全文获取类型
收费全文 | 7450篇 |
免费 | 44篇 |
国内免费 | 14篇 |
专业分类
林业 | 724篇 |
农学 | 234篇 |
基础科学 | 49篇 |
912篇 | |
综合类 | 333篇 |
农作物 | 505篇 |
水产渔业 | 460篇 |
畜牧兽医 | 3589篇 |
园艺 | 198篇 |
植物保护 | 504篇 |
出版年
2022年 | 11篇 |
2019年 | 9篇 |
2018年 | 117篇 |
2017年 | 184篇 |
2016年 | 168篇 |
2015年 | 232篇 |
2014年 | 200篇 |
2013年 | 154篇 |
2012年 | 447篇 |
2011年 | 301篇 |
2010年 | 108篇 |
2009年 | 71篇 |
2008年 | 64篇 |
2007年 | 38篇 |
2006年 | 40篇 |
2005年 | 774篇 |
2004年 | 652篇 |
2003年 | 445篇 |
2002年 | 111篇 |
2001年 | 130篇 |
2000年 | 32篇 |
1999年 | 146篇 |
1992年 | 117篇 |
1991年 | 145篇 |
1990年 | 168篇 |
1989年 | 147篇 |
1988年 | 171篇 |
1987年 | 113篇 |
1986年 | 143篇 |
1985年 | 117篇 |
1984年 | 91篇 |
1983年 | 81篇 |
1982年 | 33篇 |
1981年 | 17篇 |
1980年 | 23篇 |
1979年 | 133篇 |
1978年 | 96篇 |
1977年 | 91篇 |
1976年 | 77篇 |
1975年 | 137篇 |
1974年 | 129篇 |
1973年 | 116篇 |
1972年 | 138篇 |
1971年 | 118篇 |
1970年 | 88篇 |
1969年 | 102篇 |
1968年 | 113篇 |
1967年 | 125篇 |
1966年 | 102篇 |
1965年 | 70篇 |
排序方式: 共有7508条查询结果,搜索用时 737 毫秒
991.
992.
993.
We sampled 35 lakes in northern Wisconsin to determine the presence of Orconectes rusticus, the rusty crayfish, and related this pattern to several parameters pertaining to potential invasion routes that could influence
the distribution of these crayfish in the lakes. The presence of rusty crayfish in lakes was positively related to an index
of human use and negatively related to the length of stream connections to other lakes containing the crayfish. Humans appear
to act as vectors allowing crayfish to travel along discontinuous routes that otherwise would be inaccessible to them, and
thus, provide crayfish with spatially discontinuous corridors that increase the probability of movement by channelizing the
space between lakes. In contrast, streams correspond closely to the traditional definition of terrestrial corridors, in that
they are spatially continuous. This distribution pattern suggests colonization operating via two corridors with two spatial
scales, making management of the invasion of rusty crayfish complex. 相似文献
994.
Faith R.?KearnsEmail author N. Maggi?Kelly James L.?Carter Vincent H.?Resh 《Landscape Ecology》2005,20(1):113-125
Freshwater research and management efforts could be greatly enhanced by a better understanding of the relationship between landscape-scale factors and water quality indicators. This is particularly true in urban areas, where land transformation impacts stream systems at a variety of scales. Despite advances in landscape quantification methods, several studies attempting to elucidate the relationship between land use/land cover (LULC) and water quality have resulted in mixed conclusions. However, these studies have largely relied on compositional landscape metrics. For urban and urbanizing watersheds in particular, the use of metrics that capture spatial pattern may further aid in distinguishing the effects of various urban growth patterns, as well as exploring the interplay between environmental and socioeconomic variables. However, to be truly useful for freshwater applications, pattern metrics must be optimized based on characteristic watershed properties and common water quality point sampling methods. Using a freely available LULC data set for the Santa Clara Basin, California, USA, we quantified landscape composition and configuration for subwatershed areas upstream of individual sampling sites, reducing the number of metrics based on: (1) sensitivity to changes in extent and (2) redundancy, as determined by a multivariate factor analysis. The first two factors, interpreted as (1) patch density and distribution and (2) patch shape and landscape subdivision, explained approximately 85% of the variation in the data set, and are highly reflective of the heterogeneous urban development pattern found in the study area. Although offering slightly less explanatory power, compositional metrics can provide important contextual information. 相似文献
995.
Florent?MouillotEmail author Jean-Pierre?Ratte Richard?Joffre David?Mouillot and ?Serge Rambal 《Landscape Ecology》2005,20(1):101-112
Two hundred years of landscape changes were studied on a 3,760 ha area of central Corsica (France) representing a typical Mediterranean environment. Different historical sources, including an accurate land-cover map from 1774 and statistics on land cover from 1848 and 1913, were used. Three additional maps (1960, 1975 and 1990) were drawn, and a complete fire history from 1957 to 1997 was created. Forests expanded slowly by a border effect. Forest expansion was more rapid in unburnt sites (0.59% per year) than in burnt sites (0.23% per year), mostly because the initial amount of forests was greater. Because of the border effect, the combination of past landscape pattern and short distance colonization abilities of forest species may have allowed the shrublands to persist in some places after land abandonment. This persistence may explain the pattern of fire in the landscape, since shrubland burn more readily than forests. 相似文献
996.
Maria C.S.?NunesEmail author Maria J.?Vasconcelos José M.C.?Pereira Nairanjana?Dasgupta Richard J.?Alldredge Francisco C.?Rego 《Landscape Ecology》2005,20(6):661-673
The purpose of this study is to investigate if, or under what conditions, fires select given land cover types for burning.
If fires burn unselectively then the land cover composition (the proportional area of various land cover types) of individual
fires should approximate the land cover composition available in their neighborhood. In this study we test this hypothesis
by performing statistical analyses of a data set consisting of paired vectors with the proportions of land cover types present
in burned areas and in their respective surroundings. The statistical methods employed (a permutation technique and the Cmax statistic) are commonly used in resource selection studies where data is subject to a unit-sum constraint. The results of
the analysis of 506 fires that burned in Portugal in 1991 indicate that fires are selective, with small fires exhibiting stronger
land cover preferences than large fires. According to the results of a multiple comparison analysis performed for small fires,
there is a marked preference for shrubland followed by other forest cover types, while agriculture is clearly avoided. A similar
analysis is performed to test if fire selectivity is related to the ecological region where it occurs. The results obtained
in this study contribute to the discussion on the relative importance of fuels as a drivers of fire spread. 相似文献
997.
It is reasonable to assume that there is a relationship between the spatial distribution of forest fuels and fire hazards.
Therefore, if fire risk is to be included into numerical forest planning, the spatial distribution of risky and non-risky
forest stands should be taken into account. The present study combines a stand-level fire risk model and landscape level optimization
to solve forest planning problems in which the fire risk plays an important role. The key point of the method was to calculate
forest level fire resistance metrics from stand level indices and use these metrics as objective variables in numerical optimization.
This study shows that maximizing different landscape metrics produces very different landscape configurations with respect
to the spatial arrangement of resistant and risky stands. The landscapes obtained by maximizing different metrics were tested
with a fire spread simulator. These tests suggested that the mean fire resistance of the landscape, which is a non-spatial
metric, is the most important factor affecting the burned area. However, spatial landscape metrics that decrease the continuity
of fire resistance in the landscape can significantly improve the fire resistance of the landscape when used as additional
objective variables. 相似文献
998.
999.
1000.
In animal systems, several methods exist for the direct delivery of nucleic acids and proteins into cells for functional analysis.
Until recently, these methods have not been applied to plant systems. Now, however, several preliminary reports suggest that
both nucleic acids and proteins can also be delivered into plant cells by very simple, direct application. This promises to
open the way for high-throughput screening for gene function in a range of plant species. 相似文献