全文获取类型
收费全文 | 559篇 |
免费 | 60篇 |
国内免费 | 211篇 |
出版年
2024年 | 7篇 |
2023年 | 21篇 |
2022年 | 23篇 |
2021年 | 32篇 |
2020年 | 30篇 |
2019年 | 41篇 |
2018年 | 39篇 |
2017年 | 33篇 |
2016年 | 41篇 |
2015年 | 45篇 |
2014年 | 35篇 |
2013年 | 44篇 |
2012年 | 29篇 |
2011年 | 30篇 |
2010年 | 27篇 |
2009年 | 29篇 |
2008年 | 26篇 |
2007年 | 31篇 |
2006年 | 33篇 |
2005年 | 23篇 |
2004年 | 24篇 |
2003年 | 14篇 |
2002年 | 19篇 |
2001年 | 22篇 |
2000年 | 14篇 |
1999年 | 13篇 |
1998年 | 11篇 |
1997年 | 6篇 |
1996年 | 8篇 |
1995年 | 11篇 |
1994年 | 8篇 |
1993年 | 13篇 |
1992年 | 6篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1978年 | 3篇 |
1976年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有830条查询结果,搜索用时 50 毫秒
71.
72.
Mass mortality of macrobenthic communities in the lagoon of Hikueru atoll (French Polynesia) 总被引:2,自引:0,他引:2
Epibenthic macrofauna communities (corals, molluscs, echinoderms, and macroalgae) were investigated at Hikueru Atoll, Tuamotu
Archipelago. The very low diversity and abundance that we observed (1 coral genus, and 2 mollusc, 1 echinoderm, and 17 macroalgal
species) is likely to be the result of a mass mortality event that occurred 18 months earlier. Hydrological surveys suggest
that this high mortality was the result of a major dystrophic crisis. Other atoll lagoons in the Tuamotu Archipelago have
also been affected by high mortalities in the past, but Hikueru appears to have been the most frequently and intensely affected.
The present study confirms that mass mortalities associated with phytoplankton blooms may be one of the major disturbances
affecting coral reef ecosystems.
Accepted: 14 May 2000 相似文献
73.
《Animal : an international journal of animal bioscience》2015,9(3):544-552
Increasing input self-sufficiency is often viewed as a target to improve sustainability of dairy farms. However, few studies have specifically analysed input self-sufficiency, by including several technical inputs and without only focussing on animal feeding, in order to explore its impact on farm sustainability. To address this gap, our work has three objectives as follows: (1) identifying the structural characteristics required by specialised dairy farms located in the grassland area to be self-sufficient; (2) analysing the relationships between input self-sufficiency, environmental and economic sustainability; and (3) studying how the farms react to a decrease in milk price according to their self-sufficiency degree. Based on farm accounting databases, we categorised 335 Walloon specialised conventional dairy farms into four classes according to their level of input self-sufficiency. To this end, we used as proxy the indicator of economic autonomy – that is, the ratio between costs of inputs related to animal production, crop production and energy use and the total gross product. Classes were then compared using multiple comparison tests and canonical discriminant analysis. A total of 30 organic farms – among which 63% had a high level of economic autonomy – were considered separately and compared with the most autonomous class. We showed that a high degree of economic autonomy is associated, in conventional farms, with a high proportion of permanent grassland in the agricultural area. The most autonomous farms used less input – especially animal feeding – for a same output level, and therefore combined good environmental and economic performances. Our results also underlined that, in a situation of decrease in milk price, the least autonomous farms had more latitude to decrease their input-related costs without decreasing milk production. Their incomes per work unit were, therefore, less impacted by falling prices, but remained lower than those of more autonomous farms. In such a situation, organic farms kept stable incomes, because of a slighter decrease in organic milk price. Our results pave the way to study the role of increasing input self-sufficiency in the transition of dairy farming systems towards sustainability. Further research is required to study a wide range of systems and agro-ecological contexts, as well as to consider the evolution of farm sustainability in the long term. 相似文献
74.
《Animal : an international journal of animal bioscience》2020,14(1):171-179
Feed represents a substantial proportion of production costs in the dairy industry and is a useful target for improving overall system efficiency and sustainability. The objective of this study was to develop methodology to estimate the economic value for a feed efficiency trait and the associated methane production relevant to Canada. The approach quantifies the level of economic savings achieved by selecting animals that convert consumed feed into product while minimizing the feed energy used for inefficient metabolism, maintenance and digestion. We define a selection criterion trait called Feed Performance (FP) as a 1 kg increase in more efficiently used feed in a first parity lactating cow. The impact of a change in this trait on the total lifetime value of more efficiently used feed via correlated selection responses in other life stages is then quantified. The resulting improved conversion of feed was also applied to determine the resulting reduction in output of emissions (and their relative value based on a national emissions value) under an assumption of constant methane yield, where methane yield is defined as kg methane/kg dry matter intake (DMI). Overall, increasing the FP estimated breeding value by one unit (i.e. 1 kg of more efficiently converted DMI during the cow’s first lactation) translates to a total lifetime saving of 3.23 kg in DMI and 0.055 kg in methane with the economic values of CAD $0.82 and CAD $0.07, respectively. Therefore, the estimated total economic value for FP is CAD $0.89/unit. The proposed model is robust and could also be applied to determine the economic value for feed efficiency traits within a selection index in other production systems and countries. 相似文献
75.
76.
Arie Nissenbaum 《Hydrobiologia》1993,267(1-3):127-141
The archaeological and historical record of the Dead Sea as an economic resource is longer than that of any other hypersaline lake. Although it is completely devoid of life, except for a few bacteria and algae, the climatic and geological conditions in the Dead Sea basin have produced circumstances which made this lake important for the economy of the area. The salt which was produced by evaporation of the water, or by quarrying from the salt diapir of Mt. Sodom, on the Dead Sea coast, is referred to in the Bible and in the Talmud. It was harvested until the 1930's. Potash has been extracted from the brine, by solar processes, since 1931 and today the Dead Sea is a major source of potash and bromine. The asphalt, which is found in seepages along the shores and in large blocks, occasionally found floating on the lake, has been used by the inhabitants of the area for waterproofing baskets and for decorative purpose, since the Pre-ceramic Neolithic Period, 10 000 years ago. Later, the asphalt became a major export item to Egypt. During the Early Bronze age, 4000 years ago, it was used mostly to glue flint implements to wooden handles and in the Graeco-Roman period it was used as one of the components in the embalming of Egyptian mummies. The area around the Dead Sea was the only source of balsam, perhaps the most important incense and medication of the Ancient World. Remains of a 7th century B.C. perfume factory, were found in Ein Gedi. During later periods, until the Arab conquest in the 7th century A.D., the growing of balsam was an imperial monopoly. The area of the Dead Sea was famous, for over 2000 years, for its dates and sugar. The therapeutical and medicinal properties of Dead Sea water and the hypersaline hot springs on its shore, were famous throughout the Ancient World. For example, King Herod the Great, 2000 years ago, used to visit the area to cure his many diseases. This practice continues today, and the lakes has become a major center for treatment of psoriasis. There is pictorial, archaeological and historical evidence to support the Dead Sea's importance as a trade artery for over 2300 years. 相似文献
77.
78.
Jacques Hartmann 《Ocean Development & International Law》2018,49(3):276-299
The United Nations Convention on the Law of the Sea (LOSC) permits state parties to establish an Exclusive Economic Zone (EEZ) 200 nautical miles from their coast. Coastal states have exclusive jurisdiction over resources within the EEZ, but navigational and other high seas freedoms continue to exist. A significant number of states have, however, enacted legislation that departs from the LOSC, interfering with the navigational rights and freedoms of other states. This article analzses this development with a specific focus on the Arctic. It investigates the powers of Arctic coastal states to regulate shipping in the EEZ and thereby navigation in the Arctic Ocean. It adds to the existing literature by providing an analysis of state practice, suggesting that despite uncertainty concerning the interpretation of the LOSC Article 234 and the right to exercise legislative jurisdiction over ice-covered waters, a not insignificant number of states have claimed jurisdiction in their own EEZ beyond the rights granted in the LOSC, and are therefore not in a position to object to extensive jurisdictional claims in the Arctic. 相似文献
79.
Elisabeth Schober 《Ethnos》2018,83(3):473-488
ABSTRACTThis paper examines the dynamics behind recent land and water appropriations in Subic Bay (Philippines). The communities adjacent to the former US Naval Base Subic Bay have undergone major transformations since the US military left in 1992. Through the establishment of a Freeport Zone, the area has become a hub for foreign direct investors seeking to profit from the Philippines’ low labour costs. Today, the most important investor in Subic is a South Korean conglomerate that has built one of the largest shipyards in the world in the area. The shipyard, providing labour to tens of thousands of workers, has also led to the dislocation of hundreds of subsistence fishers. With their old fishing grounds lost due to increasing pollution and newly established water boundaries, these villagers find that both the land and water they depend upon are increasingly becoming a scarce good. 相似文献
80.