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排序方式: 共有147条查询结果,搜索用时 38 毫秒
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WF Laurance DC Useche J Rendeiro M Kalka CJ Bradshaw SP Sloan SG Laurance M Campbell K Abernethy P Alvarez V Arroyo-Rodriguez P Ashton J Benítez-Malvido A Blom KS Bobo CH Cannon M Cao R Carroll C Chapman R Coates M Cords F Danielsen B De Dijn E Dinerstein MA Donnelly D Edwards F Edwards N Farwig P Fashing PM Forget M Foster G Gale D Harris R Harrison J Hart S Karpanty WJ Kress J Krishnaswamy W Logsdon J Lovett W Magnusson F Maisels AR Marshall D McClearn D Mudappa MR Nielsen R Pearson N Pitman 《Nature》2012,489(7415):290-294
The rapid disruption of tropical forests probably imperils global biodiversity more than any other contemporary phenomenon. With deforestation advancing quickly, protected areas are increasingly becoming final refuges for threatened species and natural ecosystem processes. However, many protected areas in the tropics are themselves vulnerable to human encroachment and other environmental stresses. As pressures mount, it is vital to know whether existing reserves can sustain their biodiversity. A critical constraint in addressing this question has been that data describing a broad array of biodiversity groups have been unavailable for a sufficiently large and representative sample of reserves. Here we present a uniquely comprehensive data set on changes over the past 20 to 30 years in 31 functional groups of species and 21 potential drivers of environmental change, for 60 protected areas stratified across the world’s major tropical regions. Our analysis reveals great variation in reserve ‘health’: about half of all reserves have been effective or performed passably, but the rest are experiencing an erosion of biodiversity that is often alarmingly widespread taxonomically and functionally. Habitat disruption, hunting and forest-product exploitation were the strongest predictors of declining reserve health. Crucially, environmental changes immediately outside reserves seemed nearly as important as those inside in determining their ecological fate, with changes inside reserves strongly mirroring those occurring around them. These findings suggest that tropical protected areas are often intimately linked ecologically to their surrounding habitats, and that a failure to stem broad-scale loss and degradation of such habitats could sharply increase the likelihood of serious biodiversity declines. 相似文献
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AS Tayi AK Shveyd AC Sue JM Szarko BS Rolczynski D Cao TJ Kennedy AA Sarjeant CL Stern WF Paxton W Wu SK Dey AC Fahrenbach JR Guest H Mohseni LX Chen KL Wang JF Stoddart SI Stupp 《Nature》2012,488(7412):485-489
Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures. 相似文献
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Kiefer JR Pawlitz JL Moreland KT Stegeman RA Hood WF Gierse JK Stevens AM Goodwin DC Rowlinson SW Marnett LJ Stallings WC Kurumbail RG 《Nature》2000,405(6782):97-101
Cyclooxygenases are bifunctional enzymes that catalyse the first committed step in the synthesis of prostaglandins, thromboxanes and other eicosanoids. The two known cyclooxygenases isoforms share a high degree of amino-acid sequence similarity, structural topology and an identical catalytic mechanism. Cyclooxygenase enzymes catalyse two sequential reactions in spatially distinct, but mechanistically coupled active sites. The initial cyclooxygenase reaction converts arachidonic acid (which is achiral) to prostaglandin G2 (which has five chiral centres). The subsequent peroxidase reaction reduces prostaglandin G2 to prostaglandin H2. Here we report the co-crystal structures of murine apo-cyclooxygenase-2 in complex with arachidonic acid and prostaglandin. These structures suggest the molecular basis for the stereospecificity of prostaglandin G2 synthesis. 相似文献
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Fu WX Gong SY Qian XP Li Y Zhu ML Dong XY Li Y Chen WF 《Cellular and molecular life sciences : CMLS》2004,61(15):1935-1945
Mouse platelet basic protein (CXCL7/mPBP) was cloned from thymic stromal cells and further identification
indicated that it was expressed in thymic monocytes/macrophages (Mo/Ms). Recombinant mPBP was
chemoattractive for target cells of polymorphonuclear leucocytes, peritoneal Mo/Ms and splenic lymphocytes
with distinct potencies. CXCR2 was identified to be a cognate receptor for mPBP. Mouse thymocyte subsets of
CD4-CD8- double-negative (DN),
CD4+CD8+ double-positive (DP),
CD4+CD8- single-positive (CD4SP)
and CD4-CD8+ single-positive (CD8SP) expressed cell surface
CXCR2 with different positive percentages and expression levels. mPBP was chemoattractive for thymocyte subsets
with the potency order DN>DP> CD8SP>CD4SP, consistent with the levels of CXCR2 expressed on the
respective cells. Thus, mPBP in thymus is functionally redundant with chemokine CXCL12/ SDF-1. Moreover, our
finding that thymic Mo/Ms can produce mPBP implies that they may have other functions apart from acting as
scavengers in thymus.Received 25 March 2004; received after revision 10 May 2004; accepted 8 June 2004 相似文献