首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5845篇
  免费   176篇
  国内免费   109篇
  6130篇
  2024年   139篇
  2023年   181篇
  2022年   138篇
  2021年   218篇
  2020年   262篇
  2019年   312篇
  2018年   245篇
  2017年   237篇
  2016年   206篇
  2015年   211篇
  2014年   247篇
  2013年   303篇
  2012年   197篇
  2011年   197篇
  2010年   181篇
  2009年   249篇
  2008年   247篇
  2007年   252篇
  2006年   195篇
  2005年   201篇
  2004年   194篇
  2003年   177篇
  2002年   181篇
  2001年   136篇
  2000年   135篇
  1999年   110篇
  1998年   72篇
  1997年   63篇
  1996年   41篇
  1995年   42篇
  1994年   45篇
  1993年   39篇
  1992年   40篇
  1991年   42篇
  1990年   27篇
  1989年   24篇
  1988年   37篇
  1987年   41篇
  1986年   21篇
  1985年   31篇
  1984年   20篇
  1983年   28篇
  1982年   27篇
  1981年   28篇
  1980年   23篇
  1979年   16篇
  1978年   16篇
  1977年   11篇
  1976年   12篇
  1971年   6篇
排序方式: 共有6130条查询结果,搜索用时 31 毫秒
991.
Aim This study investigated spatial patterns of endemism in the flora of Namibia's succulent karoo in order to generate information for conservation planning. Location The study area, the Sperrgebiet, comprises the majority of Namibia's portion of the succulent karoo biome which is the south‐west corner of the country. This is an arid area that has been off limits to public access, farming and tourism for nearly a century due to restrictions imposed by the diamond industry. Methods Based on existing distribution records, areas of high concentrations of endemic plants were identified using numbers of endemics and weighted endemics according to area of occupancy. The resolution of the available data was quarter degree squares (15‐min intervals of latitude and longitude grids). Results At the scale of this study straight numbers of endemics generated similar results to the endemics weighted according to area of occupancy, which gives sparsely distributed species a higher weighting. Based on the current distribution records, 17.7% (184 species) of the Sperrgebiet's spermatophyte flora is endemic. The ‘hotspots of endemism’ comprised from north to south: Lüderitz‐Kowisberge, Klinghardt Mountains, Aurusberge‐Heioab, Witpütz, Skorpion and Obib‐Schakalsberge. Taking also areas into account that stand out because of their high proportion of local endemics, this adds Grillental and the central coastal area from Pomona to Baker's Bay to the areas of importance for plant endemism. Main conclusions The Sperrgebiet's endemic flora is special in taxonomic composition in that it does not present a subset of the total flora of this area, but shows a remarkably high representation of the families Mesembryanthemaceae and Liliaceae (sensu lato). Compared with other arid areas, the level of endemism in the Sperrgebiet is high, but not compared with the succulent karoo in general or other hotspots in the succulent karoo biome, such as the Richtersveld. The proportion of local endemics (13.5%) is high compared with some endemism hotspots in southern Africa. Hotspots of plant endemism provide an important tool to contribute to conservation planning studies. This study also highlighted the importance of centralized data bases without which these analyses would not have been possible. Further plant collecting is required to fill presently data‐deficient areas and studies at a finer spatial resolution taking habitat requirements into account are needed to elucidate some of the factors contributing to plant endemism in this area.  相似文献   
992.
Reiter  Jerome P. 《Biometrika》2008,95(4):933-946
When some of the records used to estimate the imputation modelsin multiple imputation are not used or available for analysis,the usual multiple imputation variance estimator has positivebias. We present an alternative approach that enables unbiasedestimation of variances and, hence, calibrated inferences insuch contexts. First, using all records, the imputer samplesm values of the parameters of the imputation model. Second,for each parameter draw, the imputer simulates the missing valuesfor all records n times. From these mn completed datasets, theimputer can analyse or disseminate the appropriate subset ofrecords. We develop methods for interval estimation and significancetesting for this approach. Methods are presented in the contextof multiple imputation for measurement error.  相似文献   
993.
Humpback whales were instrumented with satellite transmitters off the western Antarctic Peninsula in January of 2004–2006 to examine their movement patterns and habitat use. Whales were tracked from 4 to 80 days (mean = 36.5 days). Distance and travel rate estimates for nine individuals ranged from 223 to 4,356 km and from 17 to 75 km/day, respectively. Considerable individual variation was observed in direction, speed and range of movements. The overall pattern was characterized by short- and long-distance movements between presumed foraging areas with relatively short residency times. Travel rates were lower at these sites, characterized by erratic movements, than during traveling between them. Area usage for six individuals based on the 95% fixed kernel home range with least squares cross-validation ranged from 2,771 to 172,356 km2. The management boundary between the feeding grounds associated with Breeding Stocks G and A needs revision, as current available data suggest it should be located to the east of 50°W. This study is the first to present detailed information on the movements of humpback whales in the Southern Ocean.  相似文献   
994.
995.
1. Thirteen time series, varying from 17 to 27 years, of the abundance of Vespula germanica and V. vulgaris from lowland England are examined. The time series depend on either spring queens and workers taken in Malaise or suction traps, or collected colonies. 2. During the late 1970s and early 1980s, the abundance of V. germanica declined abruptly but that of V. vulgaris did not. 3. During the early 1980s, the 2‐year cycle of annual abundance of V. vulgaris changed to a nearly perfectly damped pattern of annual abundances. 4. The most likely factor causing these population changes was the increased use of pesticides acting directly by killing the wasps and indirectly by reducing their food resources. 5. The difference in response of the two species to increased pesticide use may be related to a difference in foraging ability.  相似文献   
996.
    
Modelling spatial and temporal patterns in ecology is imperative to understand the complex processes inherent in ecological phenomena. Log-Gaussian Cox processes are a popular choice among ecologists to describe the spatiotemporal distribution of point-referenced data. In addition, point pattern models where events instigate others nearby (i.e., self-exciting behaviour) are becoming increasingly popular to infer the contagious nature of events (e.g., animal sightings). While there are existing R packages that facilitate fitting spatiotemporal point processes and, separately, self-exciting models, none incorporate both. We present an R package, stelfi, that fits spatiotemporal self-exciting and log-Gaussian Cox process models using Template Model Builder through a range of custom-written C++ templates. We illustrate the use of stelfi's functions fitting models to Sasquatch (bigfoot) sightings data within the USA. The structure of these data is typical of many seen in ecology studies. We show, from a temporal Hawkes process to a spatiotemporal self-exciting model, how the models offered by the package enable additional insights into the temporal and spatial progression of point pattern data. We present extensions to these well-known models that include spatiotemporal self-excitation and joint likelihood models, which are better suited to capture the complex mechanisms inherent in many ecological data. The package stelfi offers user-friendly functionality, is open source, and is available from CRAN. It offers the implementation of complex spatiotemporal point process models in R for applications even beyond the field of ecology.  相似文献   
997.
    
Open science (OS) awareness and skills are increasingly becoming an essential part of everyday scientific work as e.g., many journals require authors to share data. However, following an OS workflow can seem challenging at first. Thus, instructions by journals and other guidelines are important. But how comprehensive are they in the field of ecology and evolutionary biology (Ecol Evol)? To find this out, we reviewed 20 published OS guideline articles aimed for ecologists or evolutionary biologists, together with the data policies of 17 Ecol Evol journals to chart the current landscape of OS guidelines in the field, find potential gaps, identify field-specific barriers for OS and discuss solutions to overcome these challenges. We found that many of the guideline articles covered similar topics, despite being written for a narrow field or specific target audience. Likewise, many of the guideline articles mentioned similar obstacles that could hinder or postpone a transition to open data sharing. Thus, there could be a need for a more widely known, general OS guideline for Ecol Evol. Following the same guideline could also enhance the uniformity of the OS practices carried on in the field. However, some topics, like long-term experiments and physical samples, were mentioned surprisingly seldom, although they are typical issues in Ecol Evol. Of the journals, 15 out of 17 expected or at least encouraged data sharing either for all articles or under specific conditions, e.g. for registered reports and 10 of those required data sharing at the submission phase. The coverage of journal data policies varied greatly between journals, from practically non-existing to very extensive. As journals can contribute greatly by leading the way and making open data useful, we recommend that the publishers and journals would invest in clear and comprehensive data policies and instructions for authors.  相似文献   
998.
    
Because it can lead to retaliatory killing, livestock depredation by large carnivores is among the foremost threats to carnivore conservation, and it severely impacts human well-being worldwide. Ongoing climate change can amplify these human–wildlife conflicts, but such issues are largely unexplored, though are becoming increasingly recognized. Here, we assessed how the availability of primary resources and wild prey interact to shape large carnivore selection for livestock rather than wild prey (i.e., via prey switching or apparent competition). Specifically, we combined remotely sensed estimates of primary resources (i.e., water availability and primary productivity), wild prey movement, and 7 years (2015–2021) of reports for livestock depredation by African lions (Panthera leo) in the Makgadikgadi Pans ecosystem, Botswana. Although livestock depredation did not vary between wet versus dry seasons, analyses at finer temporal scales revealed higher incidences of livestock depredation when primary production, water availability, and wild prey availability were lower, though the effects of wild prey availability were mediated by water availability. Increased precipitation also amplified livestock depredation events despite having no influence on wild prey availability. Our results suggest that livestock depredation is influenced by the diverse responses of livestock, wild prey, and lions to primary resource availability, a driver that is largely overlooked or oversimplified in studies of human–carnivore conflict. Our findings provide insight into tailoring potential conflict mitigation strategies to fine-scale changes in resource conditions to efficiently reduce conflict and support human livelihoods.  相似文献   
999.
    
Crab species are increasingly important socioeconomic resources that are threatened by human exploitation, climate change, and invasive species, such as European green crabs (Carcinus maenas). However, the continued health of their populations is often uncertain given the limited long-term population data, necessitating alternate approaches to ensure their continued viability. Furthermore, C. maenas are one of the most highly invasive and destructive marine species globally, posing a threat to local ecosystems and species, including socioeconomically important crabs and their mollusc prey. Improved understanding of C. maenas invasions and their impacts on local crab and mollusc resources is therefore vitally important. Here, we present a new method for identifying species-level presence and relative abundances of important crab species, including invasive C. maenas, from the scars they leave on their prey. We conducted controlled manipulative feeding experiments in which individuals of Dungeness crabs (Metacarcinus magister), red rock crabs (Cancer productus), and C. maenas, were allowed to attack snails (Tegula funebralis) and produce sublethal shell damage. Resulting shell damage was photographed and landmarked for geometric morphometric analyses to determine any differences in the shape of shell damage between crab species. There were statistically significant differences between the shape of shell damage created by all three crab species (p < .0001). Shell damage formed a gradient from narrow/deep (C. productus) to shallow/wide (C. maenas) with M. magister as an intermediate form. Our method provides a novel, cost-effective tool for long-term species-specific reconstructions of crab populations and assessing the broader ecological impacts of C. maenas invasions that can inform management and mitigation for these three important crab species.  相似文献   
1000.
热熔挤出技术是指将药物、增塑剂和聚合物等辅料在熔融状态下混合后,以一定的压力、速度和形状挤出形成药物产品的技术,对于改善并提高药物溶解度、改进制备缓控制剂方法具有重要意义。以生物医药产业领域的热熔挤出专利为分析样本,采用专利数据库数据及“定量+定性”分析方法,从专利产出趋势、主要竞争力量、热点技术等方面分析热熔挤出技术在生物医药产业领域中的应用,以期为该领域的研究决策提供参考。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号