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Studies of the time development of empirical networks usually investigate late stages where lasting connections have already stabilized. Empirical data on early network history are rare but needed for a better understanding of how social network topology develops in real life. Studying students who are beginning their studies at a university with no or few prior connections to each other offers a unique opportunity to investigate the formation and early development of link patterns and community structure in social networks. During a nine week introductory physics course, first year physics students were asked to identify those with whom they communicated about problem solving in physics during the preceding week. We use these students'' self reports to produce time dependent student interaction networks. We investigate these networks to elucidate possible effects of different student attributes in early network formation. Changes in the weekly number of links show that while roughly half of all links change from week to week, students also reestablish a growing number of links as they progress through their first weeks of study. Using the Infomap community detection algorithm, we show that the networks exhibit community structure, and we use non-network student attributes, such as gender and end-of-course grade to characterize communities during their formation. Specifically, we develop a segregation measure and show that students structure themselves according to gender and pre-organized sections (in which students engage in problem solving and laboratory work), but not according to end-of-coure grade. Alluvial diagrams of consecutive weeks'' communities show that while student movement between groups are erratic in the beginnning of their studies, they stabilize somewhat towards the end of the course. Taken together, the analyses imply that student interaction networks stabilize quickly and that students establish collaborations based on who is immediately available to them and on observable personal characteristics.  相似文献   
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Purpose

Life cycle assessments (LCAs) that attempt to provide advice on treatment options for phosphorus (P) containing organic waste products encounter problems related to the quantification of mineral P fertilizer substitution, P loss and crop P uptake after land application. The purpose of this study was to develop a relatively easy to use life cycle inventory model, known as PLCI, that could be used to estimate these values.

Methods

A life cycle inventory model for P was developed, which estimates the effect of an application of organic waste followed by ordinary fertilizer management in the modeling period. This was compared with a simulation without the initial waste application. The difference in mineral P fertilizer application (substitution), P loss and crop P uptake was then calculated and expressed as a proportion of the amount of waste applied. As an example, the effect of an initial application of mineral fertilizer, sewage sludge and ash on two farm types was simulated. These results were applied in an LCA case study of different sewage sludge treatment options.

Results and discussion

Farm type influenced the P fertilizer substitution, loss and crop uptake factors. The application on an arable farm showed a substitution of 28 to 31%, relatively low P loss and a large spread in crop P uptake for the different P sources, compared with the pig farm. Application on a pig farm showed no mineral P substitution. For substitution, mineral fertilizer outperformed waste product fertilizer with a short modeling period, due to higher immediate P availability, which was not the case with a long period. The LCA case study showed that the P substitution factor had an influence on the environmental impact categories climate change and depletion of reserve-based abiotic resources while the P loss factor influenced freshwater eutrophication. Application of the P loss and substitution factors generated from the PLCI model resulted in higher environmental burdens and lower savings than using conventional factors.

Conclusions

The soil P status mainly affected P substitution and loss, with the fertilizer type only having a small influence when soils had a low P status. The PLCI model can facilitate more coherent and rigorous estimates of P substitution and loss to be used in LCA studies involving application of waste products on agricultural land. This is important since P substitution and loss can have an important influence on impact categories, such as freshwater eutrophication and resource depletion.
  相似文献   
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Seed dispersal is a key process in plant community dynamics, and soil seed banks represent seed dispersal in time rather than in space. Despite their potential importance, seed bank dynamics in the Arctic are poorly understood. We investigated soil seed banks and corresponding plant community composition in three contrasting vegetation types in West Greenland, viz. dwarf shrub heaths, herb slopes and fell‐fields. Through germination testing, 31 species were documented in soil seed banks. All of these were herbaceous, while no dwarf‐shrub species, which represents the dominating growth form in the above‐ground vegetation, were emerging from the seed bank. Consequently, across vegetation types, the lowest similarity between seed bank and above‐ground vegetation was found in dwarf shrub heath. Nine plant species were exclusively found in seed bank, all of which were non‐clonal forbs. Seed bank size (total number of seeds) and species richness seemed to increase with the level of natural disturbance. Additionally, we examined the effect of different experimental light and temperature conditions on the quantity and diversity of germinating seeds. The difference in diversity in vegetation and seed bank at the species level will impact population dynamics, regeneration of vegetation after disturbances and its potential to respond to climate change.  相似文献   
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Glycoprotein D (gD) of herpes simplex virus type 1 (HSV-1) was modified to encode targeting signals known to localize proteins to either the endoplasmic reticulum (ER) or the trans-Golgi network. These motifs conferred the predicted targeting properties on gD in transfected cells as judged by immunofluorescence staining, and the exclusion of targeted gD from the cell surface was confirmed by the fact that these molecules exhibited substantially reduced activity in cell-cell fusion assays. Recombinant viruses expressing Golgi-targeted forms of gD grew to wild-type levels in noncomplementing cells, exhibited unaltered particle/infectivity ratios, and were found to contain wild-type levels of gD, whereas a recombinant expressing ER-retained gD was helper cell dependent and, when grown on noncomplementing cells, produced virions of low specific infectivity with greatly reduced levels of gD. These data imply that HSV-1 acquires its final membrane from a post-ER compartment and lend support to the view that the virus undergoes de-envelopment and reenvelopment steps during virus egress.  相似文献   
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Swidden agriculture, once the dominant form of land use throughout the uplands and much of the lowlands of Southeast Asia, is being replaced by other land uses. While change and adaptation are inherent to swiddening, the current rapid and widespread transitions are unprecedented. In this paper we review some recent findings on changes in biodiversity, especially plant diversity at various scales, as swidden farming is replaced by other land uses. We focus particularly on two areas of Southeast Asia: northern Thailand and West Kalimantan. We examine actual and potential changes in the diversity of crops that characterize regional swidden systems, as well as that of the spontaneously occurring plants that appear in swidden fields and fallows. Severe declines in plant diversity have been observed in most areas and at most spatial scales when swidden is replaced by permanent land use systems. However, shifts away from swidden agriculture do not invariably result in drastic declines or losses of biological diversity, but may maintain or even enhance it, particularly at finer spatial scales. We suggest that further research is necessary to understand the effects of swidden transitions on biodiversity.  相似文献   
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Intrauterine growth retardation (IUGR) is associated with a central fat distribution and risk of developing type 2 diabetes in adults when exposed to a sedentary Western lifestyle. Increased lipolysis is an early defect of metabolism in IUGR subjects, but the sites and molecular mechanisms involved are unknown. Twenty IUGR and 20 control (CON) subjects, aged 20-30 years, were studied before and after 10 days of bed rest using the glucose clamp technique combined with measurements of in vivo metabolism by microdialysis technique and blood flow by (133)Xe washout technique in subcutaneous abdominal (SCAAT) and femoral (SCFAT) adipose tissue. Additionally, mRNA expression of lipases was evaluated in biopsies from SCAAT. Lipolysis in SCAAT was substantially higher in IUGR than in CON subjects despite markedly lower mRNA expression of lipases. Blood flow was higher in IUGR compared with CON in both SCAAT and SCFAT. Whole body insulin sensitivity did not differ between groups and decreased after bed rest. After bed rest, SCAAT lipolysis remained higher in IUGR compared with CON, and SCFAT lipolysis decreased in CON but not in IUGR. Prior to the development of whole body insulin resistance, young men with IUGR are characterized by increased in vivo adipose tissue lipolysis and blood flow with a paradoxically decreased expression of lipases compared with CON, and 10 days of physical inactivity underlined the baseline findings. Subjects with IUGR exhibit primary defects in adipose tissue metabolism.  相似文献   
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