全文获取类型
收费全文 | 17157篇 |
免费 | 44篇 |
国内免费 | 8篇 |
专业分类
林业 | 3654篇 |
农学 | 1339篇 |
基础科学 | 140篇 |
2951篇 | |
综合类 | 790篇 |
农作物 | 2190篇 |
水产渔业 | 1832篇 |
畜牧兽医 | 1274篇 |
园艺 | 1128篇 |
植物保护 | 1911篇 |
出版年
2023年 | 2篇 |
2022年 | 19篇 |
2021年 | 24篇 |
2020年 | 17篇 |
2019年 | 27篇 |
2018年 | 2757篇 |
2017年 | 2715篇 |
2016年 | 1199篇 |
2015年 | 99篇 |
2014年 | 44篇 |
2013年 | 55篇 |
2012年 | 842篇 |
2011年 | 2196篇 |
2010年 | 2123篇 |
2009年 | 1272篇 |
2008年 | 1357篇 |
2007年 | 1612篇 |
2006年 | 70篇 |
2005年 | 135篇 |
2004年 | 133篇 |
2003年 | 181篇 |
2002年 | 86篇 |
2001年 | 27篇 |
2000年 | 63篇 |
1999年 | 15篇 |
1998年 | 7篇 |
1997年 | 3篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 16篇 |
1992年 | 10篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 16篇 |
1988年 | 16篇 |
1987年 | 4篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 6篇 |
1972年 | 3篇 |
1969年 | 5篇 |
1968年 | 5篇 |
1967年 | 2篇 |
1965年 | 2篇 |
1964年 | 3篇 |
1962年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
991.
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
992.
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
993.
A GIS-based method for the reconstruction of the late eighteenth century forest vegetation in the Prignitz region (NE Germany) 总被引:1,自引:0,他引:1
Our goal was to reconstruct the late eighteenth century forest vegetation of the Prignitz region (NE Germany) at a scale of
1:50,000. We also wanted to relate the historical forest vegetation to the actual and potential natural vegetation. For these
purposes, we selected 15 woody species and transferred relevant data found in historical records from various sources together
with the recent localities of (very) old individuals belonging to these woody species into ArcView GIS. Following multi-step
data processing including the generation of a point density layer using a moving window with kernel estimation and derivation
of vegetation units applying Boolean algebra rules together with information on site conditions, we derived 17 forest communities
corresponding to the potential natural vegetation. We were able to reconstruct the historical forest vegetation for 90% of
the forest area ca. 1780. Only two of the 17 forest communities covered large parts of the forested area. The oak forest with
Agrostis capillaris covered about 44% of the total forest area, and alder forests on fenland made up about 37%. Oak-hornbeam forests with Stellaria holostea comprised slightly less than 6% of the forest area, while all other forest communities comprised less than 1%. The historical
forest vegetation is more similar to the potential forest vegetation and quite different from the actual forest vegetation
because coniferous tree species currently cover approximately two-thirds of the actual forest area. The most beneficial result
of this study is the map of high-resolution historical vegetation units that may serve as the basis for various further studies,
e.g., modelling long-term changes in biodiversity at the landscape scale. 相似文献
994.
995.
Modelling dynamics of ecosystem services basket in Mediterranean landscapes: a tool for rational management 总被引:1,自引:1,他引:0
Natural ecosystems are life-supporting systems providing diverse ecosystem services (ESs) and benefits to human societies:
e.g., food and clean water, recreation opportunities or climate regulation. The contribution of natural and semi-natural ecosystems
to the provision of such services depends to a large extent on vegetation structure and composition, which, in turn, change
as a result of interactions between human decisions about land management, and spontaneous biological and environmental processes.
Rational management of these dynamic ecosystems requires an ability to predict short- and long-term effects of management
decisions on the desired ESs. The vegetation then contributes to, and modifies, the products and services obtained from the
land. We applied mathematical modeling to study these complex relationships. We developed a model for a Mediterranean ecosystem
which predicts the dynamics of multiple services in response to management scenarios, mediated by vegetation changes. Six
representative ESs representing different groups were selected, based on available scientific information, for a detailed
study: (1) density of geophytes, (2) potential contribution to honey production, (3) energy density of fleshy fruits foraged
by birds, (4) forage for goats, (5) forage for cattle, and (6) carbon retention in woody plants. Mean contributions to each
service by different vegetation cover types were estimated, and the overall service provided by the site was calculated as
a weighted mean of these contributions. Services were measured in their appropriate units and subsequently standardized to
a percentage of the maximum value observed in the study area. We attempted to combine all studied ESs, despite their different
nature, into one “ESs basket”. This paper presents the dynamics of simulated vegetation composition and values of services
in response to management scenarios involving grazing, fire and their combinations. Our approach can help land managers to
evaluate alternative management scenarios by presenting the “services basket” obtained from the entire managed area. 相似文献
996.
Stephanie Kramer-Schadt Tobias S. Kaiser Karin Frank Thorsten Wiegand 《Landscape Ecology》2011,26(4):501-513
With habitat loss and fragmentation having become two of the major threats to the viability of species, the question of how
to manage landscapes for species conservation has attracted much attention. In this context, the planning of stepping stones
has been proposed to increase connectivity in fragmented landscapes. We present a simulation study with a neutral landscape
approach to assess the effects of stepping stones on colonization success. To that end, we used a spatially explicit, calibrated
population model of the European lynx (Lynx lynx) coupled with structured landscapes, in which we could control the landscape parameters of dispersal habitat coverage and
contagion, as well as the number and size of stepping stones available for breeding. In general, we found that colonization
success increased with increasing habitat coverage but decreased with increasing habitat contagion, while the introduction
of stepping stones had significant effects in critical situations. Especially at low to medium dispersal habitat coverage
and high disperser mortality, stepping stones had a positive effect on colonization success when they were large enough to
produce new dispersers, but negative effects when they were small and located in a way that dispersers would be distracted
from more suitable breeding habitat patches. The latter clearly constituted a shading effect and argues for a thorough consideration
of the trade-offs related to stepping stone size and location when implementing stepping stones as a conservation measure,
especially when the number of individuals of conservation concern is low. 相似文献
997.
Protection of biodiversity and ecosystem functions requires a better understanding of spatial diversity. Here we studied diversity
patterns of true bugs and saproxylic beetles, sampled in 28 forest stands of southern Germany, using a hierarchical nested
design of five increasingly broader spatial levels: trap location, stratum, forest stand, forest site, and ecoregion. We predicted
that: (1) for large body-sized species (as a surrogate for highly mobile species) and host generalist species (low host specificity),
the proportion of β-diversity decreases from small to large spatial scales; and (2) the differences between trait-based functional
guilds in the proportion of β-diversity increase with increasing weighting of more-abundant species. Our results indicated
that the ecoregion level is the most important diversity scale for both taxa and among functional guilds sampled, followed
by the forest stand level. Specialized species were more strongly affected on the ecoregion level than generalist species.
Differences in the proportion of β-diversity between functional guilds increased with increasing weighting of abundant species.
The β-diversity patterns based on body size and host specificity were similar for true bugs, but partly contrasting for saproxylic
beetles. Our results suggest that (1) future conservation schemes should focus on establishing new conservation sites in new
ecoregions, rather than on enlarging existing protected areas; (2) host specificity might be a more meaningful trait than
body size to be considered in biodiversity studies; and (3) common conservation approaches restricted to only large, conspicuous,
but rare species might result in a mismatch of important biodiversity scales. 相似文献
998.
999.
Rebecca M. Montague-Drake David B. Lindenmayer Ross B. Cunningham John A. Stein 《Landscape Ecology》2011,26(10):1383-1394
We explored the effects of a purported ‘reverse keystone species’, the Noisy Miner (Manorina melanocephala) using a long-term, large-scale dataset. Specifically, we identify whether this aggressive bird affects the landscape distribution
patterns of other avifauna, by displacing them into, or restricting their distribution to, less productive areas, and in so
doing, adheres to ‘isoleg theory’. We sought to determine the effect of abundance of the Noisy Miner on the abundance of other
birds (individual species and groups), and determine whether that effect was consistent with varying site productivity, using
a negative binomial distribution with a logarithmic link function, and an offset variable to account for variations in search
effort. Relationships between abundance of Noisy Miners and habitat variables were examined using a Poisson distribution with
a logarithmic link function scaled for extra-variation (quasi-Poisson regression). We demonstrate that when Noisy Miner abundance
is low, many small passerine species are more abundant on high productivity sites. However, as Noisy Miner abundance increases,
small passerine abundance decreases, with this decrease most apparent on productive sites. The same patterns were not evident
for birds considered ‘non-competitors’ of the Noisy Miner. We identify that both site productivity and vegetation structure influence the abundance of the Noisy Miner. We reveal that the species increasingly tolerates ‘less
desirable’ habitat attributes with increasing site productivity. The preference of the Noisy Miner for productive areas is
likely to have deleterious impacts on the long-term survival and reproductive success of other Australian woodland bird species,
many of which have already undergone severe declines. 相似文献
1000.
Jianguo Wu 《Landscape Ecology》2011,26(10):1345-1349