Home About us Contact
|
Home About us Contact | ||
|
|
||
Several Millions (several + millions)
Selected AbstractsThe concept of the taxon cycle in biogeographyGLOBAL ECOLOGY, Issue 5 2002Robert E. Ricklefs Abstract Taxon cycles are sequential phases of expansion and contraction of the ranges of species, associated generally with shifts in ecological distribution. The important contribution of the taxon cycle to biogeographical analysis is its emphasis on evolutionary and ecological interactions among colonizing and resident species, which influence their extinction dynamics and establish patterns of geographical distribution. Taxon cycles were inferred originally from the distribution of species across island archipelagos, where a correlation was noted between gaps in island occupancy and the degree of phenotypic differentiation. This pattern implied that phases of colonization were followed by range contraction, while endemic Antillean species that were undifferentiated between islands suggested secondary expansion and the beginning of a new cycle. This interpretation was met with scepticism, but reconstruction of phylogenetic relationships from gene sequences has now permitted us to characterize taxon cycles in Lesser Antillean birds. The relative timing of phases of the cycle can be deduced from genetic divergence between island populations. We have found that taxon cycles have periods in the order of 106 years and that cycles in different lineages occur independently of each other and independently of Pleistocene climate cycles. Individual island populations may persist for several millions of years on the larger islands of the Lesser Antilles; occasional expansion phases lead to the replacement of island populations that have disappeared, thus reducing the archipelago-wide rate of extinction to nil. What drives taxon cycles is unknown, but we speculate that they may be caused by co-evolution with enemy populations, and a probable mechanism would involve infrequent mutations influencing parasite virulence and avian host disease resistance. Taxon cycles undoubtedly occur on continents, but the geographical configuration of island archipelagos reveals more clearly their presence and invites their study. [source] Generation and adaptation of computational surface meshes from discrete anatomical dataINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 6 2004Pascal J. FreyArticle first published online: 27 MAY 200 Abstract Fast and accurate scanning devices are nowadays widely used in many engineering and biomedical fields. The resulting discrete data is usually directly converted into polygonal surface meshes, using ,brute-force' algorithms, often resulting in meshes that may contain several millions of polygons. Simplification is therefore required in order to make storage, computation and display possible if not efficient. In this paper, we present a general scheme for mesh simplification and optimization that allows to control the geometric approximation as well as the element shape and size quality (required for numerical simulations). Several examples ranging from academic to complex biomedical geometries (organs) are presented to illustrate the efficiency and the utility of the proposed approach. Copyright © 2004 John Wiley & Sons, Ltd. [source] Microabrasion of Coatings Investigated by the Inclined Impact Test at Various Loads under Dry and Lubricated ConditionsPLASMA PROCESSES AND POLYMERS, Issue S1 2007Konstantinos-Dionysios Bouzakis Abstract The impact test has clearly emerged as an important technology for the prediction of cohesive, adhesive, and fatigue failure of coatings, further enabling the establishment of Smith and Woehler diagrams of PVD films. In the present paper, the abrasive wear of a well-adherent (Ti46Al54)N coating was investigated by the inclined impact test, at impact loads higher, equal to or lower than the fatigue critical impact load of the applied film. These investigations were conducted for few thousands up to several millions of impacts both under dry and lubricated conditions. The obtained results suggest that the inclined impact test is a very efficient method for detecting coating resistance against impact microabrasion, with or without lubrication and according to the test conditions for the synchronous contribution of fatigue phenomena to the film wear. [source] Identification of fiber misalignment in continuous fiber compositesPOLYMER COMPOSITES, Issue 3 2003S. C. Barwick Misaligned fibers are invariably present in nominally unidirectional high-performance composites. Such misaligned fibers are known to affect key mechanical properties of the composite, such as the longitudinal compressive strength, longitudinal tensile modulus, fatigue endurance, shear strength, and delamination resistance (1). In this paper we present a method for the automated detection of large angle fiber misalignment (, > 40°) in continuous fiber-reinforced composite materials. The method relies on the application of a series of geometrical criteria based upon measurements routinely obtained during optical scanning of polished sample cross-sections. As such, the technique is ideal for the automated identification of highly misaligned fibers in large-area (, cm2) specimens that may contain several millions of individual fiber images. The criteria applied take into account the fact that prepared cross-sections of such materials contain many damaged fibers as a result of attrition during polishing. Data obtained from three pultruded unidirectional rods reinforced with continuous carbon filaments are used to illustrate the effectiveness of this method in identifying regions where large angle misalignment occurs. [source] Improved infection control in the prevention of variant Creutzfeldt-Jakob disease in Australia: costs and benefitsAUSTRALIAN AND NEW ZEALAND JOURNAL OF PUBLIC HEALTH, Issue 6 2004Trang Vu Objective: To evaluate the costs and benefits of infection control strategies to prevent the transmission of variant Creutzfeldt-Jakob disease (vCJD) in ophthalmic surgery in Australia. Methods: The reduction in the risk of iatrogenic transmission of vCJD from feasible infection control strategies was calculated using decision analytic models. A static model calculated the direct secondary transmission for surgical eye procedures, and a simple dynamic model estimated the change in the risk of a subsequent sustained epidemic over the longer term. The expected number of vCJD infections, their cost of care and years of life lost and the estimated cost of strategies included the direct costs of infection control measures were calculated taking a health system perspective. Results: The dynamic model (Markov process) predicted that from a hypothetical pool of as many as 100 primary vCJD cases there would be less than five iatrogenic infections in the next 30 years. If there are fewer than five primary cases the model predicted no secondary cases of vCJD. The costs of providing care for a vCJD case is estimated to be about 50,000, subject to considerable uncertainty. The minimum cost for using a partial infection control strategy to prevent an iatrogenic infection is likely to be in the order of several millions of dollars. Conclusions: Substantial public health investment would need to be made in order to reduce a low risk of iatrogenic transmission of vCJD. Given the likely number of cases of iatrogenic infection, and the order of magnitude of the costs of caring for cases of vCJD, it may be difficult to justify the high cost of risk reduction strategies. [source] | ||||||||||