全文获取类型
收费全文 | 3121篇 |
免费 | 148篇 |
国内免费 | 56篇 |
专业分类
3325篇 |
出版年
2024年 | 48篇 |
2023年 | 51篇 |
2022年 | 55篇 |
2021年 | 81篇 |
2020年 | 108篇 |
2019年 | 111篇 |
2018年 | 91篇 |
2017年 | 112篇 |
2016年 | 115篇 |
2015年 | 84篇 |
2014年 | 104篇 |
2013年 | 156篇 |
2012年 | 120篇 |
2011年 | 104篇 |
2010年 | 118篇 |
2009年 | 142篇 |
2008年 | 161篇 |
2007年 | 146篇 |
2006年 | 133篇 |
2005年 | 106篇 |
2004年 | 124篇 |
2003年 | 101篇 |
2002年 | 87篇 |
2001年 | 88篇 |
2000年 | 75篇 |
1999年 | 71篇 |
1998年 | 55篇 |
1997年 | 55篇 |
1996年 | 60篇 |
1995年 | 43篇 |
1994年 | 43篇 |
1993年 | 36篇 |
1992年 | 30篇 |
1991年 | 33篇 |
1990年 | 31篇 |
1989年 | 32篇 |
1988年 | 14篇 |
1987年 | 17篇 |
1985年 | 21篇 |
1984年 | 25篇 |
1983年 | 9篇 |
1982年 | 15篇 |
1981年 | 16篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1973年 | 12篇 |
排序方式: 共有3325条查询结果,搜索用时 15 毫秒
991.
992.
ADAM D. MORRIS DARREN A. MILLER MATINA C. KALCOUNIS-RUEPPELL 《The Journal of wildlife management》2010,74(1):26-34
ABSTRACT Forest edges often have increased species richness and abundance (edge effect) and affect spatial behaviors of species and dynamics of species interactions. Landscapes of intensively managed pine (Pinus spp.) stands are characterized by a mosaic of patches and linear forest edges. Managed pine forests are a primary landscape feature of the southeastern United States, but the effects of intensive management on bat communities are poorly understood. Insectivorous bats are important top predators in nocturnal forest food webs. We examined bat foraging behavior along forest edges and in 4 structurally distinct stand types (open-canopy pine, prethinned pine, thinned pine, and unmanaged forest) within a managed pine forest in the coastal plain of North Carolina, USA. During May-August, 2006 and 2007, we recorded echolocation calls using Pettersson D240X bat detectors linked to digital recorders at 156 sites. We also sampled nocturnal flying insects at each site using Malaise insect traps. We used negative binomial count regression models to describe bat foraging behavior relative to forest edges, stand types, and prey availability. Although some species showed affinities for certain stand types and prey items, bat activity patterns were most strongly related to forest edges. Edges were used extensively by 6 aerial-hunting bat species, but avoided by Myotis species. Forest edges function similarly to natural forest gaps, by providing foraging opportunities for aerial-hunting bat species. Therefore, the maintenance of forest edges in managed pine landscapes may enhance foraging habitat for aerial-hunting bat species. 相似文献
993.
Assessing the prevalence of hybridization between sympatric Canis species surrounding the red wolf (Canis rufus) recovery area in North Carolina 总被引:1,自引:0,他引:1
Predicting spatial patterns of hybridization is important for evolutionary and conservation biology yet are hampered by poor understanding of how hybridizing species can interact. This is especially pertinent in contact zones where hybridizing populations are sympatric. In this study, we examined the extent of red wolf (Canis rufus) colonization and introgression where the species contacts a coyote (C. latrans) population in North Carolina, USA. We surveyed 22,000km(2) in the winter of 2008 for scat and identified individual canids through genetic analysis. Of 614 collected scats, 250 were assigned to canids by mitochondrial DNA (mtDNA) sequencing. Canid samples were genotyped at 6-17 microsatellite loci (nDNA) and assigned to species using three admixture criteria implemented in two Bayesian clustering programs. We genotyped 82 individuals but none were identified as red wolves. Two individuals had red wolf mtDNA but no significant red wolf nDNA ancestry. One individual possessed significant red wolf nDNA ancestry (approximately 30%) using all criteria, although seven other individuals showed evidence of red wolf ancestry (11-21%) using the relaxed criterion. Overall, seven individuals were classified as hybrids using the conservative criteria and 37 using the relaxed criterion. We found evidence of dog (C. familiaris) and gray wolf (C. lupus) introgression into the coyote population. We compared the performance of different methods and criteria by analyzing known red wolves and hybrids. These results suggest that red wolf colonization and introgression in North Carolina is minimal and provide insights into the utility of Bayesian clustering methods to detect hybridization. 相似文献
994.
利用遥感植被指数分析中国东部样带农业生态系统的生产力格局 总被引:5,自引:2,他引:5
在研究全球变化与陆地生态系统之间的关系方面 ,样带研究被认为是十分重要的手段之一。中国东部从南到北沿热量梯度出现的植被变化特点 ,形成世界上独特完整的以热量梯度驱动的植被连续带 ;加之中国地处世界上最大的“欧亚大陆”最东段 ,具有独特的大陆季风气候 ,样带内农业生态系统的各种变化具有区域代表性和典型性 ,使之成为全球变化研究中不可缺少的部分。目前 ,全球变化的研究范围和内容已经超越了传统生态系统研究所关注的斑块水平 ,要把斑块水平的生态系统研究成果拓展到区域乃至全球的空间尺度上 ,就需要在尺度转换上取得技术和方法… 相似文献
995.
We present a Bayesian approach to analyze matched \"case-control\" data with multiple disease states. The probability of disease development is described by a multinomial logistic regression model. The exposure distribution depends on the disease state and could vary across strata. In such a model, the number of stratum effect parameters grows in direct proportion to the sample size leading to inconsistent MLEs for the parameters of interest even when one uses a retrospective conditional likelihood. We adopt a semiparametric Bayesian framework instead, assuming a Dirichlet process prior with a mixing normal distribution on the distribution of the stratum effects. We also account for possible missingness in the exposure variable in our model. The actual estimation is carried out through a Markov chain Monte Carlo numerical integration scheme. The proposed methodology is illustrated through simulation and an example of a matched study on low birth weight of newborns (Hosmer, D. A. and Lemeshow, S., 2000, Applied Logistic Regression) with two possible disease groups matched with a control group. 相似文献
996.
Dr. Hassen A. Muttlak 《Biometrical journal. Biometrische Zeitschrift》1995,37(7):799-810
Ranked set sampling (RSS) as suggested by MCINTYRE (1952) and TAKAHASI and WAKIMOTO (1968) may be used to estimate the parameters of the simple regression line. The objective is to use the RSS method to increase the efficiency of the estimators relative to the simple random sampling (SRS) method. Estimators of the slope and intercept are considered. Computer simulated results are given, and an example using real data presented to illustrate the computations. 相似文献
997.
The diel activity patterns of fishes in a temperate New Brunswick stream were studied during the summer over 5 years. Young‐of‐the year Atlantic salmon Salmo salar and blacknose dace Rhinichthys atratulus were more active during the day than at night, whereas lake chub Couesius plumbeus, brook trout Salvelinus fontinalis and adult white suckers Catostomus commersonii were more active at night than during the day. Because fishes were as likely to be nocturnal as diurnal, the data suggest that more night‐time sampling is needed to provide an unbiased view of fish community structure in temperate streams. 相似文献
998.
Lisa R. Hodges Heather O'Connell Matthew J. Arduino 《Journal of microbiological methods》2010,81(2):141-146
Twelve Laboratory Response Network (LRN) affiliated laboratories participated in a validation study of a macrofoam swab protocol for the recovery, detection, and quantification of viable B. anthracis (BA) Sterne spores from steel surfaces. CDC personnel inoculated steel coupons (26 cm2) with 1-4 log10 BA spores and recovered them by sampling with pre-moistened macrofoam swabs. Phase 1 (P1) of the study evaluated swabs containing BA only, while dust and background organisms were added to swabs in Phase 2 (P2) to mimic environmental conditions. Laboratories processed swabs and enumerated spores by culturing eluted swab suspensions and counting colonies with morphology consistent with BA. Processed swabs were placed in enrichment broth, incubated 24 h, and cultured by streaking for isolation. Real-time PCR was performed on selected colonies from P2 samples to confirm the identity of BA. Mean percent recovery (%R) of spores from the surface ranged from 15.8 to 31.0% (P1) and from 27.9 to 55.0% (P2). The highest mean percent recovery was 31.0% (sd 10.9%) for P1 (4 log10 inoculum) and 55.0% (sd 27.6%) for P2 (1 log10 inoculum). The overall %R was higher for P2 (44.6%) than P1 (24.1%), but the overall reproducibility (between-lab variability) was lower in P2 than in P1 (25.0 vs 16.5%CV, respectively). The overall precision (within-lab variability) was close to identical for P1 and P2 (44.0 and 44.1, respectively), but varied greatly between inoculum levels. The protocol demonstrated linearity in %R over the three inoculum levels and is able to detect between 26 and 5 × 106 spores/26 cm2. Sensitivity as determined by culture was > 98.3% for both phases and all inocula, suggesting that the culture method maintains sensitivity in the presence of contaminants. The enrichment broth method alone was less sensitive for sampled swabs (66.4%) during P2, suggesting that the presence of background organisms inhibited growth or isolation of BA from the broth. The addition of real-time PCR testing to the assay increased specificity from > 85.4% to > 95.0% in P2. Although the precision was low at the 1 log10 inoculum level in both phases (59.0 and 50.2%), this swab processing protocol, was sensitive, specific, precise, and reproducible at 2-4 log10/26 cm2 spore concentrations. 相似文献
999.
In this paper, a two‐phase sampling estimator for a stratified population mean using two auxiliary variables x and z is considered when the stratum mean of x is unknown but that of z is known. The suggested estimator under its optimal condition is found to be more efficient than the one using only x. 相似文献
1000.
David A. Cimprich 《Journal of Field Ornithology》2009,80(1):94-100
ABSTRACT. Distance sampling applied to point count surveys (point transects) has become a common method for estimating the absolute abundance of birds. When conducting point transects, detections of focal species are typically recorded during a fixed time interval. However, count duration has varied among studies and the effect of such variation on the resulting abundance estimates is unclear. My objective was to examine the effect of count duration on abundance estimates of male Black-capped Vireos ( Vireo atricapilla ). The abundance of these vireos in a 349-ha area in central Texas was estimated using 3-, 5-, and 6-min point transects and results were then compared to actual number present as determined by banding and territory mapping. The 3-min counts provided an estimate that was 26% greater than the actual number of male Vireos present ( N = 201), but this number was within the corresponding 95% confidence interval ( N = 157–413). Confidence intervals for the 5- and 6-min counts did not include the actual number of vireos present. The shortest count duration may have provided the most accurate abundance estimate because male Black-capped Vireos are typically active, sing intermittently, and sometimes move tens of meters between songs. Thus, shorter-duration counts may also yield the most accurate abundance estimates for other species that exhibit similar behavior. However, because behavior varies among species, I recommend that investigators collect preliminary data to establish an appropriate count duration when accurate estimates of absolute, rather than relative, abundance are important. 相似文献