全文获取类型
收费全文 | 2975篇 |
免费 | 128篇 |
国内免费 | 111篇 |
出版年
2024年 | 2篇 |
2023年 | 45篇 |
2022年 | 77篇 |
2021年 | 55篇 |
2020年 | 57篇 |
2019年 | 75篇 |
2018年 | 74篇 |
2017年 | 97篇 |
2016年 | 116篇 |
2015年 | 98篇 |
2014年 | 165篇 |
2013年 | 189篇 |
2012年 | 82篇 |
2011年 | 112篇 |
2010年 | 91篇 |
2009年 | 243篇 |
2008年 | 273篇 |
2007年 | 250篇 |
2006年 | 184篇 |
2005年 | 146篇 |
2004年 | 105篇 |
2003年 | 113篇 |
2002年 | 74篇 |
2001年 | 46篇 |
2000年 | 42篇 |
1999年 | 43篇 |
1998年 | 34篇 |
1997年 | 32篇 |
1996年 | 22篇 |
1995年 | 37篇 |
1994年 | 30篇 |
1993年 | 24篇 |
1992年 | 23篇 |
1991年 | 22篇 |
1990年 | 18篇 |
1989年 | 14篇 |
1988年 | 11篇 |
1987年 | 16篇 |
1986年 | 10篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1971年 | 2篇 |
排序方式: 共有3214条查询结果,搜索用时 93 毫秒
101.
The Bankoualé Palm, Livistona carinensis is the only known species of Livistona occurring in Africa and is currently classified as vulnerable (IUCN 2004). This extreme outlier species of the genus is restricted to Yemen, Somalia and Djibouti, where all populations are in rapid
decline. In Djibouti the palm is confined to three valley systems within the upland plateau of the Goda Massif. This study
used microsatellite markers to investigate the genetic diversity and relationships within the species. At the species level
L. carinensis contained very low genetic diversity. Most variation was due to the variation between the samples from Yemen and Somalia
compared with those in Djibouti. The Djibouti populations were almost monomorphic across the nine loci tested. Interestingly,
and despite the small sample sizes, the individuals from botanic gardens collections of the Yemen and Somalia populations
were more genetically diverse than the Djibouti populations. This study indicates that the populations in Yemen and Somalia
are highly significant for the conservation of the species genetic diversity. Given the lack of genetic diversity both within
and among L. carinensis populations in Djibouti, plants could be cultivated for in-situ population enhancement from any seed that is available from
within Djibouti with no significant genetic impacts of provenance mismatch. Clearly the populations from Djibouti, Somalia
and Yemen are different genetic provenances raising some issues for the conservation and recovery of L. carinensis. 相似文献
102.
Eleven polymorphic microsatellite loci were isolated and characterized from an AC-enriched genomic library of Akebia trifoliate ssp. australis. The number of allele per locus ranged from 3 to 14. The observed heterozygosity and expected heterozygosity at population
level were 0.196–1.000 and 0.522–0.902, respectively. In addition, this set of microsatellites produced robust cross-species
amplification in other two related taxa, suggesting these microsatellite markers should provide a useful tool for genetic
and conservation studies of the Akebia species. 相似文献
103.
All crocodilians are under varying degrees of threat due to over exploitation and these species have been listed in Appendix
I or II of CITES. The lack of molecular techniques for the identification of confiscated samples makes it difficult to enforce
the law. Conclusive forensic identification of species requires a complete gene sequence which is difficult in case of degraded
samples. We have developed two novel sets of primers to amplify two partial cytochrome b gene sequences of six crocodile species i.e. Crocodylus palustris, Crocodylus porosus, Crocodylus siamensis, Crocodylus niloticus, Gavialis gangeticus and Caiman crocodilus. These partial sequences were edited to give a complete cyt b gene sequence, which can be used as an effective tool for forensic authentication of crocodile species. A phylogeny of crocodile
species was reconstructed using these sequences. The described primers hold great promise in forensic identification of crocodile
species, which can aid in the effective enforcement of law and conservation of these ancient species. 相似文献
104.
Mari Kawaji Shingo Kaneko Ryunosuke Tateno Yuji Isagi Tsuyoshi Yoneda 《Conservation Genetics》2009,10(4):1049-1051
Quercus miyagii is an endemic tree species in the Ryukyu Islands, Japan. We isolated and characterized 15 microsatellite loci in this species.
The number of alleles ranged from 2 to 16 and expected heterozygosities from 0.07 to 0.92. This set of markers is potentially
useful to investigate the genetic structure, gene flow, and the biogeographic history of Q. miyagii in the Ryukyu Islands, Japan. 相似文献
105.
Marie Pagès Nathalie Desse-Berset Christelle Tougard Laurent Brosse Catherine Hänni Patrick Berrebi 《Conservation Genetics》2009,10(1):217-224
Sturgeon disappeared from the Rhône River in the mid 70’s without certitude about which species it was and about the existence of a sympatry between European sturgeon, Acipenser sturio, and Adriatic sturgeon, A. naccarii, in this watershed. In order to reach a reliable specific determination of this extinct sturgeon population, archaeozoological remains of the Jardin d’Hiver in Arles city, on the Rhône river banks, were genetically analysed, following strict criteria of authentication for the ancient DNA work. The rich collection of Arles sturgeon bone remains stems from human activities between the 6th and the 2nd Century BC. Sequences of 86 bp of the cytochrome b gene were obtained on four bones, from different anatomical parts of the fish and from different archaeological layers. All gave A. sturio diagnostic sequences. This preliminary analysis is an essential first step in the project of sturgeon reintroduction in the Rhône River. Thus, further analyses on a larger sample are necessary to comfort this result and to solve the question of sympatry with A. naccarii. 相似文献
106.
California is a biodiversity hotspot facing unbridled human population growth, especially in Central California. One of the
poorly known, sensitive species in this area is the California legless lizard (Anniella pulchra), a fossorial worm-like reptile. We report mt and nuDNA sequences from 69 museum-vouchered samples of Anniella (A. pulchra and its sister species A. geronimensis) from 48 localities. Our genetic survey reveals substantially more genetic diversity within A. pulchra than previously reported. Our two independently evolving markers (mt and nuDNA) reveal five major lineages of A. pulchra. Two of the five major lineages of A. pulchra correspond to a north-south split found in other widespread California reptiles. These northern and southern clades also
correspond to a previous study showing variation in chromosomal number. Unlike most other Californian reptiles, A. pulchra has major genetic lineages that are endemic to Central California including two that are endemic to the San Joaquin Valley
and Carrizo Plain. Although A. pulchra is threatened throughout its range, the distinct San Joaquin lineages are seriously imperiled by urban sprawl. Some of the
localities for the newly recognized genetic lineages have already been destroyed by development. 相似文献
107.
Naofumi Nomura Kento Fujiwara Tokushiro Takaso Motomi Ito Koichi Uehara Hiroaki Setoguchi 《Conservation Genetics》2009,10(4):1093-1095
Eight microsatellite loci for the perennial herb Farfugium japonicum, including the rheophytic variety luchuense endemic to riparian areas of the Ryukyu Islands, Japan, were isolated and characterised. The number of alleles ranged from
5 to 14. The expected (H
E) and observed (H
O) heterozygosities were 0.344–0.885 and 0.121–0.754, respectively, from 69 individuals in one population. Six loci exhibited
significantly fewer heterozygotes than expected under Hardy–Weinberg equilibrium (P < 0.05). The primers amplifying microsatellite sequences in F. japonicum may provide a population genetics tool useful in the establishment of a conservation strategy. 相似文献
108.
Hans Jacquemyn Rein Brys Dries Adriaens Olivier Honnay Isabel Roldán-Ruiz 《Conservation Genetics》2009,10(1):161-168
Due to societal changes and altered demands for firewood, the traditional forest management of coppicing has been largely
abandoned. As a result, many forest herbs that are specifically adapted to regular opening of the canopy, have suffered significant
declines in abundance, and the remaining populations of these species often tend to be small and isolated. Reduced population
sizes and pronounced spatial isolation may cause loss of within-population genetic diversity and increased between-population
differentiation through random genetic drift and inbreeding. In this study, we investigated genetic diversity and genetic
structure of 15 populations of the food-deceptive orchid Orchis mascula using AFLP markers. Within-population genetic diversity significantly increased with increasing population size, indicating
genetic impoverishment in small populations. Genetic differentiation, on the other hand, was rather low (ΦST = 0.083) and there was no significant relationship between genetic and geographic distances, suggesting substantial gene
flow within the study area. However, strong differences in levels of within-population diversity and among-population differentiation
were found for populations located in forests that have been regularly coppiced and populations found in forests that were
neglected for more than 50 years and that were totally overgrown by shrubs. Our data thus indicate that a lack of coppicing
leads to decreased genetic diversity and increased differentiation in this orchid species, most likely as a result of genetic
drift following demographic bottlenecks. From a conservation point of view, this study combined with previous results on the
demography of O. mascula in relation to forest management illustrates the importance of coppicing in maintaining viable populations of forest herbs
in the long-term. 相似文献
109.
Masahiro Horinouchi 《Ichthyological Research》2009,56(2):109-125
Horizontal gradients in fish assemblage structures in and around a seagrass habitat were evaluated by visual observations.
The assemblage structures clearly differed between open microhabitats lacking seagrass, i.e., nearby sand and the area adjoining
the outer margin of the seagrass bed (referred to as “outer gap”), and microhabitats with seagrass within the overall seagrass
bed, although not differing among the latter microhabitats, including both edge and core portions. Such open microhabitats
were found to be not always inferior, but nearly equal to or even sometimes greatly superior in fish species’ diversity and/or
abundance to the microhabitats with seagrass. In particular, the outer gap was always ranked first in total species’ number
and had outstanding abundance in spring. Similar open microhabitats adjoining seagrass walls facing the sand patch within
the seagrass bed (referred to as “inner gap”) in spring were also characterized by higher fish species and individual numbers.
The 11 most abundant fishes showed four discrete distribution patterns (three recognized herein and one implied by precedent
studies), such contributing to a horizontal gradient in the fish assemblage structure. While no fishes showed a preference
for the edge or core of the seagrass bed, group-forming juveniles of several species favored gap microhabitats, suggesting
that, in addition to the traditionally recognized edge and core microhabitats, the concept of gap microhabitats should be
included in seagrass conservation ecology. 相似文献
110.
Luca Borghesio 《Plant Ecology》2009,201(2):723-731
This study focuses on the effect of fire on lowland heathlands at the extreme southern edge of their European distribution
(Vauda Nature Reserve, NW Italy). Forty-nine plots (50 m radius) were surveyed between 1999 and 2006. Each year, fire occurrences
were recorded and per cent cover of four vegetation types (grassland, heath, low shrubland, and tall shrubland) was estimated
in each plot. Vascular plant species richness was also recorded in 255, 1 m2 quadrats. After a fire, grassland vegetation expanded, but then declined rapidly as heath and shrubland recovered: 7 years
after a fire, tall shrubland encroached on to more than 40% of the plots, and grassland declined from 50% to 20% cover. Between
1999 and 2006, Betula pendula shrubland greatly expanded, while grassland decreased over most of the Reserve, even where fire frequency was high. Tall
shrubland had low plant diversity and was dominated by widespread species of lower conservation value. By contrast, early
successional vegetation (grassland and low shrubland) had higher richness and more narrowly distributed species, indication
that the development of tall shrubland causes significant species loss in the heathland. Italian lowland heathlands are characterized
by high rates of shrubland encroachment that threatens both habitat and species diversity. Burning frequencies of once in
3–6 years seem appropriate in this habitat, but burning alone might not suffice without actions to increase herbivore grazing. 相似文献