarse-scale disturbance events. The effects of spatial patterns on extinction risk tended to be reduced by long-distance dispersal. Given the high levels of spatial correlation in natural and anthropogenic disturbance processes, population vulnerability may be greatly underestimated both by classical (nonspatial) models and by those that consider spatial structure in habitat alone. Resumen:,Los modelos recientes de extinción generalmente muestran que la agregación espacial de hábitat reduce el riesgo de extinción debido a una recolonización más rápida de sitios vacíos por extinción local. Extendimos la investigación para incluir la estructura espacial en el régimen de perturbación. Desarrollamos un modelo metapoblacional espacialmente explícito en el que el patrón espacial tanto del hábitat como de los regímenes de perturbación podía variar aleatoriamente de fractal a completamente agregado (bloque) y con una amplia gama de distancias de dispersión. El incremento de la agregación espacial de la perturbación generalmente incrementó el riesgo de extinción. El riesgo relativo que enfrentan poblaciones en paisajes diferentes fue muy variable, dependiendo del régimen de perturbación. Con perturbación aleatoria, la agregación espacial de hábitat redujo el riesgo de extinción, como en estudios anteriores. Sin embargo, cuando la perturbación estaba autocorrelacionada espacialmente, esta ventaja se eliminaba o invertía debido a que las poblaciones en hábitats agregados están en riesgo de extinción masiva por eventos perturbadores a escala gruesa. Los efectos de patrones espaciales sobre el riesgo de extinción tendieron a reducirse por la dispersión de larga distancia. Debido a los altos niveles de correlación espacial en los procesos naturales y humanos de perturbación, la vulnerabilidad puede estar enormemente subestimada tanto por modelos clásicos (no espaciales) como por los que sólo consideran la estructura espacial del habitat. Los modelos que consideran la estructura espacial del hábitat solo subestiman el riesgo en comparación con modelos que consideran la estructura especial de la perturbación. [source] Contemporary kinematics of the southern Aegean and the Mediterranean RidgeGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2004Corné Kreemer SUMMARY This study focuses on the kinematics of the southern Aegean and the Mediterranean Ridge (MR). A quantification of the deformation of the MR is essential for both evaluating physical models of accretionary wedges in general and for obtaining a self-consistent model of the surface deformation over the entire Nubia,Eurasia (NU,EU) plate boundary zone in the eastern Mediterranean. Previous kinematic studies have not properly considered the deformation field south of the Hellenic arc. Although this study focuses on the deformation field of the MR, we also discuss the kinematics of the southern Aegean, because the geometry and movement of the Hellenic arc determine to a large extent the kinematic boundary conditions for kinematic studies of the MR. We calculate a continuous velocity and strain rate field by interpolating model velocities that are fitted in a least-squares sense to published Global Positioning System (GPS) velocities. In the interpolation, we use information from a detailed data set of onshore and offshore active faulting to place constraints on the expected style and direction of the model strain rate field. In addition, we use the orientations of tracks left by seamounts travelling into the wedge to further constrain the offshore deformation pattern. Our model results highlight the presence of active shear partitioning within the Mediterranean ridge. High compressional strain rates between the ridge crest and the deformation front accommodate approximately 60,70 per cent of the total motion over the wedge, and the outward growth rate of the frontal thrust is , 4 mm yr,1. Strain partitioning within the wedge leads to 19,23 mm yr,1 of dextral motion at the wedge,backstop contact of the western MR, whereas the Pliny and Strabo trenches in the eastern MR accommodate 21,23 mm yr,1 of sinistral motion. The backstop of the western MR is kinematically part of the southern Aegean, which moves as a single block [the Aegean block (AE)] at 33,34 mm yr,1 in the direction of S24°W ± 1° towards stable Nubia (NU). Our model confirms that there is a clear divergence between the western and eastern Hellenic arc and we argue for a causal relation between the outward motion of the arc and the gradient in the regional geoid anomaly. Our results suggest that a significant driving source of the surface velocity field lies south of the Hellenic arc and only for the southeastern Aegean could there be some effect as a result of gravitational collapse associated with density differences within the overriding plate. [source] Ultrastructure of bovine oocytes exposed to Taxol prior to OPS vitrificationMOLECULAR REPRODUCTION & DEVELOPMENT, Issue 8 2008Roser Morató Abstract Our objective was to document potential subcellular consequences of treatment with the microtubule stabilizer Taxol with or without subsequent vitrification of cow and calf oocytes by the open pulled straw (OPS) method. Oocytes were divided into four experimental groups for cows and four groups for calves: (1) a control group fixed immediately after maturation; (2) an OPS group cryopreserved by conventional OPS; (3) a Taxol/CPA group exposed to 1 ,M Taxol and cryoprotective agents (CPAs); and (4) a Taxol/OPS group vitrified by OPS including 1 ,M Taxol to the vitrification solution. All oocytes were processed for light and transmission electron microscopy. The main injuries were observed on the metaphase plate and the spindle. In control oocytes, the metaphase appeared as condensed chromosomes arranged in a well-organized metaphase plate and the spindle showed well organized microtubules in both cow and calf oocytes. However, in cow OPS oocytes, the metaphase plate was disorganized into scattered chromosomes or the chromosomes were condensed into a single block of chromatin. In addition, microtubules were not organized as typical spindles. In contrast, cow Taxol/OPS oocytes as well as both cow and calf Taxol/CPAs oocytes showed well-organized metaphase plates and normal spindle morphology. All calf OPS and calf Taxol/OPS oocytes displayed a single block of chromatin and no microtubules could be observed around the chromosomes. In conclusion, treatment with 1 µM Taxol before and during vitrification did not induce adverse changes in the oocyte cytoplasm or metaphase spindles in adult bovine oocytes, but stabilized the metaphase and spindle morphology. Mol. Reprod. Dev. 75: 1318,1326, 2008. © 2008 Wiley-Liss, Inc. [source] The probabilistic key-group methodINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 9 2004A. R. Yarahmadi Bafghi Abstract Key-block approaches are widely used in the analysis of rock slopes. The key-group method improves such analyses by taking into account groups of blocks instead of single blocks. Nevertheless, these stability analyses are usually carried out within a context where uncertainty may be a difficult problem to overcome. In the present paper, we propose introducing probabilistic approaches into the key-group method in order to account for uncertainty in the mechanical parameters of the problem to be solved. Both the first-order, second-moment method (FOSM) and the advanced second-moment method (ASMM) are considered herein and compared with Monte-Carlo simulations through the use of five theoretical case studies. Lastly, the probabilistic key-group method (PKGM) is qualified. Copyright © 2004 John Wiley & Sons, Ltd. [source] Modeling of the Phase Separation Behavior of Polydisperse Semi-Flexible Diblock CopolymersMACROMOLECULAR THEORY AND SIMULATIONS, Issue 8 2004Andreas John Abstract Summary: A modified random phase approximation method with a cumulant expansion for the semi-flexible structure factor of diblock copolymers was exercised to describe the phase separation behavior of semi-flexible and polydisperse diblock copolymers. Scattering curves and spinodal diagrams were calculated applying monomer specific input parameters. The influence of polydispersity was included applying basic concepts of mathematical statistics utilizing several probability density distributions in the case of the two single blocks. In contrast to semi-flexibility, the main effect of polydispersity was found to shift the spinodal up, thus to enlarge the range of existence of the homogeneous phase. Twofold Schultz-Zimm distribution of diblock copolymers. [source] Radiofrequency Neurotomy for Low Back Pain: Evidence-Based Procedural GuidelinesPAIN MEDICINE, Issue 2 2005W. Michael Hooten MD ABSTRACT Objective., This review was undertaken to outline the procedural limitations of the randomized controlled trials (RCTs) of radiofrequency (RF) neurotomy for low back pain. Second, the literature related to patient selection, diagnostic testing, and the technique of performing lumbar spine, RF neurotomy will be critically reviewed and analyzed. Based on these analyses, diagnostic and procedural guidelines will be proposed. Design., A Medline and EMBASE search identified three RCTs and two systematic reviews of RF neurotomy for low back pain. A similar search identified pertinent literature related to the method of patient selection for a diagnostic block, the medial branch and L5 dorsal ramus comparative block, and the anatomical and technical parameters of lumbar spine RF neurotomy. Results., Substantial procedural shortcomings were identified in all three RCTs. In the systematic reviews, these procedural limitations were not accounted for by the quality assessment of study design which resulted in an inaccurate estimate of clinical effectiveness. Analysis using likelihood ratios showed that screening criteria could increase the probability of zygapophysial joint pain before performing diagnostic blocks. Similar analysis showed that comparative medial branch blocks, rather than single blocks, must be used before RF neurotomy. Anatomical studies demonstrated that the shorter distal compared with the circumferential radius of the RF lesion necessitates placement of the electrode parallel to the course of the nerve along the base of the superior articular process. Conclusions., The evidence-based procedural guidelines provide consistent criteria for multisite studies that could enroll a sufficiently large homogenous study cohort. [source] The Prevalence of Facet Joint-Related Chronic Neck Pain in Postsurgical and Nonpostsurgical Patients: A Comparative EvaluationPAIN PRACTICE, Issue 1 2008Laxmaiah Manchikanti MD ,,Abstract Background: Facet (zygapophysial) joints may be clinically important sources of chronic cervical spinal pain. Previous studies have demonstrated the value and validity of controlled, comparative local anesthetic blocks in the diagnosis of facet joint pain, and reported an overall prevalence of 36% to 67% facet joint involvement in cervical spinal pain. The reports of lumbar facet joint-involvement in postsurgery syndrome have been shown to be highly variable with prevalence ranging from 8% to 32%. To date, however, the prevalence of postsurgical facet joint-related pain in the cervical spine has not been evaluated. In light of this, the present retrospective study was conducted to assess and compare the prevalence of chronic postsurgical facet joint cervical spinal pain to nonsurgical, chronic cervical facet joint pain. Methods: Patients presenting with chronic neck pain were studied. The procedures were performed by a single physician in an interventional pain management ambulatory surgery center. The prevalence of cervical facet joint pain in postsurgical patients was assessed and compared to nonsurgical patients. Results: A total of 251 patients (45 postsurgery vs. 206 nonsurgical patients) with chronic persistent neck pain were evaluated using controlled, comparative local anesthetic blocks in accordance with IASP criteria. The prevalence of the cervical facet joint pain and false-positive rate of single blocks in postsurgical patients were 36% and 50% compared with 39% and 43% in nonsurgical patients. Conclusions: Cervical facet joints are clinically important pain generators in a significant proportion of patients with chronic persistent neck pain after surgical intervention(s). The prevalence of cervical facet joint pain was similar in both postsurgical and nonsurgical patients.,, [source] | |||||||||||||