全文获取类型
收费全文 | 24033篇 |
免费 | 2763篇 |
国内免费 | 2637篇 |
出版年
2024年 | 61篇 |
2023年 | 501篇 |
2022年 | 506篇 |
2021年 | 821篇 |
2020年 | 872篇 |
2019年 | 1040篇 |
2018年 | 839篇 |
2017年 | 986篇 |
2016年 | 1032篇 |
2015年 | 929篇 |
2014年 | 1114篇 |
2013年 | 1578篇 |
2012年 | 1017篇 |
2011年 | 1000篇 |
2010年 | 944篇 |
2009年 | 1306篇 |
2008年 | 1294篇 |
2007年 | 1424篇 |
2006年 | 1174篇 |
2005年 | 1150篇 |
2004年 | 979篇 |
2003年 | 890篇 |
2002年 | 828篇 |
2001年 | 756篇 |
2000年 | 735篇 |
1999年 | 591篇 |
1998年 | 546篇 |
1997年 | 484篇 |
1996年 | 435篇 |
1995年 | 380篇 |
1994年 | 333篇 |
1993年 | 315篇 |
1992年 | 328篇 |
1991年 | 273篇 |
1990年 | 212篇 |
1989年 | 220篇 |
1988年 | 148篇 |
1987年 | 137篇 |
1986年 | 146篇 |
1985年 | 177篇 |
1984年 | 171篇 |
1983年 | 100篇 |
1982年 | 145篇 |
1981年 | 121篇 |
1980年 | 104篇 |
1979年 | 89篇 |
1978年 | 43篇 |
1977年 | 34篇 |
1975年 | 27篇 |
1973年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
991.
992.
Ángel Vale Danny Rojas Julio C. Álvarez Luis Navarro 《Botanical journal of the Linnean Society. Linnean Society of London》2013,172(3):345-357
The geographical distribution, population structure and pollination ecology are key aspects in the conservation and management of rare orchids. Here, we address these aspects and the main threats affecting the endangered Cuban orchid Broughtonia cubensis. This rewardless orchid is self‐compatible, but pollinator dependent. However, seed production can be negatively affected by insect‐mediated selfing. Three species of small bee (genera Ceratina and Lasioglossum) act as pollinators. As in the case of other nectarless orchids, we detected two species of plant producing large amounts of nectar in the area, the floral morphology of which closely resembles that of B. cubensis. The simultaneous flowering of these species could positively affect the reproductive success of B. cubensis. Nonetheless, the fitness of this orchid in natural conditions is low, possibly related to strong pollen limitation. To the problems arising from reduced fitness is added the fact that its historical distribution range has been greatly reduced in recent years. Throughout this study, we have detected dramatic reductions in the population sizes, in some cases as a result of human plundering, but also as a consequence of hurricanes. Based on the results of this study, we propose some guidelines to manage and conserve this orchid. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 172 , 345–357. 相似文献
993.
Andreas Rudh Martin F. Breed Anna Qvarnström 《Biological journal of the Linnean Society. Linnean Society of London》2013,108(1):116-126
For prey, many behavioural traits are constrained by the risk of predation. Therefore, shifts between warning and cryptic coloration have been suggested to result in parallel changes in several behaviours. In the present study, we tested whether changes in chromatic contrast among eight populations of the strawberry poison‐dart frog, Dendrobates pumilio, co‐vary with behaviour, as expected if selection is imposed by predators relying on visual detection of prey. These eight populations are geographically isolated on different island in the Bocas del Toro region of Panama and have recently diverged morphologically and genetically. We found that aggression and explorative behaviour were strongly correlated and also that males tended to be more aggressive and explorative if they belonged to populations with conspicuously coloured individuals. We discuss how evolutionary switches between predator avoidance strategies and associated behavioural divergence between populations may affect reproductive isolation. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ●● , ●●–●●. 相似文献
994.
Ada Wróblewska 《Botanical journal of the Linnean Society. Linnean Society of London》2013,173(1):64-76
Surveys of genetic diversity patterns of self‐incompatible clonal polyploid plant species are still scarcer than those of diploid plant species. Therefore, I studied the phylogeographical history of Linnaea borealis subsp. borealis to shed light on the colonization history of this clonal self‐incompatible polyploid plant in Eurasia using selected regions of plastid DNA and genetic diversity patterns of 22 populations of this species employing AFLP markers. I also addressed the question of whether the genetic diversity patterns in L. borealis subsp. borealis in Eurasia are similar to those of earlier published studies of clonal self‐incompatible diploid or polyploid plants. This survey revealed that the shallow phylogeographical history (six plastid haplotypes forming one haplogroup, 100% bootstrap support) and moderate genome‐wide diversity estimated using AFLP markers (Fragpoly = 10.8–38.9%, I = 0.060–0.180, FST = 0.289) were general characteristics of L. borealis subsp. borealis in its Eurasian range. The sampling strategy, in most cases at 1–2‐m or even 3–5‐m intervals, showed that a balance between vegetative and sexual reproduction and limited pollen dispersal among compatible mates can be important for genetic diversity patterns in populations of this taxon. Despite the fact that one‐half of the investigated populations were strongly isolated, they still preserved similar levels of genetic diversity across the geographical range. I found no support for the hypothesis that a bottleneck and/or inbreeding had accompanied habitat fragmentation as factors shaping genetic diversity. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 173 , 64–76. 相似文献
995.
Craig I. Peter Steven D. Johnson 《Botanical journal of the Linnean Society. Linnean Society of London》2013,171(4):713-729
Non‐rewarding plants use a variety of ruses to attract their pollinators. One of the least understood of these is generalized food deception, in which flowers exploit non‐specific food‐seeking responses in their pollinators. Available evidence suggests that colour signals, scent and phenology may all play key roles in this form of deception. Here we investigate the pollination systems of five Eulophia spp. (Orchidaceae) lacking floral rewards. These species are pollinated by bees, notably Xylocopa (Anthophorinae, Apidae) or Megachile (Megachilidae) for the large‐flowered species and anthophorid (Anthophorinae, Apidae) or halictid (Halictidae) bees for the small‐flowered species. Spectra of the lateral petals and ultraviolet‐absorbing patches on the labella are strongly contrasting in a bee visual system, which may falsely signal the presence of pollen to bees. All five species possess pollinarium‐bending mechanisms that are likely to limit pollinator‐mediated self‐pollination. Flowering times extend over 3–4 months and the onset of flowering was not associated with the emergence of pollinators, some of which fly year round. Despite sharing pollinators with other plants and lacking rewards that would encourage fidelity, the Eulophia spp. exhibited relatively high levels of pollen transfer efficiency compared with other rewarding and deceptive orchids. We conclude that the study species employ generalized food deception and exploit food‐seeking bees. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013 , 171 , 713–729. 相似文献
996.
Sclerocarya birrea subsp. caffra (marula), a keystone large tree species in southern Africa and heavily utilized by people and herbivores, is declining at an unprecedented rate in the Kruger National Park (KNP), South Africa. Due to increasing concerns over large tree declines in the KNP, mirroring those in eastern Africa over the last 50 years, and local extinctions of marula populations in the north‐western KNP, adult trees in the southern KNP were monitored between 2001 and 2010. Annual mortality rates across all adult size classes ranged between 3.2 and 4.6%. Loss of adult marula trees in some areas over the last decade exceeded 25%. Rainfall had a strong influence on mortality rates temporally and spatially, but the distance to the nearest perennial river had little influence. Overall, marula populations continue to decline and further local extinctions are possible. Given the clearly unacceptable trends of decline, the next step is to utilize this information (i) to develop predictive models for the future dynamics of marula populations incorporating all drivers; (ii) to advocate active management of refugia within the park where marula populations are protected from elephant impacts; and in combination with (iii) continued monitoring. 相似文献
997.
Steve Garner 《Ethnic and racial studies》2013,36(2):358-359
Within the politics of nationalism and nation-building, the emigration of ethnic and religious minorities, whether voluntary or involuntary, appears to be a commonly occurring practice. After the collapse of the Ottoman Empire in the early twentieth century, modern Turkey still carried the legacy of a multi-ethnic, multi-religious diversity in which its Armenian, Greek and Jewish communities had official minority status based upon the 1923 Treaty of Lausanne. However, throughout the twentieth century, Turkey's non-Muslim minority populations have undergone a mass emigration experience in which thousands of their numbers have migrated to various countries around the globe. While in the 1920s the population of non-Muslims in the country was close to 3 per cent of the total, today it has dropped to less than two per thousand. This article analyses the emigration of non-Muslim people from Turkey and relates this movement to the wider context of nation-building in the country. 相似文献
998.
An investigation of population structure and ecology of the African buffalo (Syncerus caffer Sparrman, 1779) was carried out in the Chebera Churchura National Park, Ethiopia during the wet and dry seasons of 2005–2006. Sample counts of African buffaloes were carried out in an area of 1,215 km2. The estimated population of buffalo was 2,617 individuals. Males comprised 35.0%, while females 52.8% of the population. The remaining 12% of the population was young of both sexes and of all ages of unknown sex. It was difficult to categorize the young into male and female in the field, as their primary sexual characteristics were not easily visible. Male to female sex ratio was 1.00 : 1.51. Age structure was dominated by adults, which constituted 72.06% of the total population. Subadults comprised 22.02%, and young accounted for 5.9% of the population. Larger herds of up to 27 individuals were observed during the wet season and smaller herds of eight individuals during the dry season. The mean herd size during wet and dry seasons was 24.81 and 7.77, respectively. The African buffaloes were distributed in four habitat types such as grasslands, woodlands, montane forests and riverine habitat in the study area. They were observed more in the riverine vegetation types during the dry season. Relative abundance of food resources, green vegetation cover and water availability in the area were the major factors governing their distribution in the present study area. They spent a greater proportion of the time in feeding and resting/ruminating activities. On the average, 49.7% of the daytime was spent in feeding, and 38.75% of the day on resting (lying down and standing). Morning and the late afternoon activity peaks were more pronounced during the dry season than the wet season. 相似文献
999.
1000.
Michal Kravel-Tovi 《Ethnic and racial studies》2013,36(4):737-756
A large number of non-Jewish immigrants from the former Soviet Union have arrived in Israel since the late 1980s. This article explores how the Israeli State has responded to this perceived demographic threat by endorsing a pro-Jewish conversion policy targeted at this population of new citizens. By analysing a variety of ethnographic and textual materials, I trace the organizational processes and discursive practices through which conversion has been crafted into a ‘national mission’: an all-encompassing state endeavour whose impetus is a national-Zionist biopolitics. The Foucauldian concept of biopolitics offers a novel way to understand the interface between religious conversion and the nation-state. Specifically, it positions the concept of population as a primary analytical category, thereby enabling us to understand religious conversion as a mechanism of national population policy. 相似文献