首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   428篇
  免费   26篇
  国内免费   31篇
测绘学   7篇
大气科学   13篇
地球物理   78篇
地质学   53篇
海洋学   118篇
天文学   1篇
综合类   15篇
自然地理   200篇
  2023年   3篇
  2022年   10篇
  2021年   5篇
  2020年   10篇
  2019年   14篇
  2018年   15篇
  2017年   20篇
  2016年   16篇
  2015年   22篇
  2014年   29篇
  2013年   47篇
  2012年   14篇
  2011年   25篇
  2010年   25篇
  2009年   25篇
  2008年   24篇
  2007年   26篇
  2006年   29篇
  2005年   11篇
  2004年   12篇
  2003年   15篇
  2002年   16篇
  2001年   8篇
  2000年   11篇
  1999年   9篇
  1998年   10篇
  1997年   6篇
  1996年   2篇
  1995年   2篇
  1994年   1篇
  1993年   4篇
  1992年   2篇
  1991年   3篇
  1990年   3篇
  1989年   2篇
  1987年   3篇
  1986年   2篇
  1985年   2篇
  1982年   1篇
  1981年   1篇
排序方式: 共有485条查询结果,搜索用时 31 毫秒
41.
Pollution of the water environment by mining waters is a problem not only in Poland, but worldwide. This study investigated the mollusc communities in seven mining subsidence reservoirs affected by coal mine output (the Katowicka Upland, Upper Silesia, Southern Poland). The objective of the survey was to determine the relationship between the molluscs and their environments and to evaluate the ecological-conservation value of freshwater habitats which support rare and vulnerable molluscs. From 1993 to 2005, 23 mollusc species were recorded. Our result confirmed an invasion by Potamopyrgus antipodarum (Gray, 1843), whose density varied from 2 to 2422 individuals/m2 in the waters of the Katowicka Upland. A few rare and vulnerable species were found, e.g. Acroloxus lacustris (Linnaeus, 1758), Hippeutis complanatus (Linnaeus, 1758), Ferrissia wautieri (Mirolli, 1960), Anodonta anatina (Linnaeus, 1758), Musculium lacustre (O.F. Müller, 1774) or Pisidium casertanum (Poli, 1791). Principal component analysis showed a positive correlation between mollusc density and pH, the concentration of chlorides, the total hardness, alkalinity and total dissolved solids, and a negative correlation between the number of species and phosphates. Because of the distinctive environmental features of the mining subsidence reservoirs in Czułów, the research area has provided a refuge for wildlife.  相似文献   
42.
The study by the eddy covariance technique in the alpine shrub meadow of the Qing-hai-Tibet Plateau in 2003 and 2004 showed that the net ecosystem carbon dioxide exchange (NEE) exhibited noticeable diurnal and annual variations, with more distinct daily changes during the warmer seasons. The CO2 emission of the shrub ecosystem culminated in April and September while the CO2 absorption capacity reached a maximum in July and August. The absorbed carbon dioxide during the two consecutive years was 231.4 and 274.8 g CO2·m-2 respectively, yielding an average of 253.1 gCO2·m-2 per year: that accounts for a large proportion of absorbed CO2 in the region. Obviously, the diurnal carbon flux was negatively related to temperature, radiation and other atmospheric factors. Still, minute discrepancies in kurtosis and duration of carbon emission/absorption were detected between 2003 and 2004. It was found that the CO2 flux in the daytime was similarly affected by photosynthetic photon flux density in both years. Temperature appears to be the most important determinant of CO2 flux: specifically, the high temperature during the plant growing season inhibits the carbon absorption capacity. One potential explanation is that soil respiration is enhanced under such condition. Analysis of biomass revealed that the annual net carbon fixed capacity of aboveground and belowground biomass was 544.0 in 2003 and 559.4 g Cm"2 in 2004, which coincided with the NEE absorption capacity (63.1 g C·m-2 in 2003 and 74.9 g C·m-2 in 2004) in the corresponding plant growing season.  相似文献   
43.
Abstract

Abstract Water balances for a re-vegetated xerophyte shrub (Caragana korshinskii) area were compared to that of a bare surface area by using auto-weighing type lysimeters during the 1990–1995 growing seasons at the southeast Tengger Desert, Shapotou, China. The six-year experiment displayed how major daily water balance components might vary for a bare and a re-vegetated sand dune area. Evapotranspiration from the C. korshinskii lysimeter represented a major part of the water balance. The average annual ET/P ratios varied between 69 and 142%. No seepage was observed for the vegetated lysimeter. For the bare lysimeter, on the other hand, 48 mm or 27% of observed rainfall per year occurred as seepage. These results suggest that re-vegetating large sandy areas with xerophytic shrubs could reduce soil water storage by transpiration. Also, the experimental results indicate that re-vegetating large sandy areas could significantly change groundwater recharge conditions. However, from a viewpoint of desert ecosystem reconstruction, it appears that natural rainfall can sustain xerophytic shrubs such as C. korshinskii which would reduce erosion loss of sand. However, re-vegetation has to be balanced with recharge/groundwater needs of local populations.  相似文献   
44.
The grazing turbinid gastropod Lunella smaragda was sampled regularly over 3.5 y from precise sites of different microtopography and height on the platform shore at Goat Island Beach near Leigh, Northland, New Zealand. Growth is linear for over 3 y, and the year‐classes distinct. The position of the different size‐classes is related to both the shelter afforded by the microtopography, and to the height on the shore. The populations in the mid‐eulittoral turf flats, low eulittoral bare rock areas, and the sublittoral fringe are distinct, and there is a general movement down the shore with age and size. Wave action apparently dislodges the animals from higher areas when they grow to a critical size and transports them to sites lower on the shore, where wave disturbance is less. Field experiments with marked animals and laboratory studies with a wave tank confirm that the wave‐effected distribution is size related. However measurements showed that the ratio of foot attachment area to the shell area presented to the wave does not vary with animal size. The possible benefits of wave dislodgment and wave‐effected distribution are considered.  相似文献   
45.
:In the context of high-quality development, the conflicting relationship between tourist areas and surrounding communities has become a severe obstacle to the realization of high-quality economic development. As a theoretical tool to describe the relationship between tourist areas and surrounding communities, the concept of tourism island is of great value in identifying, assessing, and managing the conflicts between tourist areas and surrounding communities. Therefore, it has gradually become a new topic in geography, sociology, economics, and other related disciplines. On the basis of distinguishing and analyzing the three interrelated concepts of tourism enclave, tourism bubble, and tourism island, this article systematically reviewed the research progress from the four aspects of tourism island types, key characteristics, causes of formation, and impacts, and found that the current research still has obvious deficiencies in theoretical framework, empirical research, and governance system. Finally, this article proposed three specific directions for further research based on the shortcomings of the current research. Based on the practice of Chinese tourism development, a theoretical framework of tourism island should be constructed, the formation processes and impacts of tourism island should be explored using diversified methods of analysis, and a multi-dimensional, multi-level, and multi-process regulatory mechanism should be developed. Overall, this article presented a systematic review of the previous research literature on tourism island to show their results and quality; it also clarified the starting point for further research and laid a foundation for new research works. © 2023, Editorial office of PROGRESS IN GEOGRAPHY. All rights reserved.  相似文献   
46.
Andean grasslands ecosystems are fragile environments with rigorous climatologic conditions and low and variable food for the grazing. The Apolobamba area is located in the Bolivian Andean Mountains. Its high grasslands provide a natural habitat for wild and domestic camelids such as vicuna(Vicugna vicugna) and alpaca(Lama pacos). The botanical diversity plays an essential role in maintaining vital ecosystem functions. The objectives of this research were to determine the seasonal changes in soil properties, to study the vegetation changes during the wet and dry seasons and the influence of soil properties and camelid densities on the vegetation in the Apolobamba grasslands. Four zones with different vicuna populations were selected to be studied. The following soil parameters were determined: total organic carbon, total nitrogen, available phosphorous, cation exchange capacity, exchangeable cations, pH and texture. The vegetation season changes were studied through botanical identification, above-ground biomass, plant cover and species richness. Results showed that some soil properties such as C/N ratio, CEC, silt and clay percentages kept stable against the seasonal changes. Generally, soil nutrients were relatively higher during the dry season in the surface and subsurface. The results did not point out the predominant vegetation growth during the wet season. The seasonal vegetation growth depended on each species. Thegood soil fertility corresponded to the highest plant cover. Soil fertility presented no influence on the above-ground biomass of the collected species. The negative influence of camelid grazing on soil properties could not be assessed. However, overgrazing could affect some plant species. Therefore, protection is needed in order to preserve the biodiversity in the Andean mountain grasslands.  相似文献   
47.
Wildfires cause enormous damage worldwide, particularly in Victoria, Australia, with growing populations in fire-prone ecosystems. Broad-scale prescribed burning is an established, yet controversial, wildfire management policy in Victoria and Australia. But does broad-scale prescribed burning reduce fire damage? The answer depends on how damage is measured. We propose that different perceptions about the efficacy of broad-scale prescribed burning derive from different disciplinary measures of damage (e.g., biodiversity, area burned, or fatalities). Although broad-scale prescribed burning reduces subsequent wildfire size, there is little evidence that it saves human lives and homes. Evidence suggests that small-scale fuel reduction may be more effective at saving lives. The prescribed burning debate might be better focused on what damage we wish to limit, and which measures are most effective at limiting this damage. In this context, the current policy focus on broad-scale prescribed burning targets may distract efforts from more effective lifesaving strategies.  相似文献   
48.
The management of marine ecosystems requires adequate knowledge of both environmental and human dimensions, as well as their interrelationships. In this study, the aquaculture and fisheries activities are analyzed in one of most important fishing regions in Europe, Galicia (NW Spain). In particular, the intensity and characteristics of the fishing dependency are evaluated in terms of income and employment. Thus, nine marine economic activities for nine Maritime Zones have been defined, considering the social and economic relevance of the provided ecosystem services to these communities. This paper highlights the entire Galician coast as fishing-dependent, independently of the urbanization level. Furthermore, the contribution of different fishery segments to the income and employment of these coastal communities is reviewed, including fluctuations on whether the activity is small or large-scale. Finally, this study establishes a strong relationship among the marine activities, which generate most employment, and the regulatory framework of the Regional Government. Therefore, the conclusions are relevant to design and implement policies that affect Galicia׳s Region and all its related marine ecosystems.  相似文献   
49.
DETERMINANTS OF STRUCTURE IN AQUATIC NON-PELAGIC PROTOZOAN COMMUNITIES   总被引:2,自引:0,他引:2  
Although microbes have traditionally been used as models for testing ecological theory, research on the organization of these communities has largely been isolated from other areas of community ecology. Early studies indicated that microbial populations can form dynamic, interacting assemblages. Observations of distributional patterns suggest that certain deterministic forces regulate community structure. It appears that protozoans are tolerant of a wide range of environmental conditions although evidence for this is largely based on gross environmental analyses. Laboratory studies have suggested the importance of negative and positive biotic interactions in determining community membership, but little field evidence for this exists. The importance of transport processes in controlling community composition is being increasingly recognized. All three types of processes likely act to regulate the colonization and successional dynamics of these communities. A simple model is presented to promote the devel  相似文献   
50.
Medicinal plants and fungi play important roles in the health of Maliseet people of northern Maine, USA. A critical aspect of exercising choice in health care for this community is the ability to locate and have access to these plants. Habitat suitability modeling is a form of geospatial technology that can enhance health sovereignty by identifying locations in which populations of medicinal plants can be conserved or established. However, use of this technology within indigenous communities has been limited. Focusing on the medicinal plant muskrat root, Acorus americanus (Raf.) Raf., we generate a habitat suitability model for eastern Aroostook County, Maine (1,055,653.659 ha) that also takes community needs into consideration. Drawing on participatory ethnographic data as well as environmental characteristics, our model combines ecological and sociocultural parameters to identify previously unknown populations of A. americanus that are accessible to tribal elders. Our model successfully predicted 95% of A. americanus locations in our field validation data set of ∼71,000 ha. Results suggest that approximately 0.6% of our study area contains suitable habitat to plant muskrat root that could also meet tribal members' gathering needs for the future. Increasing the number of potential collection sites gives communities options for gathering, thereby enhancing health sovereignty. Broadly, our work suggests that, when done in partnership with communities, different forms of geospatial technology can be beneficial tools for efforts to promote health sovereignty.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号