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41.
The looming mass extinction of biodiversity in the humid tropics is a major concern for the future, yet most reports of extinctions in these regions are anecdotal or conjectural, with a scarcity of robust, broad-based empirical data. Here we report on local extinctions among a wide range of terrestrial and freshwater taxa from Singapore (540 km2) in relation to habitat loss exceeding 95% over 183 years. Substantial rates of documented and inferred extinctions were found, especially for forest specialists, with the greatest proportion of extinct taxa (34-87%) in butterflies, fish, birds and mammals. Observed extinctions were generally fewer, but inferred losses often higher, in vascular plants, phasmids, decapods, amphibians and reptiles (5-80%). Forest reserves comprising only 0.25% of Singapore's area now harbour over 50% of the residual native biodiversity. Extrapolations of the observed and inferred local extinction data, using a calibrated species-area model, imply that the current unprecedented rate of habitat destruction in Southeast Asia will result in the loss of 13-42% of regional populations over the next century, at least half of which will represent global species extinctions. 相似文献
42.
A small-molecule nitroimidazopyran drug candidate for the treatment of tuberculosis 总被引:32,自引:0,他引:32
Stover CK Warrener P VanDevanter DR Sherman DR Arain TM Langhorne MH Anderson SW Towell JA Yuan Y McMurray DN Kreiswirth BN Barry CE Baker WR 《Nature》2000,405(6789):962-966
Mycobacterium tuberculosis, which causes tuberculosis, is the greatest single infectious cause of mortality worldwide, killing roughly two million people annually. Estimates indicate that one-third of the world population is infected with latent M. tuberculosis. The synergy between tuberculosis and the AIDS epidemic, and the surge of multidrug-resistant clinical isolates of M. tuberculosis have reaffirmed tuberculosis as a primary public health threat. However, new antitubercular drugs with new mechanisms of action have not been developed in over thirty years. Here we report a series of compounds containing a nitroimidazopyran nucleus that possess antitubercular activity. After activation by a mechanism dependent on M. tuberculosis F420 cofactor, nitroimidazopyrans inhibited the synthesis of protein and cell wall lipid. In contrast to current antitubercular drugs, nitroimidazopyrans exhibited bactericidal activity against both replicating and static M. tuberculosis. Lead compound PA-824 showed potent bactericidal activity against multidrugresistant M. tuberculosis and promising oral activity in animal infection models. We conclude that nitroimidazopyrans offer the practical qualities of a small molecule with the potential for the treatment of tuberculosis. 相似文献
43.
Protein deprivation causes reversible impariment of mucosal immune response to cholera toxoid/toxin in rat gut 总被引:2,自引:0,他引:2
Scretory antibodies may be the major defence against mucosal infections, especially those due to viruses and non-invasive pathogens such as Vibrio cholerae and toxinogenic Escherichia coli. The high incidence of mucosal infections in malnourished protein-deficient children may result from defective antibody production, but evidence for this is conflicting. We report here that protein deficiency markedly impairs the mucosal immune reponse to cholera toxiod/toxin (CT), a protein antigen, in rats and that this impairment is rapidly reversed by refeeding. 相似文献
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Much of western North America is dominated by dense, monotypic, late seral stands of big sagebrush ( Artemisia tridentata Nutt.). These stands often have depauperate understories with limited species richness, diversity, and herbaceous cover. The National Park Service at Dinosaur National Monument, Colorado, is using both strategic and natural prescribed fire in Wyoming big sagebrush ( Artemisia tridentata ssp. wyomingensis Beetle and Young) communities to foster intra-community (α -scale) and landscape diversity. This study analyzed an accumulated foliar cover data set between paired burn and control areas on 6 different sites during the last 20 years. Across the monitoring period, mean total vegetation cover of all combined sites was 44% control and 42% burn. Total vegetation cover in burn areas was higher than or equal to paired control areas within 2-3 years post-burn. Shrubs were essentially eliminated in burn areas, but perennial grass cover was 10-35% higher. Mean number of species on all sites and years combined was 17 control and 18 burn. Species richness was different on only 1 site-year, Dry Woman 1995 ( P = 0.001, 15 control, 9 burn). Species similarity by site and between treatments ranged from 44% to 75%. Differences in Shannon-Weiner diversity index values between paired sites occurred in 6 of 20 years ( P < 0.05). Index value differences on these 6 sites were due to a large annual grass component in burn areas. Prescribed burning successfully shifted late successional sagebrushdominated communities to earlier herbaceous-dominated successional stages without lowering total vegetation cover, while maintaining -scale diversity and species richness. 相似文献
49.
Calls of the red squirrel ( Tamiasciurus hudsonicus; n = 122) were recorded in wild populations from 15 localities in Arizona, New Mexico, Colorado, Utah, Wyoming, Montana, Idaho, and Washington. Computer-generated audiospectrograms of 20- or 30-second samples from a calling bout of each individual were analyzed. Eighteen bark types (distinct forms of the bark call) were identified plus a 19th category that included rarely used, longer bark calls. The frequency of use of each bark type within the sample was recorded for each squirrel. Differences in frequency of use of the various bark types were found among subspecies, within subspecies, and within populations; additionally, the southern subspecies utilized a reduced number of bark types. The large number of different bark types and the variation in bark type usage within populations suggest the potential for communication of such information as individual identification, behavioral states, or gender identification. 相似文献
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Pryor WR Rymer AM Mitchell DG Hill TW Young DT Saur J Jones GH Jacobsen S Cowley SW Mauk BH Coates AJ Gustin J Grodent D Gérard JC Lamy L Nichols JD Krimigis SM Esposito LW Dougherty MK Jouchoux AJ Stewart AI McClintock WE Holsclaw GM Ajello JM Colwell JE Hendrix AR Crary FJ Clarke JT Zhou X 《Nature》2011,472(7343):331-333
Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling between Enceladus and Saturn like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io-Jupiter coupling, for example, create an auroral footprint in Jupiter's ionosphere. Auroral ultraviolet emission associated with Enceladus-Saturn coupling is anticipated to be just a few tenths of a kilorayleigh (ref. 12), about an order of magnitude dimmer than Io's footprint and below the observable threshold, consistent with its non-detection. Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon's footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters--and as such is probably indicative of variable plume activity. 相似文献