Assuntos
Biodiversidade , Conservação dos Recursos Naturais , Ecologia , Espécies em Perigo de Extinção , Plantas , Animais , Classificação , Conservação dos Recursos Naturais/economia , Conservação dos Recursos Naturais/estatística & dados numéricos , Ecologia/economia , Ecologia/métodos , Espécies em Perigo de Extinção/estatística & dados numéricos , Plantas/classificação , Política PúblicaRESUMO
The World Conservation Union (IUCN) Red List is widely recognized as the most authoritative and objective system for classifying species by their risk of extinction. Red List Indices (RLIs) illustrate the relative rate at which a particular set of species change in overall threat status (i.e. projected relative extinction-risk), based on population and range size and trends as quantified by Red List categories. RLIs can be calculated for any representative set of species that has been fully assessed at least twice. They are based on the number of species in each Red List category, and the number changing categories between assessments as a result of genuine improvement or deterioration in status. RLIs show a fairly coarse level of resolution, but for fully assessed taxonomic groups they are highly representative, being based on information from a high proportion of species worldwide. The RLI for the world's birds shows that that their overall threat status has deteriorated steadily during the years 1988-2004 in all biogeographic realms and ecosystems. A preliminary RLI for amphibians for 1980-2004 shows similar rates of decline. RLIs are in development for other groups. In addition, a sampled index is being developed, based on a stratified sample of species from all major taxonomic groups, realms and ecosystems. This will provide extinction-risk trends that are more representative of all biodiversity.
Assuntos
Biodiversidade , Conservação dos Recursos Naturais/métodos , Demografia , Densidade Demográfica , Anfíbios/fisiologia , Animais , Aves/fisiologia , Bases de Dados Factuais , Geografia , Agências Internacionais , Dinâmica PopulacionalRESUMO
The X-ray storage phosphor materials BaFC1 and BaFBr differ greatly in their 137Ba nuclear electric quadrupole interaction, despite a common crystal structure. The quadrupole coupling constant in BaFCl is relatively small and increases rapidly with temperature, in an unusual manner: [CQ[ = 1.7 MHz at T = 296 K, CQ-1(delta CQ/delta T)p = 1.7 x 10(-3) K-1. The coupling BaFBr is [CQ[ = 17.0 MHz at 296 K. The difference in quadrupole coupling at room temperature is explained using an electrostatic point-charge model of the crystals.
Assuntos
Compostos de Bário/química , Bário/química , Espectroscopia de Ressonância Magnética , Brometos/química , Cloretos/química , Cristalografia , Eletroquímica , Fluoretos/química , Isótopos , Espectroscopia de Ressonância Magnética/métodos , Temperatura , Raios XRESUMO
This is a theoretical study of nuclear magnetic resonance (NMR) spectra to be obtained from a powder specimen spinning in the magnetic field. The nuclear system is assumed to be magnetically dilute and to exhibit a combination of electric quadrupole interaction, magnetic shielding anisotropy and heteronuclear spin coupling. Formulas are given for the line positions and centreband linewidths of all transitions of the quadrupolar nucleus, in terms of scalar products of local interaction tensors. A number of unusual effects are predicted.
