全文获取类型
收费全文 | 16661篇 |
免费 | 18篇 |
专业分类
林业 | 3638篇 |
农学 | 1301篇 |
基础科学 | 141篇 |
2801篇 | |
综合类 | 713篇 |
农作物 | 2145篇 |
水产渔业 | 1825篇 |
畜牧兽医 | 1120篇 |
园艺 | 1119篇 |
植物保护 | 1876篇 |
出版年
2023年 | 2篇 |
2022年 | 12篇 |
2021年 | 16篇 |
2020年 | 19篇 |
2019年 | 25篇 |
2018年 | 2759篇 |
2017年 | 2717篇 |
2016年 | 1193篇 |
2015年 | 76篇 |
2014年 | 28篇 |
2013年 | 40篇 |
2012年 | 808篇 |
2011年 | 2145篇 |
2010年 | 2112篇 |
2009年 | 1262篇 |
2008年 | 1325篇 |
2007年 | 1581篇 |
2006年 | 34篇 |
2005年 | 103篇 |
2004年 | 105篇 |
2003年 | 153篇 |
2002年 | 60篇 |
2001年 | 5篇 |
2000年 | 41篇 |
1999年 | 1篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 12篇 |
1992年 | 7篇 |
1990年 | 1篇 |
1989年 | 7篇 |
1988年 | 11篇 |
1987年 | 2篇 |
1977年 | 4篇 |
1972年 | 1篇 |
1969年 | 1篇 |
1968年 | 4篇 |
1967年 | 1篇 |
1945年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
In the absence of access to formal credit, informal contracts with independent investors give the small ranchers of the Lower Amazon an acceptable means through which to surmount the high investment hurdle of starting a cattle herd. These contracts – called sociedades – allow small ranchers to raise an outside investor's cattle in return for a portion of the offspring and are commonplace in the cattle production systems of the Amazon. But, notwithstanding a vast literature on cattle production in the Amazon, informal contracts have been largely overlooked. This paper presents the results of a field survey and financial analysis for informal contracts on small ranches in the Lower Amazon. In the results, we suggest that informal contracts are an important means of cattle herd start-up and herd production for small ranchers. Internal rates of returns in cattle production under these contracts are in the range of –7 to –12% for the small rancher and 12% for the investor. The net present value of the contract to the small rancher ranges from –R$1219 to –R$8599 and from R$1681 to R$8845 for the investor, for a 10-year period, depending on herd size. Financial returns contracts are sensitive to the contract design – e.g., to who pays health costs – and to beef prices. The small ranchers have a negative IRR, lose money, and bear all the risk of loss, yet persist in using this form of herd development. We surmise that this is due to the non-financial benefits of cattle ownership and the lack of access to formal credit structures. In conclusion, although outside the formal economy and apparently financially unrewarding, these contracts are an important mechanism by which the small ranchers on the Amazon Floodplains create cattle herds. 相似文献
993.
994.
Fahad S. Khan Qamar U. Zaman Young K. Chang Aitazaz A. Farooque Arnold W. Schumann Ali Madani 《Precision Agriculture》2016,17(2):155-167
Wild blueberry (Vaccinium angustifolium Ait.) fields in the north east Canada are naturally grown in a course textured thin layer of soil and below this layer is a soilless layer of gravel. The root zone depth of this crop varies from 10 to 15 cm. Investigating the depth to the gravel layer below the course textured soil is advantageous, as it affects the water holding capacity of the root zone. Water and nutrient management are the two primary determinants of crop yield and the amount of leaching. The objective of this study was to estimate the depth to the gravel layer using DualEM-2 instrument. A C++ program written in Visual Studio 2010 was used to develop mathematical models for estimating the depth to the gravel layer from the outputs of DualEM-2 sensor. Two wild blueberry fields were selected in central Nova Scotia, Canada to evaluate the performance of DualEM-2 instrument in estimating the rootzone depth above the gravel layer. The mid points of squares created by grid lines were used as the sampling points at each experimental site. The actual depth to the interface was measured manually at selected grid points (n = 50). The apparent ground conductivity (ECa) values of DualEM-2 were recorded and the depth to the interface was estimated for the same sampling points within the selected fields. The fruit yield samples were also collected from the same grid points to identify the impact of the depth to the gravel layer on crop yield. After calibrations, comprehensive surveys were conducted and the actual and estimated depths to the interface were established. The interpolated maps of fruit yield, and the actual (zin) and estimated (\( {\text{z}}_{\text{in}}^{*} \)) depths to the interface were created in ArcGIS 10 software. Results indicated that the zin was significantly correlated with \( {\text{z}}_{\text{in}}^{*} \) for the North River (R 2 = 0.73; RMSE = 0.27 m) and the Carmel (R 2 = 0.45; RMSE = 0.20 m) sites. Results revealed that the areas with shallow depth to the gravel layer were low yielding, indicating that the variation in the depth to the gravel layer can have an impact on crop productivity. Non-destructive estimations of the depth to the gravel layer can be used to develop erosion control strategies, which will result in an increased crop production. 相似文献
995.
996.
The erection of dams alters habitat and longitudinal stream connectivity for migratory diadromous and potamodromous fish species
and interrupts much of organismal exchange between freshwater and marine ecosystems. In the US, this disruption began with
colonial settlement in the seventeenth century but little quantitative assessment of historical impact on accessible habitat
and population size has been conducted. We used published surveys, GIS layers and historical documents to create a database
of 1356 dams, which was then analyzed to determine the historical timeline of construction, use and resultant fragmentation
of watersheds in Maine, US. Historical information on the anadromous river herring was used to determine natural upstream
boundaries to migration and establish total potential alewife spawning habitat in nine watersheds with historic populations.
Dams in Maine were constructed beginning in 1634 and by 1850 had reduced accessible lake area to less than 5% of the virgin
892 km2 habitat and 20% of virgin stream habitat. There is a near total loss of accessible habitat by 1860 that followed a west-east
pattern of European migration and settlement. Understanding historic trends allows current restoration targets to be assessed
and prioritized within an ecosystem-based perspective and may inform expectations for future management of oceanic and freshwater
living resources. 相似文献
997.
We studied the effects of anthropogenic edges on predation and parasitism of forest bird nests in an agriculturally fragmented
landscape and a continuously forested landscape in Ontario, Canada. Nesting data were collected at 1937 nests across 10 species
in the fragmented landscape from 2002–2008, and 464 nests across 4 species in the continuously forested landscape from 2006–2008.
Brood parasitism only occurred in the fragmented landscape, and was positively related to the proportion of rural grassland
and row crop habitats within 500-m of nests. Daily nest survival was negatively related to the density of roads within 500-m
of nests in the fragmented landscape, but was not influenced by distance to anthropogenic edge in either landscape. Predation
rates were higher in the fragmented landscape for Ovenbird and Rose-breasted Grosbeak nests, but did not differ between landscapes
for Veery and American Redstart nests. Uniformly high predation in the fragmented landscape may be a result of (1) matrix
predators that penetrate deep (>300 m) into the forest interior, or (2) the additive effect of forest-dependent and matrix-associated
predators that results in high predation pressure in both edge and interior habitats. Further research focused on the identification
of nest predators, their population dynamics, and habitat use is required to understand the underlying mechanisms leading
to uniformly high nest predation in fragmented landscapes. 相似文献
998.
The tallgrass prairie of North America has undergone widespread habitat loss and fragmentation (<4% remains). The Flint Hills
region of Kansas and Oklahoma is the largest tallgrass prairie remaining and therefore provides an opportunity to study the
population genetic structure of grassland species in a relatively contiguous landscape and set a baseline for evaluating changes
when the habitat is fragmented. We adopted a landscape genetics approach to identify how landscape structure affected dispersal,
population genetic structure, and landscape connectivity of the Eastern Yellowbelly Racer (Coluber constrictor flaviventris) across a 13,500-km2 landscape in northeastern Kansas, USA. The racer population had high allelic diversity, high heterozygosity, and was maintaining
migration-drift equilibrium. Autocorrelation between genetic and geographic distance revealed that racers exhibited restricted
dispersal within 3 km, and isolation-by-distance. Significant isolation-by-distance occurred at broad regional scales (>100 km),
but because of sufficient gene flow between locations, we were unable to define discrete subpopulations using Bayesian clustering
analyses. Resistance distance, which considers the permeability of habitats, did not explain significant variation in genetic
distance beyond Euclidean distance alone, suggesting that racers are not currently influenced by landscape composition. In
northeastern Kansas, racers appear to be an abundant and continuously distributed snake that perceives the landscape as well
connected with no cover type currently impeding snake dispersal or gene flow. 相似文献
999.
Relative importance of management vs. design for implementation of large-scale ecological networks 总被引:1,自引:0,他引:1
Ecological networks (ENs) are used to mitigate landscape-scale habitat loss, and are managed and designed to conserve regional
biodiversity. In our study region in southern Africa, ENs of isolated grassland remnants are specifically set aside and managed
for conservation, and are complemented by corridor-like power line servitudes which are maintained by regular mowing. Using
grasshoppers, a sensitive and reliable bioindicator taxon, we determine whether ENs effectively conserve biodiversity. We
used cluster analysis and variation partitioning to select the best subset of environmental variables which explained the
patterns of species composition. We then compared the relative importance of environmental variables grouped by the scale
of their influence: local-scale variables affected by management practices vs. landscape-scale variables affected by design
of ENs. Management was consistently and significantly 2–5 times more influential than design in determining grasshopper assemblages
within ENs and servitudes. Servitudes had a higher proportion of bare ground, lower proportion of tall grasses and higher
abundance of grasshoppers relative to ENs. Three grasshopper species were strongly associated with servitudes and exhibited
traits consistent with early colonizers. As management actions are primarily responsible for vegetation succession, the use
of ENs for conservation efforts should first focus on appropriate management strategies, such as fire regime and grass height
management before altering the landscape structure (e.g. increasing connectivity or enlarging patches). The conservation implications
of these results are that, if ENs are managed and designed for heterogeneity and to simulate multiple successional stages,
they may be beneficial for biodiversity conservation. 相似文献
1000.
Salvador Pueyo 《Landscape Ecology》2011,26(3):305-309
There is an ongoing controversy on the use of the patch-size distribution as an early warning signal for abrupt shifts to
a desertified state in Mediterranean arid landscapes. This controversy started with Kéfi et al.’s suggestion that, when approaching
the transition point to widespread desertification, vegetation patches would switch from a power-law (PL) to a truncated power-law
(TPL) distribution. Here I show that, for fundamental reasons, no untruncated power law can be found in this context, irrespective
of the level of degradation. This result does not deny the importance of the findings by Kéfi et al., but means that these
have to be reinterpreted by moving from the PL/TPL dichotomy to other categorizations of the patch-size distribution. Physical
constraints on patch-size distributions have general interest for landscape ecology. 相似文献