全文获取类型
收费全文 | 9148篇 |
免费 | 1026篇 |
国内免费 | 600篇 |
专业分类
10774篇 |
出版年
2024年 | 46篇 |
2023年 | 256篇 |
2022年 | 274篇 |
2021年 | 330篇 |
2020年 | 430篇 |
2019年 | 479篇 |
2018年 | 430篇 |
2017年 | 403篇 |
2016年 | 380篇 |
2015年 | 386篇 |
2014年 | 420篇 |
2013年 | 626篇 |
2012年 | 376篇 |
2011年 | 377篇 |
2010年 | 313篇 |
2009年 | 494篇 |
2008年 | 513篇 |
2007年 | 524篇 |
2006年 | 459篇 |
2005年 | 420篇 |
2004年 | 359篇 |
2003年 | 294篇 |
2002年 | 275篇 |
2001年 | 291篇 |
2000年 | 224篇 |
1999年 | 233篇 |
1998年 | 190篇 |
1997年 | 156篇 |
1996年 | 109篇 |
1995年 | 105篇 |
1994年 | 132篇 |
1993年 | 64篇 |
1992年 | 65篇 |
1991年 | 51篇 |
1990年 | 69篇 |
1989年 | 35篇 |
1988年 | 22篇 |
1987年 | 28篇 |
1986年 | 22篇 |
1985年 | 14篇 |
1984年 | 23篇 |
1983年 | 21篇 |
1982年 | 19篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1979年 | 6篇 |
1978年 | 1篇 |
1977年 | 5篇 |
1974年 | 2篇 |
1972年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
871.
872.
Mourthé IM Guedes D Fidelis J Boubli JP Mendes SL Strier KB 《American journal of primatology》2007,69(6):706-712
Many arboreal primates descend to the ground, a custom that may occur more frequently in disturbed habitats, and in the presence of researchers to whom the primates are habituated. In this paper, we describe opportunistic observations of ground use in two groups of northern muriquis (Brachyteles hypoxanthus) at the RPPN Feliciano Miguel Abdala, Minas Gerais, Brazil. Members of both groups were observed drinking, resting, feeding, playing, and traveling on the ground to different degrees, and variation in the levels of habituation of the two groups may be responsible for the differences in the distribution of their terrestrial activities. The potential increase in vulnerability to predation or disease owing to ground use has implications for the conservation of this critically endangered species. 相似文献
873.
Frank R. Thomas 《Human ecology: an interdisciplinary journal》2007,35(2):179-194
Focusing on contemporary shellfish exploitation among several atoll communities in Kiribati, Micronesia, this paper examines
the relationship between human foragers and their invertebrate prey via the prey choice or diet breadth model derived from
optimal foraging theory. Shellfish, like many other reef organisms, are relatively sedentary and predictable, but these characteristics
make them susceptible to over-harvesting. The research reveals that shellfish gatherers are foraging in a manner that matches
the predictions of optimal foraging theory. The work adds to our understanding of optimal foraging decisions in atoll settings
by critically evaluating the depiction of atoll dwellers as conservationists.
相似文献
Frank R. ThomasEmail: Email: |
874.
David L. Stokes 《Human ecology: an interdisciplinary journal》2007,35(3):361-369
Human preferences will increasingly determine many species’ prospects for survival. However, aside from a small number of
survey-based studies of preference among disparate taxa, human species preferences have received little attention. I determined
human aesthetic preferences among a relatively homogenous group, the penguins, from representation in all recently published,
comprehensive, popular books of photographs of penguins (n = 4 books; 304 photographs). Representation of visually distinguishable types of penguins, measured by total photograph area,
was highly skewed and rankings were highly concordant across books, suggesting large and commonly held differences in aesthetic
appeal. Multiple regression analysis indicated that amount of warm color was the only significant determinant of representation,
and warm color was highly correlated (r
2 = 0.96) with mean representation of the penguin types. Body size and neotenic form, traits found to influence human preferences
among other animals, were not significant, suggesting that the bases of human species preferences differ by species type.
The results of this study indicate that human aesthetic preferences discriminate finely among species and may be based on
minor features. Conservationists must be vigilant to the potential for aesthetic responses to influence conservation efforts. 相似文献
875.
876.
Research on dugong–seagrass interactions in Indonesia was done during the period 1990 until 2005 in respectively East Aru,
Maluku Province and East Kalimantan, Indonesia. This research investigated intensive rotational grazing by dugongs in intertidal
inshore Halodule univervis seagrass meadows, dugong movements in relation to these grazing swards, and analyses of parameters explaining the temporal
and spatial patterns of grazing in these meadows. In this paper, we report the findings of this long-term study. The patterns
of movement and the results of snorkelling surveys confirmed a practice of regular recropping of restricted grazing swards
by small feeding assemblages of dugongs. Dugong grazing showed a significant correlation with carbohydrate content of the
below-ground biomass and no significant relation with total N. The timing of dugong grazing in these intertidal meadows coincides
with high below-ground biomass and high carbohydrate content in the rhizomes of H. uninervis in the upper 0–4 cm sediment layer. Our findings support the hypothesis that temporal dugong grazing is ruled by carbohydrate
content in below-ground biomass. The mechanisms of rotational grazing in restricted grazing swards are not yet well understood,
and the maximisation of carbohydrates does not fully explain this phenomenon. Our research confirms that intertidal H. univervis seagrass meadows form a crucial resource for dugong survival. These relatively unknown biotopes need therefore more attention
in research and conservation programmes. 相似文献
877.
1. Parasites and infectious diseases have become a major concern in conservation biology, in part because they can trigger or accelerate species or population declines. Focusing on primates as a well-studied host clade, we tested whether the species richness and prevalence of parasites differed between threatened and non-threatened host species. 2. We collated data on 386 species of parasites (including viruses, bacteria, protozoa, helminths and arthropods) reported to infect wild populations of 36 threatened and 81 non-threatened primate species. Analyses controlled for uneven sampling effort and host phylogeny. 3. Results showed that total parasite species richness was lower among threatened primates, supporting the prediction that small, isolated host populations harbour fewer parasite species. This trend was consistent across three major parasite groups found in primates (helminths, protozoa and viruses). Counter to our predictions, patterns of parasite species richness were independent of parasite transmission mode and the degree of host specificity. 4. We also examined the prevalence of selected parasite genera among primate sister-taxa that differed in their ranked threat categories, but found no significant differences in prevalence between threatened and non-threatened hosts. 5. This study is the first to demonstrate differences in parasite richness relative to host threat status. Results indicate that human activities and host characteristics that increase the extinction risk of wild animal species may lead simultaneously to the loss of parasites. Lower average parasite richness in threatened host taxa also points to the need for a better understanding of the cascading effects of host biodiversity loss for affiliated parasite species. 相似文献
878.
1. Range size, population size and body size, the key macroecological variables, vary temporally both within and across species in response to anthropogenic and natural environmental change. However, resulting temporal trends in the relationships between these variables (i.e. macroecological patterns) have received little attention. 2. Positive relationships between the local abundance and regional occupancy of species (abundance-occupancy relationships) are among the most pervasive of all macroecological patterns. In the absence of formal predictions of how abundance-occupancy relationships may vary temporally, we outline several scenarios of how changes in abundance within species might affect interspecific patterns. 3. We use data on the distribution and abundance of 73 farmland and 55 woodland bird species in Britain over a 32-year period encompassing substantial habitat modification to assess the likelihood of these scenarios. 4. In both farmland and woodland habitats, the interspecific abundance-occupancy relationship changed markedly over the period 1968-99, with a significant decline in the strength of the relationship. 5. Consideration of intraspecific dynamics shows that this has been due to a decoupling of abundance and occupancy particularly in rare and declining species. Insights into the intraspecific processes responsible for the interspecific trend are obtained by analysis of temporal trends in the distribution of individuals between sites, which show patterns consistent with habitat quality declines. 6. This study shows that a profitable approach to ascertaining the nature of human impacts is to link intra- and interspecific processes. In the case of British farmland and woodland birds, changes to the environment lead to species-specific responses in large-scale distributions. These species-specific changes are the driver of the observed changes in the form and strength of the interspecific relationship. 相似文献
879.
880.
拟南芥CBF/DREB途径的研究进展及其在植物基因工程中的应用 总被引:4,自引:0,他引:4
细胞膜感知低温环境信号,通过信号转导激活以CBF/DREB信号途径为主、兼与其他途径交谈和交叠所构成的信号网络,继而调控下游相应功能蛋白的表达,赋予植物低温抗性。文章重点介绍了拟南芥CBF/DREB信号传导途径的研究进展、CBF基因在高等植物中的保守性,以及构建CBF转基因抗冻植物所取得的成果,包括CBF的表达调控与作用的分子机制、影响CBF途径的一些重要蛋白和环境因子、组成性启动子以及冷诱导特异性启动子驱动CBF基因表达所获得的抗冻转基因植物的研究进展。同时,对该研究领域未来可能的发展方向进行了展望。 相似文献