全文获取类型
收费全文 | 1829篇 |
免费 | 220篇 |
国内免费 | 505篇 |
出版年
2023年 | 36篇 |
2022年 | 40篇 |
2021年 | 59篇 |
2020年 | 77篇 |
2019年 | 89篇 |
2018年 | 75篇 |
2017年 | 97篇 |
2016年 | 85篇 |
2015年 | 73篇 |
2014年 | 101篇 |
2013年 | 95篇 |
2012年 | 80篇 |
2011年 | 87篇 |
2010年 | 67篇 |
2009年 | 106篇 |
2008年 | 113篇 |
2007年 | 112篇 |
2006年 | 119篇 |
2005年 | 81篇 |
2004年 | 75篇 |
2003年 | 76篇 |
2002年 | 66篇 |
2001年 | 62篇 |
2000年 | 59篇 |
1999年 | 48篇 |
1998年 | 49篇 |
1997年 | 43篇 |
1996年 | 61篇 |
1995年 | 70篇 |
1994年 | 39篇 |
1993年 | 51篇 |
1992年 | 44篇 |
1991年 | 35篇 |
1990年 | 42篇 |
1989年 | 28篇 |
1988年 | 24篇 |
1987年 | 15篇 |
1986年 | 9篇 |
1985年 | 14篇 |
1984年 | 8篇 |
1983年 | 10篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1958年 | 2篇 |
1950年 | 1篇 |
排序方式: 共有2554条查询结果,搜索用时 15 毫秒
1.
华北地区针叶林下凋落物层化学性质的研究 总被引:10,自引:0,他引:10
森林凋落物中贮积了大量的有机、无机养分物质,它是森林土壤自然肥力的重要来源之一。在森林生态系统中,养分物质的内部循环主要是通过凋落物来实现的。在分解、转化过 相似文献
2.
Communities change with time. Studying long-term change in community structure permits deeper understanding of community dynamics, and allows us to forecast community responses to perturbations at local (e.g. fire, secondary succession) and global (e.g. desertification, global warming) spatial scales. Monitoring efforts exploring the temporal dynamics of indicator taxa are therefore a critical part of conservation agendas. Here, the temporal dynamics of the Otongachi leaf litter ant community, occurring in a cloud forest in coastal Ecuador, were explored. By sampling this community six times over eleven years, I assessed how the ant fauna caught by Winkler traps (more diverse and cryptic fauna) and caught by pitfall traps (larger, more mobile fauna) changed over time. The Otongachi leaf litter ant community was dynamic. Although species richness in the community remained constant, temporal turnover of species was high: on average, 51% of the ant species in Winkler traps, and 56% of those in pitfall traps, were replaced with other ant species from one year to the other. Shifts in the rank abundance of species in the community were also large across the eleven years and, on average, shifts in the rank abundance of species collected by Winkler traps doubled those occurring in pitfall traps from one census to the other. In spite of these trends, the Otongachi ant fauna showed no (Winkler) or weak (pitfall) evidence of directional change (towards a new community). Thus, this tropical ant community can be divided in two community compartments. The Winkler compartment composed by a more diverse and cryptic ant fauna appears to be resilient and stable in time. The pitfall compartment composed by larger and more mobile ants may be prone to respond to disturbance. This study suggests that 1) species appearing/disappearing from a site may be rather the rule, difficult to separate from responses to ecological stress. 2) Conclusions made in short-term studies, or studies comparing two (e.g. before and after) snapshots of a community, should thus be revisited. Finally, 3) the ant fauna caught by pitfall traps (a rather simple and cheap survey method) is the most likely community compartment to indicate ecological perturbation. This study adds to the growing evidence that using ants as ecological indicators should incorporate long-term temporal dynamics. 相似文献
3.
Alan B. Darlington Anna Halinska James F. Dat T. J. Blake 《Trees - Structure and Function》1997,11(4):223-228
Plant responses to saturation vapour pressure deficit (SVPD) were studied by subjecting black spruce [Picea mariana (Mill) B.S.P.] and jack pine seedlings (Pinus banksiana Lamb.) to humid (0.3 – 0.8 kPa) or dry (2.0 – 2.5 kPa SVPD) regimes for 4 weeks using a computer-controlled environmental
system to control diurnal variation in SVPD. Dry matter accumulation in needles was not altered by increasing SVPD. However,
root growth declined by 60% which increased shoot to root ratio and reduced total seedling dry weight in both black spruce
and jack pine. Relative growth rate of jack pine also declined to about half the rate of plants grown under humid conditions.
In situ root marking studies showed that the decline in root growth of jack pine under the high SVPD was the result of reduced
lateral root initiation, whereas root elongation was unaffected by humidity. A 4-week exposure to dry air increased abscisic
acid (ABA) levels in needles, but not roots, of jack pine whereas ABA levels in black spruce were not altered. A short (3-day)
exposure failed to increase needle ABA levels in either species. These results suggest that the responses of conifers to dry
air were not the result of ABA accumulation.
Received: 24 March 1996 / Accepted: 30 May 1996 相似文献
4.
5.
6.
7.
G. Bucci T. L. Kubisiak W. L. Nance P. Menozzi 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1997,95(4):643-654
We built a “consensus” partial linkage map based on RAPD markers using 48 sibships of eight megagametophytes each from a
natural population of Norway spruce. A RAPD linkage map for a single individual from the same population had previously been
constructed. Using 30 random decamers that had yielded 83 RAPD markers in the single-tree map, eight megagametophytes for
each of the 48 sibships were screened. The linkage relationship among markers was estimated considering each family of eight
megagametophytes as a progeny of a phase-unknown backcross mating between a heterozygous mother and a fictitious ‘recessive’
father. Markers were assigned to windows using LOD=2.0 and θ=0.4 as thresholds, and ordered using a criterion of interval
support ≥2.0. For eight “windows” of recombination selected on the single-tree map, we investigated the consistency of marker
order in the two maps. We adopted restrictive criteria for rejecting co-linearity between the two locus orders. For each window
we imposed the most likely locus order obtained from one data set to the other (and vice versa), obtaining two symmetrical
log-likelihood differences. We considered the hypothesis of co-linearity rejected when both symmetrical differences were significant
(ΔLOD>3.0). By bootstrapping a subset of markers for each window (highly informative, ‘framework’ loci chosen on the previous
single-tree map using a matrix correlation method) the sampling variability of the single-tree and population maps was estimated.
As expected the population map was affected by a larger variability than the single-tree map. Heterogeneity in pairwise recombination
fractions among groups of sibship revealed a (possible) alternative genomic arrangement detected within a single recombination
window.
Received : 4 January 1997 / Accepted : 24 January 1997 相似文献
8.
Mayumi YOSHIMURA 《Entomological Science》2012,15(2):145-154
Natural and human‐made disasters such as floods and logging occur in and around rivers. Stream‐dwelling aquatic insects respond to these disturbances in various ways. Primary consumers among them rely greatly on algae and leaf litter from riparian vegetation as food. Therefore, once a disturbance such as a flood has occurred, insects may find it difficult to find food in a stream, and the aquatic insect assemblage can be impacted greatly as a result. Disturbances in riparian areas also increase fine sediment loads into streams, damaging habitat and altering the aquatic insect assemblage. Deforestation impacts not only terrestrial but also aquatic animals. In this review paper, aquatic insect assemblages are assessed according to alterations in land use in and around streams. Following this paper, it is expected that clarification of aquatic insect fauna and their life cycles will progress and that the distribution and habitat use of aquatic insects will be afforded greater attention in forest management. 相似文献
9.
EMMA DESPLAND MARIA GUNDERSEN SIMON P. DAOUST BRIAN J. MADER NATHALIE DELVAS PAUL J. ALBERT ERIC BAUCE 《Physiological Entomology》2011,36(1):39-46
The pattern of feeding of Eastern spruce budworm Choristoneura fumiferana (Clem.) (Lepidoptera, Tortricidae) is compared on foliage from white spruce Picea glauca (Moench) Voss. (Pinaceae) trees previously determined to be susceptible and resistant to defoliation by budworm. No differences are observed in electrophysiological responses from taste sensilla to aqueous extracts of the two foliage types, nor is there a preference for either extract type in a choice test. Acetone extracts from the two foliage types are both preferred to a control sucrose solution, although neither elicits a preference relative to the other. These results suggest that there is no difference in phagostimulatory power of internal leaf contents of the two foliage types. Longer‐term observation of feeding behaviour in a no‐choice situation shows no difference in meal duration, confirming the lack of difference in phagostimulatory power. However, on average, intermeal intervals are twice as long on the resistant foliage, leading to an overall lower food consumption during the assay. This result suggests an anti‐digestive or toxic effect of the resistant foliage that slows behaviour and limits food intake. Previous research has shown that waxes of the resistant foliage deter initiation of feeding by the spruce budworm and that this foliage contains higher levels of tannins and monoterpenes. The data suggest that the resistant foliage contains a post‐ingestive second line of defence against the spruce budworm. 相似文献
10.