全文获取类型
收费全文 | 1393篇 |
免费 | 111篇 |
国内免费 | 149篇 |
专业分类
测绘学 | 65篇 |
大气科学 | 95篇 |
地球物理 | 185篇 |
地质学 | 261篇 |
海洋学 | 394篇 |
天文学 | 1篇 |
综合类 | 81篇 |
自然地理 | 571篇 |
出版年
2024年 | 2篇 |
2023年 | 28篇 |
2022年 | 33篇 |
2021年 | 70篇 |
2020年 | 60篇 |
2019年 | 50篇 |
2018年 | 44篇 |
2017年 | 79篇 |
2016年 | 74篇 |
2015年 | 52篇 |
2014年 | 95篇 |
2013年 | 117篇 |
2012年 | 60篇 |
2011年 | 80篇 |
2010年 | 84篇 |
2009年 | 58篇 |
2008年 | 77篇 |
2007年 | 76篇 |
2006年 | 68篇 |
2005年 | 57篇 |
2004年 | 51篇 |
2003年 | 57篇 |
2002年 | 45篇 |
2001年 | 33篇 |
2000年 | 43篇 |
1999年 | 31篇 |
1998年 | 24篇 |
1997年 | 27篇 |
1996年 | 12篇 |
1995年 | 18篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
排序方式: 共有1653条查询结果,搜索用时 15 毫秒
991.
992.
Studies on the species composition and biodiversity of free-living marine nematodes in the southern Huanghai Sea 总被引:1,自引:0,他引:1
1 IntroductionThe conservation, persistentuse and basic re-search ofbiodiversity are increasingly becom ing oneofthe centraltopics concerned by internationalaca-dem iccirclesandothergroupsdueto itshugevaluetom ankind living and suffered seriousthreats. Fr… 相似文献
993.
Fish Biodiversity in the Vermetid Reef Of Shiqmona (Israel) 总被引:1,自引:0,他引:1
Abstract. A study of the structure of the fish community associated with a vermetid reef at Shiqmona, Israel, revealed the highest fish biodiversity (36 species) of any habitat along the Mediterranean coast of Israel. Despite the prevalence of fish of Erythrean origin in the Levantine littoral, the benthic fish community at the site is entirely autochthonous. The families richest in members were the Blenniidae and the Gobiidae. Eighteen species are benthic, sixteen supra-benthic and two pelagic. 相似文献
994.
995.
This study examines the economic efficiency of implementing a residential water conservation program in a small Pennsylvania public water utility. Local demographic data and results from similar programs elsewhere were used to estimate potential water conservation for three programs: rebates for low-flow toilets, rebates for high-efficiency washing machines, and in-home water audits. Future water supply and wastewater treatment demand were estimated. The net present value of constructing, operating, and maintaining new capacity infrastructure (water supply, water storage tanks, and wastewater treatment plant capacity), as well as the costs of the conservation programs, was calculated for a range of scenarios that included future demand growth, construction costs, interest rates, and levels of participation in the conservation programs. Results indicate that investing in residential water conservation would be cost-effective for a small number of scenarios that generally combine higher growth rates, higher interest rates, higher construction costs, and relatively modest public participation in conservation. The results are affected by the timing of necessary supply-side investments relative to the most significant water conservation gains, the relatively inexpensive local water supply costs, and the relatively high local wastewater treatment costs. Water conservation might be cost-effective for a wider range of conditions in communities facing scarce and expensive water supply options. 相似文献
996.
生物土壤结皮对荒漠昆虫多样性的影响 总被引:4,自引:6,他引:4
生物土壤结皮广泛分布在干旱半干旱地区与寒区荒漠,是荒漠生态系统的主要组成和景观特征之一,其重要性已被大量的研究报道所证实。然而,关于生物土壤结皮与昆虫种类多样性之间关系的研究却很少。本文以腾格里沙漠东南缘的沙坡头地区半固定沙丘柠条-油蒿群落和固定沙丘柠条-油蒿群落为观测样地,选择具有不同类型生物土壤结皮分布的植被群落为观测样方。昆虫的调查采用100 m×100 m的样方,利用样筐和网捕法收集昆虫,记录昆虫数量,采集标本在室内进行鉴定。结果表明:与无结皮覆盖的植被区相比,生物土壤结皮在地表的覆盖显著地增加了昆虫的多样性和种的丰富度,其中以苔藓和地衣为主的结皮覆盖的植被样方中昆虫种的多样性和丰富度显著地高于以蓝藻和藻类为主的结皮样方。生物土壤结皮对荒漠昆虫多样性的贡献可能是由于稳定了土表、改善了植被系统中的土壤环境,为昆虫,特别是幼虫阶段提供相对适宜的土壤生境或部分食物来源。 相似文献
997.
A floristic quadrat study of sparsely wooded plains on western Melville Island was conducted and used to define and describe floristic and structural vegetation types and identify their associated environmental correlates. The results were compared with an existing land‐unit survey of the area in order to identify the relative merits of these schemes for conservation planning. All schemes identify a range of broad structural types including Acacia shrubland, Grevillea pteridifolia low woodland, Lophostemon lactifluus low woodland, and Banksia dentata low woodland. However, some of the structural and land‐unit types are composed of two plant communities with distinctive environmental relations. Despite being structurally distinctive, some community types are floristically indistinguishable from surrounding forest dominated by Eucalyptus miniata and E. tetrodonta. The number of species captured by randomly selecting sites from each classificatory type revealed little difference between schemes. All the classificatory approaches were less effective at species capture than a simple iterative procedure. Some of the plant communities seem to be unique and require preservation initiatives in the face of current forestry operations and planned horticultural development. 相似文献
998.
Mark Morrison 《社会与自然资源》2014,27(1):70-87
Connectivity initiatives typically require conservation action on private lands as a complement to public reserve systems, so they are more likely to succeed if underpinned by knowledge of the social factors influencing landholders' participation. Using a combination of social survey and market research techniques, we identify spatially referenced landholder segments. Using a number of behavioral and attitudinal constructs, including three psychographic constructs not previously used for identifying segments—sense of place, lifestyle motives, and nature values—we identify three lifestyler and two farmer segments, and show that the characteristics of lifestyler segments are more nuanced than previously thought. Preferences for four corridor connectivity initiatives are found to differ across landholder segments, which indicates the importance using a targeted approach for designing connectivity conservation programs. The results highlight the importance for achieving corridor connectivity of the Blue Collar Lifestyler segment, a lower sociodemographic segment not previously identified in the literature. 相似文献
999.
1000.
As land use change continues to increase throughout the Amazon basin, there is a pressing need to accurately map, quantify and assess the effects of different factors on forest cover change (FCC). Land tenure may sometimes have important effects on forest cover, yet such effects remain poorly understood in Amazonia, particularly outside Brazil. In this paper we assess whether significant differences in trends of FCC can be partially explained by different land tenure arrangements, using a case study in southwestern Beni (Bolivian Amazon). We examine spatio-temporal dynamics of FCC across four land tenure systems (indigenous titled territory, protected area, logging concession, and private land) by classifying forests using a time-series of Landsat satellite imagery consisting of four dates (1986, 1996, 2001, 2009). Specifically, we unravel (1) trends in early growth and old-growth forest extent, including changes in total cover area, annual change rates, and spatial change dynamics, and (2) trends in old-growth forest fragmentation. To better understand the association between land tenure and FCC, we qualitatively assess the potential role that other underlying and proximate drivers may have had in FCC over the study period. We found that private lands underwent, by far, the largest FCC, that indigenous territories and the protected area had little FCC, and that logging concessions were responsible for the lowest FCC. Our findings suggest that land tenure played a key role in FCC except in private areas, where many other drivers had operated. Our study sheds light into the potential role of land tenure in FCC and has important implications for public policies aimed at socioeconomic development and environmental conservation in the Amazon. We give some policy recommendations drawn from a biocultural conservation perspective that could contribute to implement more inclusive conservation policies in the region. 相似文献