全文获取类型
收费全文 | 17081篇 |
免费 | 54篇 |
专业分类
林业 | 3644篇 |
农学 | 1312篇 |
基础科学 | 139篇 |
2868篇 | |
综合类 | 796篇 |
农作物 | 2138篇 |
水产渔业 | 1812篇 |
畜牧兽医 | 1377篇 |
园艺 | 1117篇 |
植物保护 | 1932篇 |
出版年
2023年 | 7篇 |
2022年 | 18篇 |
2021年 | 24篇 |
2020年 | 15篇 |
2019年 | 13篇 |
2018年 | 2759篇 |
2017年 | 2716篇 |
2016年 | 1208篇 |
2015年 | 84篇 |
2014年 | 41篇 |
2013年 | 36篇 |
2012年 | 829篇 |
2011年 | 2179篇 |
2010年 | 2124篇 |
2009年 | 1276篇 |
2008年 | 1371篇 |
2007年 | 1618篇 |
2006年 | 64篇 |
2005年 | 119篇 |
2004年 | 135篇 |
2003年 | 172篇 |
2002年 | 71篇 |
2001年 | 28篇 |
2000年 | 57篇 |
1999年 | 8篇 |
1997年 | 5篇 |
1996年 | 3篇 |
1994年 | 5篇 |
1993年 | 16篇 |
1992年 | 13篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 12篇 |
1987年 | 3篇 |
1979年 | 3篇 |
1977年 | 5篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1972年 | 6篇 |
1971年 | 4篇 |
1969年 | 4篇 |
1968年 | 6篇 |
1965年 | 3篇 |
1939年 | 3篇 |
1929年 | 2篇 |
1916年 | 2篇 |
1913年 | 2篇 |
1912年 | 2篇 |
1911年 | 2篇 |
1896年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
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
992.
Jianguo Wu 《Landscape Ecology》2011,26(10):1345-1349
993.
The extent of noise and its impacts continues to grow globally indicating a different approach from regulating individual
noise sources is needed. We pose the argument that soundscapes, or the acoustic environment, should be managed as a common-pool
resource (CPR). Using CPR theory, we argue that soundscapes possess key features of CPRs: (1) multiple soundscape users, (2)
difficulty of exclusion, and (3) subtractability and degradation. Using Ostrom’s Social-Ecological Systems (SES) framework,
we describe the main elements of soundscapes to consider for their sustainable management. In order to assess noise issues
and challenges in managing national park soundscapes, we conducted interviews with U.S. National Park Service managers at
parks identified as having air tour overflight impacts. While most managers indicated that aircraft overflights posed the
most serious impacts to park resources and visitor experiences, the park units also experienced several other types of noise
impacts including traffic on park roads, park maintenance operations, and different types of motorized recreational vehicles.
Addressing single sources of noise is necessary, as is the case with air tour overflights, but we argue that a more comprehensive
approach is needed to protect park soundscapes. From this study several SES framework variables emerged that need to be addressed
for sustainable management, such as the lack of clear soundscape boundaries, availability of acoustic monitoring and data,
and the number and types of soundscape users. Based on CPR theory and using the SES framework, the challenges and a potential
new approach for sustainable management are discussed. 相似文献
994.
María Uriarte Charles B. Yackulic Yili Lim Javier A. Arce-Nazario 《Landscape Ecology》2011,26(8):1151-1164
There is a pressing need to understand the consequences of human activities, such as land transformations, on watershed ecosystem
services. This is a challenging task because different indicators of water quality and yield are expected to vary in their
responsiveness to large versus local-scale heterogeneity in land use and land cover (LUC). Here we rely on water quality data
collected between 1977 and 2000 from dozens of gauge stations in Puerto Rico together with precipitation data and land cover
maps to (1) quantify impacts of spatial heterogeneity in LUC on several water quality indicators; (2) determine the spatial
scale at which this heterogeneity influences water quality; and (3) examine how antecedent precipitation modulates these impacts.
Our models explained 30–58% of observed variance in water quality metrics. Temporal variation in antecedent precipitation
and changes in LUC between measurements periods rather than spatial variation in LUC accounted for the majority of variation
in water quality. Urbanization and pasture development generally degraded water quality while agriculture and secondary forest
re-growth had mixed impacts. The spatial scale over which LUC influenced water quality differed across indicators. Turbidity
and dissolved oxygen (DO) responded to LUC in large-scale watersheds, in-stream nitrogen concentrations to LUC in riparian
buffers of large watersheds, and fecal matter content and in-stream phosphorus concentration to LUC at the sub-watershed scale.
Stream discharge modulated impacts of LUC on water quality for most of the metrics. Our findings highlight the importance
of considering multiple spatial scales for understanding the impacts of human activities on watershed ecosystem services. 相似文献
995.
Positive effects of forest fragmentation,independent of forest amount,on bat abundance in eastern Ontario,Canada 总被引:1,自引:0,他引:1
While studies have found that bat abundance is positively related to the amount of forest cover in a landscape, the effects
of forest fragmentation (breaking apart of forest, independent of amount) are less certain, with some indirect evidence for
positive effects of fragmentation. However, in most of these studies, the variables used to quantify fragmentation are confounded
with forest amount, making it difficult to interpret the results. The purpose of this study was to examine how forest amount
and forest fragmentation independently affect bat abundance. We conducted acoustic bat surveys at the centers of 22 landscapes
throughout eastern Ontario, Canada, where landscapes were chosen to avoid a correlation between forest amount and forest fragmentation
(number of patches) at multiple spatial scales, while simultaneously controlling for other variables that could affect bat
activity. We found that the effects of forest amount on bat relative abundance were mixed across species (positive for Lasiurus borealis, negative for Perimyotis subflavus and Lasionycteris noctivagans). When there was evidence for an effect of forest fragmentation, independent of forest amount, on bat relative abundance,
the effect was positive (Myotis septentrionalis, Myotis lucifugus and Lasiurus borealis). We suggest that the mechanism driving the positive responses to fragmentation is higher landscape complementation in more
fragmented landscapes; that is, increased access to both foraging and roosting sites for these bat species. We conclude that
fragmented landscapes that maximize complementation between roosting and foraging sites should support a higher diversity
and abundance of bats. 相似文献
996.
997.
Luis J. Villanueva-Rivera Bryan C. Pijanowski Jarrod Doucette Burak Pekin 《Landscape Ecology》2011,26(9):1233-1246
In this paper we present an introduction to the physical characteristics of sound, basic recording principles as well as several
ways to analyze digital sound files using spectrogram analysis. This paper is designed to be a “primer” which we hope will
encourage landscape ecologists to study soundscapes. This primer uses data from a long-term study that are analyzed using
common software tools. The paper presents these analyses as exercises. Spectrogram analyses are presented here introducing
indices familiar to ecologists (e.g., Shannon’s diversity, evenness, dominance) and GIS experts (patch analysis). A supplemental
online tutorial provides detailed instructions with step by step directions for these exercises. We discuss specific terms
when working with digital sound analysis, comment on the state of the art in acoustic analysis and present recommendations
for future research. 相似文献
998.
Soundscape ecology is being proposed as a new synthesis that leverages two important fields of study: landscape ecology and
acoustic ecology. These fields have had a rich history. Running “in parallel” for over three decades now, soundscape ecology
has the potential to unite these two (among other) fields in ways that provide new perspectives on the acoustics of landscapes.
Each of us was involved in the “birth” of these two fields. We each reflect here on the rich history of landscape ecology
and acoustic ecology and provide some thoughts on the future of soundscape ecology as a new perspective. 相似文献
999.
1000.
Populations at the periphery of a species’ range often show reduced genetic variability within populations and increased genetic
divergence among populations compared to those at the core, but the mechanisms that give rise to this core-periphery pattern
in genetic structure can be multifaceted. Peripheral population characteristics may be a product of historical processes,
such as founder effects or population expansion, or due to the contemporary influence of landscape context on gene flow. We
sampled collared lizards (Crotaphytus collaris) at four locations within the northern Flint Hills of Kansas, which is at the northern periphery of their range, to determine
the genetic variability and extent of genetic divergence among populations for ten microsatellite loci (n = 229). We found low genetic variability (average allelic richness = 3.37 ± 0.23 SE; average heterozygosity = 0.54 ± 0.05
SE) and moderate population divergence (average FST = 0.08 ± 0.01 SE) among our sample sites relative to estimates reported in the literature at the core of the species’ range
in Texas. We also identified differences in dispersal rates among sampling locations. Gene flow within the Flint Hills was
thus greater than for other peripheral populations of collared lizards, such as the Missouri glade system where most of the
mesic grasslands have been converted to forest since the last glacial retreat, which appears to have greatly impeded gene
flow among populations. Our findings signify the importance of considering landscape context when evaluating core-peripheral
trends in genetic diversity and population structure. 相似文献