全文获取类型
收费全文 | 8136篇 |
免费 | 640篇 |
国内免费 | 705篇 |
出版年
2024年 | 9篇 |
2023年 | 160篇 |
2022年 | 134篇 |
2021年 | 182篇 |
2020年 | 212篇 |
2019年 | 268篇 |
2018年 | 220篇 |
2017年 | 268篇 |
2016年 | 259篇 |
2015年 | 248篇 |
2014年 | 275篇 |
2013年 | 416篇 |
2012年 | 236篇 |
2011年 | 289篇 |
2010年 | 259篇 |
2009年 | 390篇 |
2008年 | 456篇 |
2007年 | 467篇 |
2006年 | 504篇 |
2005年 | 434篇 |
2004年 | 422篇 |
2003年 | 363篇 |
2002年 | 300篇 |
2001年 | 278篇 |
2000年 | 247篇 |
1999年 | 225篇 |
1998年 | 192篇 |
1997年 | 204篇 |
1996年 | 182篇 |
1995年 | 140篇 |
1994年 | 174篇 |
1993年 | 150篇 |
1992年 | 132篇 |
1991年 | 122篇 |
1990年 | 101篇 |
1989年 | 75篇 |
1988年 | 80篇 |
1987年 | 62篇 |
1986年 | 61篇 |
1985年 | 47篇 |
1984年 | 41篇 |
1983年 | 34篇 |
1982年 | 27篇 |
1981年 | 31篇 |
1980年 | 14篇 |
1979年 | 21篇 |
1978年 | 22篇 |
1977年 | 13篇 |
1976年 | 12篇 |
1973年 | 7篇 |
排序方式: 共有9481条查询结果,搜索用时 15 毫秒
991.
广州外来入侵植物 总被引:10,自引:4,他引:6
根据野外调查及相关文献资料,初步探讨了广州市外来入侵植物的种类组成、分布生境、生长特性、危害以及来源.结果表明:广州市现有外来人侵植物73种,隶属于27科59属,其中菊科(19种)、禾本科(8种)、豆科(7种)和苋科(7种)植物占了入侵植物总种数的56.2%.生活型分析发现,在广州有分布的外来入侵植物中,陆生草本植物最多(58种),其次为灌木(10种)、水生草本(3种),乔木种类最少(2种),它们主要生长在人类干扰比较频繁的低海拔地区.另外,基于对外来入侵植物目前在野外的生长和分布状况等生态学特性进行分析,可大致将它们对本地生物多样性危害程度分为3级,其中危害严重的有11种,危害中等的有25种,危害较轻的有37种.从原产地分析来看,61种来自于热带美洲地区,占广州外来入侵植物总种数的83.6%.与国内其他地区外来入侵植物进行比较发现,广州和广西共有种数最多,有62种,然而,对已经造成严重危害的外来植物种类,在数量上广州与香港、深圳和广西相同种类较多,分别有8种、7种和6种.对广州市目前外来植物的入侵风险(潜在威胁)的评估结果表明,对一些在其它地区已经造成严重危害的外来植物,应当加强监控,防止入侵扩散并造成重大生态灾难.还对广州市外来植物入侵的主要途径及防治策略做了简要分析和说明. 相似文献
992.
993.
外来入侵植物的氮代谢及其土壤氮特征 总被引:7,自引:1,他引:6
研究了4种外来入侵植物(五爪金龙、南美蟛蜞菊、金腰箭和马缨丹)和1种本地植物鸡矢藤(对照)的氮代谢及其土壤氮特征.结果表明:外来人侵植物的组织硝酸还原酶活性、根际土壤NH4-N、NO3-N含量、蛋白酶活性和脲酶活性均较高,分别为鸡矢藤的1.65~4.34、1.56~2.15、1.72~3.11、1.43~3.23和1.41~3.33倍,而植物组织硝态氮含量则较低,仅为鸡矢藤的17.5%~50.6%.相关分析表明:植物组织硝酸还原酶活性与根际土壤总氮、NH4-N、NO3-N含量呈显著正相关(P<0.05),与蛋白酶活性和脲酶活性呈极显著正相关(P<0.01).这说明,外来植物入侵使土壤氮代谢加快,氮的生物有效性增强,氮同化能力提高,并且较好地将植物体氮素代谢与土壤氮素代谢协调起来.因此,较强的氮素同化能力与加速土壤氮素的转化可能是植物成功入侵的重要机制之一. 相似文献
994.
A. G. Viktorov 《Russian Journal of Plant Physiology》2008,55(6):738-747
The critical review of experimental conclusions stating environmental safety of transgenic plants reveals that the methods and test materials employed in some studies were inadequate to the goals pursued. A large-scale application of transgenic Bt-plants may result in long-lasting negative impact on the environment. First, the cultivation of these plants leads to accumulation of Bt-toxins in soil. Second, the decomposition of transgenic plants takes significantly longer time compared to that of isogenic lines. Third, the biological activity of soils under transgenic crops is lower than in the control plots. The transfer of δ-endotoxin-encoding genes to the genome of agricultural crops affects simultaneously several entirely different traits of genetically modified plants, thus exerting pleiotropic effects. This gives rise to a paradoxical situation: the genetically engineered crops selected on the trait of resistance to herbivorous insects of the order Lepidoptera become more attractive for herbivores from the other order, Homoptera. 相似文献
995.
The signaling role of action potential (AP) in higher plants is considered. The principles underlying realization of this role and the significance of AP-induced short-term effector response are discussed. The notion is put forward that the effect of propagating AP on plant cells is similar to nonspecific component of the cell functional response to external stimuli. 相似文献
996.
997.
In plants, Rab proteins represent the largest family of monomeric GTP-binding proteins (mG-proteins). As distinct from animal cells comprising 40 subfamilies of Rab proteins, which are the key regulators of intracellular vesicular transport, numerous Rab proteins in Arabidopsis and other plant species could be grouped in only eight subfamilies on the basis of their functional properties. The available data concerning the involvement of these mG-proteins in the control of vesicle trafficking agree generally with the paradigms accepted for other eukaryotes. On the other hand, these proteins play an important role in plant responses to abiotic and biotic factors, indicating specific for plants functions of Rab proteins. 相似文献
998.
A snail prefers native over exotic freshwater plants: implications for the enemy release hypotheses 总被引:1,自引:0,他引:1
1. One of the most popular single‐factor hypotheses that have been proposed to explain the naturalization and spread of introduced species is the enemy release hypothesis (ERH). Most studies have considered how specialist consumers might affect the success of exotics, but the importance of generalist herbivores has received little attention 2. In this study, we compared the palatability of native and exotic freshwater plants in both phylogenetic pairings and in region‐wide assays, using a generalist consumer (the pond snail Radix swinhoei). 3. Our study indicated that, when plants were paired by taxonomic relatedness, the snail preferred native over exotic freshwater plants by a 2 : 1 ratio. The snail also preferred native over exotic plants when tested across 20 native and seven exotic species found growing in Liangzi Lake. 4. Snails consumed more biomass of native than exotic plants, providing support for the ERH and suggesting that exotic species have a potential advantage in most lakes of the middle and lower reaches of the Yangtze River in China, thus helping their invasion. 相似文献
999.
WYATT F. CROSS ALAN P. COVICH TODD A. CROWL JONATHAN P. BENSTEAD ALONSO RAMÍREZ 《Freshwater Biology》2008,53(12):2504-2519
1. Freshwater shrimps often dominate the biomass of tropical island streams and are known to have strong effects on stream ecosystem structure and function, but little effort has been dedicated toward quantifying basic energetic and life history attributes such as growth, production and longevity. Such information is critical for understanding both the role of shrimps in ecosystem dynamics and the gravity of threats to shrimp populations posed by human activities such as shrimp harvesting, dam construction and water withdrawal. 2. We quantified growth rates and secondary production of dominant freshwater shrimps for 3 years in two Puerto Rican headwater streams that differ in food web structure because of the presence or absence of predatory fishes that are excluded from reaches above waterfalls. Using growth data, we constructed a minimum longevity model to explore the likely minimum life spans of the two dominant taxa (Atya spp. and Xiphocaris elongata). Finally, we used a bioenergetics model to quantify annual consumption rates of major basal resources by the two taxa. 3. Daily growth rates ranged from ?0.001 to 0.011 day?1, were inversely related to body size, and were higher for small individuals of X. elongata than Atya spp. Mean annual shrimp biomass and secondary production were an order of magnitude higher in the stream that lacked predatory fishes (biomass: 4.34 g AFDM m?2; production: 0.89 g AFDM m?2 year?1) than in the stream with predatory fishes (biomass: 0.12 g AFDM m?2; production: 0.02 g AFDM m?2 year?1). Production : biomass ratios ranged from 0.01 to 0.38. 4. Our longevity model predicted a minimum life span of 8 years for Atya spp. and 5 years for X. elongata in the stream lacking predatory fishes. In contrast, due to a larger average size of X. elongata in the stream with predatory fishes, our model predicted a minimum life span of 11 years. Actual life spans of these taxa are likely to be much longer based on long‐term observations of marked individuals. 5. Estimated consumption rates from the bioenergetics model indicated that Atya spp. and X. elongata are important processors of organic matter resources in streams where they occur at high densities. Atya spp. and X. elongata appeared capable of consuming a large proportion of algal and insect production and the proportion of direct leaf litter inputs consumed was also appreciable (c. 40–60%). However, the consumption of suspended fine particulate organic matter (SFPOM) by Atya spp. is probably only a minor proportion of total SFPOM flux in these streams. 6. Our study suggests that geomorphic features such as waterfalls may play an important role in controlling the distribution and production of freshwater shrimps through their effects on predatory fish movement. Spatial differences in shrimp densities result in landscape‐scale variation in the significance to ecosystem processes of these long‐lived organisms, particularly as processors of major organic matter resources. 相似文献
1000.
A GIS-based simulation program to predict multi-species size-structure dynamics for natural forests in Hokkaido, northern Japan 总被引:2,自引:0,他引:2
A simulation program that runs on a geographic information system (GIS) was developed to predict the multi-species size-structure dynamics of forest stands. Because important characteristics of a forest stand, including woody biomass accumulation, carbon storage, commercial value of timber, and functions for environmental conservation, can be inferred from the size structures of the component populations, management plans can be made from the predictions of the size-structure dynamics. For example, the simulation can incorporate various forms of thinning; forest managers can then try several thinning plans in simulated forest stands and choose the appropriate plan that achieves the best results. Using GIS to predict the size-structure dynamics of forest stands is of practical importance, because GIS has been used widely in forest management and can easily handle spatial distributions of environmental information (e.g., climate, geology, soils) that may influence tree performance. To predict size-structure dynamics, the program numerically solves a continuum equation that describes size-structure dynamics based on growth and mortality rates of individual trees. When predicting size-structure dynamics of a forest stand, the program obtains the environmental information of the stand from a database stored in the GIS and calculates environmental factors such as warmth index and potential evapotranspiration/precipitation ratio that influence growth and mortality rates. The simulation program calculates growth and mortality rates using published growth and mortality models that incorporate the effects of size of the individual, competition between trees, and abiotic environmental factors. To demonstrate the effects of abiotic environmental factors on the multi-species size-structure dynamics, sensitivity analyses were conducted. The size-structure dynamics varied in a way that was predictable from the responses of the growth and mortality rates to variations in the abiotic environmental factors. To demonstrate the size-structure dynamics in different locations, five test runs of the simulation program were also performed using the same initial size-structure and five different sets of abiotic environmental conditions from five locations. At the end of the simulation, the predicted size structures differed because the growth and mortality rates differed among the five locations. Finally, the response of the size structure to thinning was clarified. The result showed how the size structure of a component species in a forest stand is dependent on the presence of other species. 相似文献