全文获取类型
收费全文 | 365篇 |
免费 | 55篇 |
国内免费 | 25篇 |
出版年
2024年 | 1篇 |
2023年 | 19篇 |
2022年 | 11篇 |
2021年 | 21篇 |
2020年 | 12篇 |
2019年 | 34篇 |
2018年 | 17篇 |
2017年 | 25篇 |
2016年 | 17篇 |
2015年 | 14篇 |
2014年 | 19篇 |
2013年 | 19篇 |
2012年 | 15篇 |
2011年 | 14篇 |
2010年 | 18篇 |
2009年 | 19篇 |
2008年 | 23篇 |
2007年 | 19篇 |
2006年 | 14篇 |
2005年 | 13篇 |
2004年 | 10篇 |
2003年 | 16篇 |
2002年 | 11篇 |
2001年 | 18篇 |
2000年 | 9篇 |
1999年 | 11篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1994年 | 4篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1983年 | 1篇 |
排序方式: 共有445条查询结果,搜索用时 15 毫秒
81.
Conservation International's biodiversity hotspots are areas of high vascular plant endemism combined with high levels of habitat destruction and land use change. Although such hotspots have also been shown to be centres for terrestrial vertebrate endemism, much less is known about how well these areas function as hotspots for other less well‐studied groups, including the hyperdiverse arthropods, other invertebrates and fungi. Because there is a close evolutionary and ecological relationship between insects and plants, we suggest that the potential role of plants as umbrella species for herbivorous insects, potentially herbivorous fungi and nematodes, and parasitic insects should be explored. Finally, we reflect on the increasing social, economic, human conflict and governance issues and the impacts of increasing land use change and global climate change that threaten the biodiversity hotspot system. 相似文献
82.
S.O. Koch S.L. Chown A.L.V. Davis S. Endrödy-Younga A.S. van Jaarsveld 《Journal of Insect Conservation》2000,4(1):45-56
Despite being the focus of an international research effort spanning decades, the spatial distribution of southern African scarab beetles remains poorly documented. As well as reinforcing the magnitude of the challenge facing biodiversity scientists, this raises real concerns about best practice conservation strategies in the absence of detailed distribution information. However, dung beetles appear to be well represented in established conservation areas. This apparent contradiction could be ascribed to anthropogenic transformation, successful conservation efforts, the presence of dung generalists and reserve-biased or mesic-biased dung beetle collection efforts. It is suggested that all of the above contribute to the observed pattern to varying degrees. The implications of selecting areas that are either rich in species, contain rare species or contain taxonomically distinct species from a group whose taxonomy is well known but for which inadequate distribution data exist are explored. Best practice, in the face of inadequate data, appears to revolve around a subtle interplay between advantages and disadvantages associated with data interpolation techniques, reserve selection algorithms that use criteria more robust than database rarity (such as taxonomic distinctiveness) and the long-term economic costs of proceeding with the data at hand versus investing in biological surveys. 相似文献
83.
This study examined the inter-annual variation in macroinvertebrate assemblages in six wilderness streams in central Idaho over a 6-year period (1990–1995). Benthic macroinvertebrates and associated environmental correlates were sampled during baseflow each summer. Little environmental change, as assessed using coefficients of variation (CVs) for substrate size and embeddedness, width, depth and periphyton standing crops, occurred in the streams over the period of study. There was also little temporal change in macroinvertebrate assemblages based on the relative abundance of the 10 most abundant taxa, all shredder taxa and all plecopteran taxa. CVs for individual taxa were substantially greater than those of most community measures, with rare taxa contributing 30–50% of the variation for any one stream. Frequency distributions for taxa CVs excluding rare taxa were more normally distributed. Differences in assemblage structure among streams were attributed to stream size (shift in shredder assemblages) and temperature (shift in plecopteran taxa). These data indicate a long-term (multi-year) persistence in the macroinvertebrate composition of these pristine streams, thus supporting the premise that such streams are excellent references for use in long-term biomonitoring programs. 相似文献
84.
85.
86.
87.
Hyeyeong Choe James H. Thorne Robert Hijmans Changwan Seo 《Ecology and evolution》2019,9(3):1353-1363
Increasingly large presence‐only survey datasets are becoming available for use in conservation assessments. Potentially, these records could be used to determine spatial patterns of plant species rarity and endemism. We test the integration of a large South Korean species record database with Rabinowitz rarity classes. Rabinowitz proposed seven classes of species rarity using three variables: geographic range, habitat specificity, and local population size. We estimated the range size and local abundance of 2,215 plant species from species occurrence records and habitat specificity as the number of landcover types each species’ records were found in. We classified each species into a rarity class or as common, compared species composition by class to national lists, and mapped the spatial pattern of species richness for each rarity class. Species were classed to narrow or wide geographic ranges using 315 km, the average from a range size index of all species (Dmax), based on maximum distance between observations. There were four classes each within the narrow and wide range groups, sorted using cutoffs of local abundance and habitat specificity. Nationally listed endangered species only appeared in the narrow‐range classes, while nationally listed endemic species appeared in almost all classes. Species richness in most rarity classes was high in northeastern South Korea especially for species with narrow ranges. Policy implications. Large presence‐only surveys may be able to estimate some classes of rarity better than others, but modification to include estimates of local abundance and habitat types, could greatly increase their utility. Application of the Rabinowitz rarity framework to such surveys can extend their utility beyond species distribution models and can identify areas that need further surveys and for conservation priority. Future studies should be aware of the subjectivity of the rarity classification and that regional scale implementations of the framework may differ. 相似文献
88.
IS50-mediated inverse transposition: specificity and precision 总被引:4,自引:0,他引:4
The IS50 elements, which are present as inverted repeats in the kanamycin-resistance transposon, Tn5, can move in unison carrying with them any interstitial DNA segment. In consequence, DNA molecules such as a lambda::Tn5 phage genome are composed of two overlapping transposons - the kan segment bracketed by IS50 elements (Tn5), and lambda bracketed by IS50 elements. During direct transposition, mediated by IS50 "O" (outside) ends, the kan gene is moved and the lambda vector is left behind. During inverse transposition, mediated by the "I" (inside) ends of the IS50 elements, the lambda vector segment is moved and the kan gene is left behind. Direct transposition is several orders of magnitude more frequent than inverse transposition (Isberg and Syvanen, 1981; Sasakawa and Berg, 1982). We assessed the specificity and precision of the rare events mediated by pairs of I ends by mapping and sequencing independent inverse transpositions from a lambda::Tn5 phage into the amp and tet genes of plasmid pBR322. Using restriction analyses, 32 and 40 distinct sites of insertion were found among 46 and 72 independent inverse transpositions into the amp and tet genes, respectively. Eleven sites were used in two or more insertion events, and the two sites in tet used most frequently corresponded to major hotspots for the insertion of the Tn5 (by direct transposition). The sequences of 22 sites of inverse transposition (including each of the sites used more than once) were determined, in eleven cases by analyzing both pBR322-IS50 junctions, and in eleven others by sequencing one junction. The sequence of the "I" end of IS50 was preserved and 9-bp target sequence duplications were present in every case analyzed. GC pairs were found at each end of the target sequence duplication in ten of the eleven sites used more than once, and also in seven of the other eleven sites. Our data indicate that transposition mediated by pairs of "I" ends is similar in its specificity and precision to the more frequent transposition mediated by IS50 "O" ends. 相似文献
89.
Thermodynamic Approach to Evaluate the Criticality of Raw Materials and Its Application through a Material Flow Analysis in Europe
下载免费PDF全文
![点击此处可从《Journal of Industrial Ecology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
This paper makes a review of current raw material criticality assessment methodologies and proposes a new approach based on the second law of thermodynamics. This is because conventional methods mostly focus on supply risk and economic importance leaving behind relevant factors, such as the physical quality of substances. The new approach is proposed as an additional dimension for the criticality assessment of raw materials through a variable denoted “thermodynamic rarity,” which accounts for the exergy cost required to obtain a mineral commodity from bare rock, using prevailing technology. Accordingly, a given raw material will be thermodynamically rare if it is: (1) currently energy intensive to obtain and (2) scarce in nature. If a given commodity presents a high risk in two of the three dimensions (economic importance, supply risk, and thermodynamic rarity), it is proposed to be critical. As a result, a new critical material list is presented, adding to the 2014 criticality list of the European Commission (EC) Li, Ta, Te, V, and Mo. With this new list and using Sankey diagrams, a material flow analysis has been carried out for Europe (EU‐28) for 2014, comparing the results when using tonnage and thermodynamic rarity as units of measure. Through the latter, one can put emphasis on the quality and not only on the quantity of minerals traded and domestically produced in the region, thereby providing a tool for improving resource management. 相似文献
90.
Vehicles and Critical Raw Materials: A Sustainability Assessment Using Thermodynamic Rarity
下载免费PDF全文
![点击此处可从《Journal of Industrial Ecology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Abel Ortego Alicia Valero Antonio Valero Eliette Restrepo 《Journal of Industrial Ecology》2018,22(5):1005-1015
The changing material composition of cars represents a challenge for future recycling of end‐of‐life vehicles (ELVs). Particularly, as current recycling targets are based solely on mass, critical metals increasingly used in cars might be lost during recycling processes, due to their small mass compared to bulk metals such as Fe and Al. We investigate a complementary indicator to material value in passenger vehicles based on exergy. The indicator is called thermodynamic rarity and represents the exergy cost (GJ) needed for producing a given material from bare rock to the market. According to our results, the thermodynamic rarity of critical metals used in cars, in most cases, supersedes that of the bulk metals that are the current focus of ELV recycling. While Fe, Al, and Cu account for more than 90% of the car's metal content, they only represent 60% of the total rarity of a car. In contrast, while Mo, Co, Nb, and Ni account for less than 1% of the car's metal content, their contribution to the car's rarity is larger than 7%. Rarity increases with the electrification level due to the greater amount of critical metals used; specifically, due to an increased use of (1) Al alloys are mainly used in the car's body‐in‐white of electric cars for light‐weighting purposes, (2) Cu in car electronics, and (3) Co, Li, Ni, and rare earth metals (La, Nd, and Pr) in Li‐ion and NiMH batteries. 相似文献