全文获取类型
收费全文 | 2398篇 |
免费 | 200篇 |
国内免费 | 184篇 |
出版年
2023年 | 39篇 |
2022年 | 35篇 |
2021年 | 68篇 |
2020年 | 80篇 |
2019年 | 102篇 |
2018年 | 105篇 |
2017年 | 85篇 |
2016年 | 89篇 |
2015年 | 81篇 |
2014年 | 94篇 |
2013年 | 137篇 |
2012年 | 88篇 |
2011年 | 97篇 |
2010年 | 77篇 |
2009年 | 93篇 |
2008年 | 118篇 |
2007年 | 113篇 |
2006年 | 120篇 |
2005年 | 103篇 |
2004年 | 89篇 |
2003年 | 74篇 |
2002年 | 77篇 |
2001年 | 67篇 |
2000年 | 62篇 |
1999年 | 69篇 |
1998年 | 46篇 |
1997年 | 39篇 |
1996年 | 43篇 |
1995年 | 37篇 |
1994年 | 43篇 |
1993年 | 33篇 |
1992年 | 27篇 |
1991年 | 36篇 |
1990年 | 24篇 |
1989年 | 38篇 |
1988年 | 33篇 |
1987年 | 37篇 |
1986年 | 20篇 |
1985年 | 20篇 |
1984年 | 21篇 |
1983年 | 11篇 |
1982年 | 20篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1979年 | 13篇 |
1978年 | 10篇 |
1977年 | 10篇 |
1976年 | 7篇 |
1973年 | 8篇 |
1972年 | 7篇 |
排序方式: 共有2782条查询结果,搜索用时 15 毫秒
1.
Patterns generated from ecological surveys are rarely tested in similar habitats to assess the accuracy of predictions. Testing empirically derived predictions provides a strong tool for establishing the consistency of general patterns in ecology. We test the consistency of beetle community associations with habitat complexity in open canopy forests and make both community and morphospecies-level comparisons with results from a previous study. We use Normalized Difference Vegetation Indices (NDVIs) from remote sensing as a surrogate for habitat complexity. The positive relationships between NDVIs and site-based beetle species richness and abundance were consistent in open canopy forests both south and north of Sydney, Australia. NDVIs were also useful for predicting differences in beetle composition in open canopy forests. Taxon-specific responses to NDVI differences in 'southern forests' were very similar to responses in 'northern forests', most likely reflecting beetle trophic roles. This study shows that NDVIs can be used as rapid biodiversity indicators, when integrated with identified faunal responses to vegetation structure, provided that the lower vegetation strata may be measured by remote sensing. 相似文献
2.
Sheryl Coombs Ruth A. Conley 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1997,180(4):401-415
Extracellular, single unit recording techniques were used to measure the responses of posterior lateral line nerve fibers
to a 50-Hz dipole source that slowly changed its location along the length of the fish. The flow-field equations for a dipole
source were used to model the pressure gradient pattern and thus, the expected excitation pattern along a linear array of
lateral line receptor organs for different source locations. Finally, excitation patterns were similarly modeled along the
left and right side of the fish's head for actual steps taken by sculpin in approach pathways to the 50-Hz dipole source.
Spatial histograms of posterior lateral line nerve fiber responses to different locations of the dipole source could be predicted
from pressure gradient patterns modeled from the flow-field equations, confirming that the modeling approach applied to behavioral
results was a good predictor of excitation patterns likely to be encoded by the lateral line periphery. An examination of
how modeled excitation patterns changed from one position to the next in typical approach pathways and how patterns differed
between positions from which successful and unsuccessful strikes were launched suggests that approach and strike strategies
can indeed be explained by the information available in excitation patterns. In particular, changes in the spatial distribution
of pressure gradient directions (polarities), available only when the source is lateral (as opposed to directly in front of
the fish), appear to enhance the ability of sculpin to determine source distance. Without such information, misses are more
likely to occur and successful strikes are more likely to be launched from short distances only.
Accepted: 23 October 1996 相似文献
3.
4.
Genetic and spatial structure within a swift fox population 总被引:1,自引:0,他引:1
5.
Spatial distribution of ant workers and, notably their aggregation/segregation behaviour, is a key-element of the colony social
organization contributing to the efficiency of task performance and division of labour. In polymorphic species, specialized
worker castes notably differ in their intrinsic aggregation behaviour. In this context, knowing the preponderant role of minors
in brood care, we investigate how a stimulus such as brood can influence the spatial patterns of Pheidole pallidula worker castes. In a homogeneous area without brood, it was shown that minors display only a low level of aggregation while
majors form large clusters in the central area. Here we find out that these aggregation patterns of both minors and majors
can be deeply influenced by the presence of brood. For minors, it nucleates or enhances the formation of a large stable cluster.
Such high sensitivity of minors to brood stimuli fits well with their role as main brood tenders in the colony. For majors,
interattraction between individuals still remains the prevailing aggregation factor while brood strongly influences the localisation
of their cluster. We discuss how the balance between interattraction and sensitivity to environmental stimuli determines the
mobility of each worker castes and, consequently, the availability of minors and majors to participate in everyday colony
tasks. Moreover, we will evoke the functional value of majors’ cluster location close to the brood, namely with respect to
social regulation of the colony caste ratio.
Received 30 May 2005; revised 11 January 2006; accepted 13 January 2006. 相似文献
6.
7.
M. Matabos S. Plouviez S. Hourdez D. Desbruyères P. Legendre A. Warén D. Jollivet E. Thiébaut 《Journal of Biogeography》2011,38(3):575-594
Aim Deep‐sea hydrothermal vents have now been reported along all active mid‐ocean ridges and back‐arc basins, but the boundaries of biogeographic entities remain questionable owing to methodological issues. Here we examine biogeographic patterns of the vent fauna along the East Pacific Rise (EPR) and determine the relative roles of regional and local factors on the distribution of biodiversity associated with mussel beds along a poorly explored zone, the southern EPR (SEPR). Location East Pacific Rise. Methods A species list of macrobenthic invertebrates along the EPR was compiled from the literature and supplemented with data recovered during the French research cruise BIOSPEEDO carried out in 2004 along the SEPR. Biogeographic patterns were assessed by combining the identification of morphological species with a molecular barcoding approach. A multivariate regression tree (MRT) analysis was performed to identify any geographic breaks, and an empirical distribution of species richness was compared with predictions provided by a mid‐domain effect model. Macrofaunal community structure associated with mussel beds along the SEPR was analysed in relation to environmental factors using cluster and canonical redundancy analyses. Results Sequencing of the cytochrome c oxidase subunit I gene revealed the occurrence of several cryptic species complexes along the EPR, with the equator separating the southern and northern clades. Furthermore, during the BIOSPEEDO cruise at least 10 still unnamed species were collected between 7°25′ S and 21°33′ S. The shift in community structure identified by MRT analysis was located south of 17°34′ S or south of 13°59′ S, depending on the data used, suggesting that the southern part of the SEPR (17°25′–21°33′ S) constitutes a biogeographic transition zone in the vent fauna along the EPR. At a regional scale, latitude combined with the type of venting was significantly correlated with the community structure associated with mussel beds. Main conclusions Together, the molecular data, in situ observations, and the distribution of species suggest that the high diversity of vent fauna species presently observed between 17°25′ S and 21°33′ S is probably a result of the overlap of several distinct biogeographic provinces. We argue that this area thus constitutes a biogeographic vent fauna transition zone along the EPR. 相似文献
8.
9.
10.
The potential for and constraints on the evolution of compensatory ability in Asclepias syriaca 总被引:2,自引:0,他引:2
To investigate the potential for and constraints on the evolution of compensatory ability, we performed a greenhouse experiment
using Asclepias syriaca in which foliar damage and soil nutrient concentration were manipulated. Under low nutrient conditions, significant genetic
variation was detected for allocation patterns and for compensatory ability. Furthermore, resource allocation to storage was
positively, genetically correlated both with compensatory ability and biomass when damaged, the last two being positively,
genetically correlated with each other. Thus, in the low nutrient environment, compensatory ability via resource allocation
to storage provided greater biomass when damaged. A negative genetic correlation between compensatory ability and plant biomass
when undamaged suggests that this mechanism entailed an allocation cost, which would constrain the evolution of greater compensatory
ability when nutrients are limited. Under high nutrient conditions, neither compensatory ability nor allocation patterns predicted
biomass when damaged, even though genetic variation in compensatory ability existed. Instead, plant biomass when undamaged
predicted biomass when damaged. The differences in outcomes between the two nutrient treatments highlight the importance of
considering the possible range of environmental conditions that a genotype may experience. Furthermore, traits that conferred
compensatory ability did not necessarily contribute to biomass when damaged, demonstrating that it is critical to examine
both compensatory ability and biomass when damaged to determine whether selection by herbivores can favor the evolution of
increased compensation.
Received: 2 April 1999 / Accepted: 21 September 1999 相似文献