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1.
An S I R S epidemiological model with vital dynamics in a population of varying size is discussed. A complete global analysis is given which uses a new result to establish the nonexistence of periodic solutions. Results are discussed in terms of three explicit threshold parameters which respectively govern the increase of the total population, the existence and stability of an endemic proportion equilibrium and the growth of the infective population. These lead to two distinct concepts of disease eradication which involve the total number of infectives and their proportion in the population.Partially supported by NSF Grant No. DMS-8703631. This work was done while this author was visiting the University of VictoriaResearch supported in part by NSERC A-8965 相似文献
2.
Marcia Ricci 《Biodiversity and Conservation》2006,15(10):3111-3130
Was realized field studies and ex situ propagation on the vascular flora of Juan Fernández Archipelago during 15 year period.
To evaluate the conservation status of a total of 133 species and subspecies of vascular endemic plants I used a IUCN classification
founding: 2 species extinct, 1 extinct in it natural habitat, 52 critically endangered, 37 endangered and 9 vulnerable. Thus,
73.8% are contained in a threat category; only 24 taxon can be considered to be of a lesser conservation concern. The largest
threat of extinction is a reduction in individuals in local populations resulting in small, isolated populations. This habitat
fragmentation and a reduction in endemic flora has also impacted endemic fauna. Besides, during this period was propagated
in nurseries a total of 80 of these species and subspecies (60%). It seem clear the necessity to continue to actions conserve
this particular ecosystem. 相似文献
3.
China is among the world's richest countries in terms of plant biodiversity. Besides the abundant flora, containing some 33,000
vascular plants (30,000 angiosperms, 250 gymnosperms, and 2600 pteridophytes), there is extraordinary ecosystem diversity,
as well as a large pool of both wild and cultivated germplasms. China is also considered one of the main centers of origin
and diversification for seed plants on Earth, and is especially profuse in phylogenetically primitive taxa and/or paleoendemics
due to the refuge role glaciation played during the Quaternary period. The collision with the Indian subcontinent significantly
enriched Chinese flora and led to the formation of many neoendemisms. However, flora distribution remains uneven, and some
local floristic hotspots are found across China, such as Yunnan, Sichuan and Taiwan. Unfortunately, this biodiversity faces
enormous threats, which have increased substantially over the last 50 years. The combined effects of habitat destruction and/or
fragmentation, environmental contamination, over-exploitation of natural resources, and to a lesser extent, introduction of
exotic species, have caused irreparable damage to China's plant biodiversity. Burgeoning economic and population growth have
also contributed to this deterioration. It is believed that up to 5000 flora species are currently endangered in China, with
some taxa having already become extinct. Although in recent years government authorities have made some efforts to preserve
biodiversity, much work remains to be done. While China has established an extensive network of nature reserves and protected
areas, encompassing more than 16% of the total land area, insufficient budgetary and staffing commitments are common limitations
in their management structures. Ex-situ conservation is also deficient, primarily because the botanical gardens are not representative of several local floras, nor
are they often of adequate size or representative of endangered species. The lack of effective and efficient environmental
legislation and education are also problems that continue to accelerate the loss of plant biodiversity in China. 相似文献
4.
基于鸟类特有种亚种分化的保护优先性 总被引:2,自引:0,他引:2
我们以中国鸟类特有种为代表,基于鸟类物种分化程度来探讨多样性保护优先区。本文参照105种中国特有鸟种的分化等级来制作物种地理分布图。依据生物种的概念,给单型种赋值“1”,对具有n各亚种分化的物种赋值“n”。利用GIS叠加与制图功能对物种的分布做图以反映不同区域的物种分化等级。结果发现分布中心赋值很高并以同心圆形式向周边递减,反映了物种在中国西南部横断山区至秦岭高度分化的地理格局,并由此向外递减。作为全球25个生物多样性热点之一的横断山区,可能不仅是物种种级而且是种下级多样性的热点。因此,该地区的多样性保护优先性,不仅要考虑目前物种多样性分布的格局而且要考虑其未来发展 相似文献
5.
根据文献资料和标本馆及化石记录,讨论了壳斗科栲属植物的现代分布和地史分布。现代栲属植物有110~134种,主要分布在东亚及东南亚,其中印度支那地区有82种栲属植物,是世界栲属植物分布最集中的地区,马来西亚地区是栲属特有种最丰富的事实是支持马来西亚地区与其它地区的区系交流十分有限的论据。中国栲属植物最丰富的地区是滇黔桂地区(29种)。栲属植物现代多样化中心在马来西亚和中南半岛。排除Chrysolepis后,栲属的分布类型应属热带亚洲分布。栲属在地质历史上有着比现在广泛的分布,最早、最可靠的栲属化石记录发现于北美始新世地层,欧洲和日本始新世也有栲属的化石记录,化石记录表明栲属起源的时间不晚于古新世,所有的壳斗科及栲属的化石都发现于北半球,现代分布也主要在北半球,壳斗科及栲属起源于北半球可以确认,由于化石证据与现代植物学的研究结果有较大差异以及关键地区化石证据的不足,具体的起源地尚不能肯定。 相似文献
6.
Viral hepatitis A, as other endemic diseases, represents a public health priority worldwide. To study long-time scale human pathogens through individual-based simulations, the development of a dynamic network of contacts is required. In this work, we introduce an individual-based model accounting for the birth and death of the individuals, the generation of new households, and the educational career of the individuals, in order to investigate viral hepatitis A dynamics in the most affected Italian areas. Intervention options such as targeted vaccination, social distancing measures (e.g., closure of day care centers and kindergartens) and improvements in standards of living and hygiene are evaluated. Results show that a very low vaccination coverage is sufficient to control hepatitis A in Italy, while its elimination is not possible since new cases are continuously imported from high endemicity areas outside the country. Finally, the considered social distancing measures can be counterproductive since the fraction of recovered individuals does not decline while the age at infection increases, thus augmenting the probability of developing acute symptoms. 相似文献
7.
The purpose of this paper was to analyze the diversity patterns of Cactaceae at a global scale, to identify those countries
where conservation actions should be performed. In order to do this, the species richness and the number of endemic species
for 34 American countries were determined. With these data, the relationship between the total number of species or the number
of endemic species and the area of the countries were analyzed. In addition, a complementarity analysis was conducted to determine
the most important countries for cactus conservation. Results showed that Mexico had the highest number of total and endemic
species followed by Argentina, Bolivia, Brazil, and Peru, among others. There was a significant positive relationship between
both, the total and endemic species, and the area of the countries. Despite this fact, the cactus diversity in Mexico, Argentina,
Peru, Bolivia, Chile, and Costa Rica was higher than expected according to their area. Further, these countries also presented
the highest proportions of endemic species. The complementarity analysis indicated that 24 countries are necessary to preserve
all cactus species. However, 94% of all species could be preserved with only 10 countries. Considering the diversity patterns
and the complementarity analysis, three important groups for cactus conservation were identified: (1) Mexico, Argentina, Peru,
Bolivia, Chile, and Costa Rica, (2) Paraguay and Cuba, and (3) Brazil and USA. Conservation efforts should be focused on these
countries in order to preserve cactus diversity. 相似文献
8.
一个具暂时免疫且总人数可变的传染病动力学模型 总被引:10,自引:3,他引:7
建立了一个具常恢复率和接触率依赖于总人数的SIRS传染病动力学模型,讨论了系统平衡点的存在性和稳定性,对双线性传染率的特殊情形,给出了传染病平衡点的全局稳定性结论,推广和改进了已有的相应结果。 相似文献
9.
10.
To understand the differences in the spawning sites among Cyprininae fishes in Lake Biwa, we conducted periodic sampling of
larvae and juveniles at three sites (irrigation ditch, St. 1; river, St. 2; and satellite lake, St. 3). On the basis of species/subspecies
identification by using RAPD analysis, we examined the species composition of the larvae and juveniles at these three sampling
sites. The number of specimens was 616, 68, and 117 at St. 1, St. 2, and St. 3, respectively. Based on morphological and genetic
identification, the specimens were found to include nine fish species/subspecies, namely, Carassius auratus grandoculis, Carassius cuvieri, Carassius auratus langsdorfii, Cyprinus carpio, Sarcocheilichthys sp., Silurus asotus, Oryzias latipes, Odontobutis obscura obscura, and Rhinogobius sp. The species composition at the three sites also differed. Among the Cyprininae fishes, C. auratus grandoculis, C. auratus langsdorfii, and Cyprinus carpio were found in abundance at St. 1; C. cuvieri was not collected from St. 1 but was found at the other two sites, particularly St. 3. Among the other fishes, Rhinogobius sp. was collected at St. 1 and St. 3, whereas the other four occurred only at St. 1. These results suggest that the selection
of spawning sites by C. cuvieri differs to a certain extent from that of the other Cyprininae fishes, and the irrigation ditch in the lake is an important
habitat for the larvae and juveniles of native fish species. 相似文献