全文获取类型
收费全文 | 803篇 |
免费 | 78篇 |
国内免费 | 23篇 |
出版年
2024年 | 2篇 |
2023年 | 21篇 |
2022年 | 11篇 |
2021年 | 23篇 |
2020年 | 41篇 |
2019年 | 36篇 |
2018年 | 27篇 |
2017年 | 35篇 |
2016年 | 37篇 |
2015年 | 21篇 |
2014年 | 33篇 |
2013年 | 39篇 |
2012年 | 35篇 |
2011年 | 27篇 |
2010年 | 29篇 |
2009年 | 43篇 |
2008年 | 57篇 |
2007年 | 38篇 |
2006年 | 41篇 |
2005年 | 33篇 |
2004年 | 43篇 |
2003年 | 21篇 |
2002年 | 28篇 |
2001年 | 22篇 |
2000年 | 13篇 |
1999年 | 15篇 |
1998年 | 19篇 |
1997年 | 10篇 |
1996年 | 19篇 |
1995年 | 6篇 |
1994年 | 13篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有904条查询结果,搜索用时 93 毫秒
81.
ABSTRACT. Although breeding populations of Piping Plovers are well studied, their winter distribution is less clear. We studied the seasonal abundance of nonbreeding Piping Plovers ( Charadrius melodus ) during the winters of 2003–2004 and 2004–2005 on Little St. Simons Island (LSSI), Georgia. Our objectives were to determine the relative abundance of individuals from three breeding populations at LSSI, and identify possible differences among populations in arrival time, winter movements, or departure time. We observed up to 100 Piping Plovers during peak migration, and approximately 40 plovers wintered at LSSI. From July 2004 to May 2005, approximately 20% of the Great Lakes breeding population used LSSI. Plovers were not present at LSSI during June. All breeding populations of Piping Plovers had similar patterns of temporal occurrence on LSSI, suggesting no need for population-specific management plans at this site. Our results suggest that LSSI is among the most important wintering sites on the Atlantic coast for Piping Plovers, especially for individuals from the endangered Great Lakes population. 相似文献
82.
Christopher B. Ruff Clark Spencer Larsen Wilson C. Hayes 《American journal of physical anthropology》1984,64(2):125-136
Structural characteristics of the femur are compared in preagricultural (2200 B.C.–A.D. 1150) and agricultural (A.D. 1150–1550) subsistence strategy groups from the Georgia coast. Using an automated technique, cross-sectional geometric properties used in structural analyses (areas, second moments of area) were determined at midshaft and distal to the lesser trochanter in 20 adults from each group. A significant decline in magnitude of almost every geometric property occurs in the agricultural group. The differences between groups are reduced but still significant for many properties after standardizing for bone length differences. In addition, a remodeling of the femoral cortex to one of relatively smaller medullary and subperiosteal diameter, as well as a more circular cross-sectional shape, is characteristic of agricultural femora. Thus, while the relative cross-sectional area of bone remains the same, the spatial distribution of bone area is different in the two groups. The results strongly suggest a relative reduction in mechanical loadings of the femur in the agricultural group, implying different levels and possibly types of activity involving the lower limb in the two groups. The data are also compared with similar data available for the Pecos Pueblo (agricultural) sample. The comparison indicates that types of activity may have been more similar in the two agricultural samples, but that general levels of activity were more similar in the Pecos Pueblo and Georgia coast preagricultural samples. 相似文献
83.
This article examines transformations associated with changes in resource use and land cover dynamics in the community of São Manoel, Maranhão state, in the eastern Brazilian Amazon. The shifting cultivator peasants in São Manoel integrate swidden fields for annual cropping, the extraction of babassu palm products, and pastures for cattle ranching. Since the early twentieth century, predominant vegetative cover patterns have been altered from species-rich mature forests to secondary succession with babassu dominant to pasture or swidden fields containing palm stands of various densities. A grounded political ecology of resource use in the area suggests that management strategies and the resulting land cover dynamics integrate site-specific decisions of peasant producers. I discuss the trajectory of production strategies in São Manoel since the establishment of the community in the 1920s, and identify the multiple dimensions affecting resource use and environmental outcomes, with an emphasis on the period following land struggles and the recovery of peasant tenure rights in the mid-1980s. The analysis indicates that socionatural trajectories that optimize resource use and address the socioeconomic needs of the community include the maintenance of palm/pastures associations. 相似文献
84.
Wendy R.?TownsendEmail author A.?Randall Borman Eduardo?Yiyoguaje Luis?Mendua 《Biodiversity and Conservation》2005,14(11):2743-2755
The Cofán Indians of Aguarico and Zábalo Rivers in Ecuador in 1990 began a turtle recuperation effort by raising turtle hatchlings
(Podocnemis expansa and P. unifilis) in small pools. To evaluate the effectiveness of this program, the Cofán developed and tested three methods to document
changes in the populations of turtles. The first method consisted of interviews with people from neighbouring communities
about their perceptions of changes in turtle abundance. The second method was a direct count of turtles observed along the
rivers from canoe. The third method was an indirect count, involving turtle nest censusing along community beaches. Two of
the methods yielded numerical, statistically positive turtle population trends. The turtle program has influenced the community's
perception of turtle conservation issues so that, instead of hunting the adults and eggs, they now protect them. The training
in record-keeping permits them to make a transparent history of the equitability of access to the turtle egg resource. The
Cofán monitoring experience strengthened their successful petition to the Ecuadorian government for the rights to manage their
territorial lands within the Cuyabeno Wildlife Reserve, and to receive protected area status and management rights to other
ancestral lands. The experience gained by the Zábalo monitors is now serving as a model for the design and implementation
of the park monitoring system of the newly-established Cofán Park Ranger Corps, which will be patrolling three Ecuadorian
Ecological Reserves which overlap Cofán inhabited areas. 相似文献
85.
The annual and lunar reproductive cycle of the widely distributed edible sea urchin Tripneustes gratilla (L) was examined through measurements of gonad index, histological examination of gametogenesis, and induction of spawning with KCl injections. The population density and morphological characteristics of urchins at Diani, Kanamai, and Vipingo reef lagoons were also studied as well as the effects of seawater temperature and light on reproduction. Gonad growth started early during the northeast monsoon and reached a peak in June at the beginning of the southeast monsoon followed by a sharp decrease in gonad size of 50% in July and August towards the end of the southeast monsoons. Histological examination of gonads, revealed many different stages of gametogenesis with gametes present throughout the year, indicating continuous reproduction. There was a significant relationship between gonad index and lunar day with spawning occurring between lunar day 7 and 21, but spawning was not in perfect synchrony in the population. The population density of urchins at each reef is variable from year to year and was highest on average at Vipingo. Urchins at Kanamai had the lowest gonad indices, the largest jaws and smallest individuals an indication of food limitation. The gonads (roe) of T. gratilla at all three sites, were perpetually ‘runny’ an attribute that is not suitable for urchin fisheries. Studies to develop techniques to improve roe quality are recommended. 相似文献
86.
Vona G Falchi A Moral P Calò CM Varesi L 《American journal of physical anthropology》2005,127(3):361-369
New data were obtained on mitochondrial DNA (mtDNA) from Guahibo from Venezuela, a group so far not studied using molecular data. A population sample (n = 59) was analyzed for mtDNA variation in two control-region hypervariable segments (HV1 and HV2) by sequencing. The presence or absence of a 9-bp polymorphism in the COII/tRNA(Lys) region was studied by direct amplification and electrophoretic identification. Thirty-eight variable sites were detected in regions HV1 and HV2, defining 26 mtDNA lineages; 23.7% of these were present in a single individual. The 9-bp deletion was found in 3.39% of individuals. Nucleotide and haplotype diversities were relatively high compared with other New World populations. The identified sequence haplotypes were classified into four major haplogroups (A-D) according to previous studies, with high frequencies for A (47.46%) and C (49.15%), low frequency for B (3.39%), and an absence of D. 相似文献
87.
Ready JS Sampaio I Schneider H Vinson C Dos Santos T Turner GF 《Journal of evolutionary biology》2006,19(4):1139-1148
Laboratory mate choice experiments have confirmed species status for cichlid fish in the African Great Lakes that differ in colour and little else. Colour differences between allopatric populations of the South American cichlid genus Apistogramma are known for many species, yet the status of such populations has not been previously tested. Analysis of the genetic relationships and mate choice characteristics of populations previously described as Apistogramma caetei from eastern Amazonia indicates genetic differentiation into at least three allopatric lineages, which also show strong prezygotic isolation through female mate choice, confirming them as Biological species. If future studies confirm that this result is indicative of a general trend, the species richness of the South American cichlid fishes may presently be seriously underestimated. 相似文献
88.
Jordan P O'Neill S 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2010,365(1559):3875-3888
Many recent studies of cultural inheritance have focused on small-scale craft traditions practised by single individuals, which do not require coordinated participation by larger social collectives. In this paper, we address this gap in the cultural transmission literature by investigating diversity in the vernacular architecture of the Pacific northwest coast, where communities of hunter-fisher-gatherers constructed immense wooden long-houses at their main winter villages. Quantitative analyses of long-house styles along the coastline draw on a range of models and methods from the biological sciences and are employed to test hypotheses relating to basic patterns of macro-scale cultural diversification, and the degree to which the transmission of housing traits has been constrained by the region's numerous linguistic boundaries. The results indicate relatively strong branching patterns of cultural inheritance and also close associations between regional language history and housing styles, pointing to the potentially crucial role played by language boundaries in structuring large-scale patterns of cultural diversification, especially in relation to 'collective' cultural traditions like housing that require substantial inputs of coordinated labour. 相似文献
89.
Ibraheem Borie Mohammad Ibraheem 《Journal of phycology》2010,46(4):818-824
Five cyanobacterial species (Phormidium sp., Nostoc sp., Anabaena sp. Aphanothece conferta, and Synechocystis aquatilis) isolated from the Suez Canal coast at the city of Ismailia (Egypt) were tested for biodegradation of four hydrocarbon (HC) compounds: two aliphatic compounds (n‐octadecane and pristine) and two aromatic compounds (phenanthrene and dibenzothiophene). High degradation efficiencies for the two aliphatic compounds were measured for A. conferta (64% for n‐octadecane and 78% for pristine) and S. aquatilis (85% for n‐octadecane and 90% for pristane). However, the other biodegradation percentages ranged between weak and moderate percentages. 相似文献
90.
Oil and gas concessions now cover vast swaths of the western Amazon, including protected areas and indigenous territories. The Yasuní-ITT Initiative, Ecuador's innovative proposal to leave nearly a billion barrels of oil locked beneath Yasuní National Park, is the first major effort to reverse this trend. We provide a concise overview of the initiative, including an in-depth look at its widely lauded goals of protecting biodiversity, respecting indigenous peoples' territory, and combating climate change. We also discuss the proposal's lingering caveats, such as technical questions regarding the generation of financial resources to replace the forgone oil revenue. We conclude that the Yasuní-ITT Initiative is a potentially precedent-setting advance towards avoiding damaging oil and gas development in sensitive areas. 相似文献