3,081 research outputs found
Abundance, spatial variance and occupancy: arthropod species distribution in the Azores
Copyright © 2006 British Ecological Society.1. The positive abundance-occupancy and abundance-variance relationships are two of the most widely documented patterns in population and community ecology. 2. Recently, a general model has been proposed linking the mean abundance, the spatial variance in abundance, and the occupancy of species. A striking feature of this model is that it consists explicitly of the three variables abundance, variance and occupancy, and no extra parameters are involved. However, little is known about how well the model performs. 3. Here, we show that the abundance-variance-occupancy model fits extremely well to data on the abundance, variance and occupancy of a large number of arthropod species in natural forest patches in the Azores, at three spatial extents, and distinguishing between species of different colonization status. Indeed, virtually all variation about the bivariate abundance-occupancy and abundance-variance relationships is effectively explained by the third missing variable (variance in abundance in the case of the abundance-occupancy relationship, and occupancy in the case of the abundance-variance relationship). 4. Introduced species tend to exhibit lower densities, less spatial variance in these densities, and occupy fewer sites than native and endemic species. None the less, they all lie on the same bivariate abundance-occupancy and abundance-variance, and trivariate abundance-variance-occupancy, relationships. 5. Density, spatial variance in density, and occupancy appear to be all the things one needs to know to describe much of the spatial distribution of species
Synthesis and magnetic properties of CoPt–poly(methylmethacrylate) nanostructured composite material
We have prepared nanometer-sized CoPt particles dispersed in a poly~methyl methacrylate~PMMA!matrix, as a novel nanostructured magnetic plastic, through a soft chemical processing route. In this work, CoPt nanoparticles were successfully synthesized from a solution phase reduction system in the presence of capping ligands and stabilizing agents at high temperature. The CoPt nanoparticles were annealed at 400 °C for 3 h, and were subsequently re-dispersed inmethylmethacrylate~monomer! . The polymerization was induced by a UV source and the hardness of final product was adjusted by varying the amount of monomeric cross-link agent. Annealed bare CoPt nanoparticles as a ‘‘core’’ material and CoPt–PMMA composite material were characterized by using energy dispersive spectroscopy, transmission electron microscopy, and x-ray diffraction, indicating that we are able to prepare CoPt nanoparticles with 10 nm in diameter ~after annealing by employing this high temperature colloidal processing method. Magnetic investigation of this CoPt–PMMA material indicates an intrinsic coercivity of 300 Oe at 300 K and 1665 Oe at 5 K
Synthesis and magnetic properties of CoPt–poly(methylmethacrylate) nanostructured composite material
We have prepared nanometer-sized CoPt particles dispersed in a poly~methyl methacrylate~PMMA!matrix, as a novel nanostructured magnetic plastic, through a soft chemical processing route. In this work, CoPt nanoparticles were successfully synthesized from a solution phase reduction system in the presence of capping ligands and stabilizing agents at high temperature. The CoPt nanoparticles were annealed at 400 °C for 3 h, and were subsequently re-dispersed inmethylmethacrylate~monomer! . The polymerization was induced by a UV source and the hardness of final product was adjusted by varying the amount of monomeric cross-link agent. Annealed bare CoPt nanoparticles as a ‘‘core’’ material and CoPt–PMMA composite material were characterized by using energy dispersive spectroscopy, transmission electron microscopy, and x-ray diffraction, indicating that we are able to prepare CoPt nanoparticles with 10 nm in diameter ~after annealing by employing this high temperature colloidal processing method. Magnetic investigation of this CoPt–PMMA material indicates an intrinsic coercivity of 300 Oe at 300 K and 1665 Oe at 5 K
Mass spectrometry reveals the assembly pathway of encapsulated ferritins and highlights a dynamic ferroxidase interface
Self-assembly of FePt nanoparticles into nanorings
The application of nanoparticles as quantum dots in nanoelectronics demands their arrangement in ordered arrays. Shape controlled self-assembly is a challenge due to the difficulties of obtaining proper self-assembling parameters, such as solvent concentration, organic ligands, and nanoparticle size. In this article, hard magnetic FePt nanoparticles were synthesized using a combination approach of reduction and thermal decomposition. The nanoparticles are about 4.5 nm and appeared as truncated octahedral enclosed by the
{100} and {111}
crystal facets of fcc structure. The nanoparticles are of hexagonal close packing and orient randomly in the self-assembly nanoarrays. By diluting the solution for large-area self-assembly, monolayer, submonolayer, and multilayer nanorings of FePt nanoparticles were formed. The nanoring formation is determined by hydrodynamics, surface effects, and interaction between the FePt nanoparticles and substrates
Distinct Roles for Drosophila Dicer-1 and Dicer-2 in the siRNA/miRNA Silencing Pathways
AbstractThe RNase III enzyme Dicer processes RNA into siRNAs and miRNAs, which direct a RNA-induced silencing complex (RISC) to cleave mRNA or block its translation (RNAi). We have characterized mutations in the Drosophila dicer-1 and dicer-2 genes. Mutation in dicer-1 blocks processing of miRNA precursors, whereas dicer-2 mutants are defective for processing siRNA precursors. It has been recently found that Drosophila Dicer-1 and Dicer-2 are also components of siRNA-dependent RISC (siRISC). We find that Dicer-1 and Dicer-2 are required for siRNA-directed mRNA cleavage, though the RNase III activity of Dicer-2 is not required. Dicer-1 and Dicer-2 facilitate distinct steps in the assembly of siRISC. However, Dicer-1 but not Dicer-2 is essential for miRISC-directed translation repression. Thus, siRISCs and miRISCs are different with respect to Dicers in Drosophila
Stochastic Substitute Training: A Gray-box Approach to Craft Adversarial Examples Against Gradient Obfuscation Defenses
It has been shown that adversaries can craft example inputs to neural
networks which are similar to legitimate inputs but have been created to
purposely cause the neural network to misclassify the input. These adversarial
examples are crafted, for example, by calculating gradients of a carefully
defined loss function with respect to the input. As a countermeasure, some
researchers have tried to design robust models by blocking or obfuscating
gradients, even in white-box settings. Another line of research proposes
introducing a separate detector to attempt to detect adversarial examples. This
approach also makes use of gradient obfuscation techniques, for example, to
prevent the adversary from trying to fool the detector. In this paper, we
introduce stochastic substitute training, a gray-box approach that can craft
adversarial examples for defenses which obfuscate gradients. For those defenses
that have tried to make models more robust, with our technique, an adversary
can craft adversarial examples with no knowledge of the defense. For defenses
that attempt to detect the adversarial examples, with our technique, an
adversary only needs very limited information about the defense to craft
adversarial examples. We demonstrate our technique by applying it against two
defenses which make models more robust and two defenses which detect
adversarial examples.Comment: Accepted by AISec '18: 11th ACM Workshop on Artificial Intelligence
and Security. Source code at https://github.com/S-Mohammad-Hashemi/SS
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