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   24347篇
  免费   261篇
  国内免费   127篇
安全科学   586篇
废物处理   1024篇
环保管理   2920篇
综合类   4736篇
基础理论   6616篇
环境理论   18篇
污染及防治   6240篇
评价与监测   1452篇
社会与环境   1006篇
灾害及防治   137篇
  2021年   156篇
  2019年   165篇
  2018年   309篇
  2017年   325篇
  2016年   485篇
  2015年   387篇
  2014年   576篇
  2013年   1786篇
  2012年   661篇
  2011年   985篇
  2010年   770篇
  2009年   770篇
  2008年   1010篇
  2007年   1017篇
  2006年   894篇
  2005年   773篇
  2004年   739篇
  2003年   719篇
  2002年   726篇
  2001年   841篇
  2000年   647篇
  1999年   372篇
  1998年   301篇
  1997年   315篇
  1996年   314篇
  1995年   369篇
  1994年   355篇
  1993年   310篇
  1992年   314篇
  1991年   315篇
  1990年   326篇
  1989年   301篇
  1988年   268篇
  1987年   256篇
  1986年   244篇
  1985年   222篇
  1984年   279篇
  1983年   228篇
  1982年   289篇
  1981年   248篇
  1980年   203篇
  1979年   216篇
  1978年   206篇
  1977年   175篇
  1976年   165篇
  1975年   161篇
  1974年   174篇
  1973年   183篇
  1972年   166篇
  1971年   161篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
Conspicuous secondary sexual traits may have evolved as handicap-revealing signals or as badges of status. We present results of an experiment using males of the sexually dimorphic house sparrow (Passer domesticus), that support the idea that the male-specific bib can be both a handicap-revealing signal and a reliable badge indicating the physical condition of the bird. In a test of the immunocompetence handicap hypothesis, wild-caught adult male house sparrows were studied in captivity. Birds implanted with elevated doses of testosterone were more dominant, had higher circulating levels of both testosterone and corticosterone and they also harboured relatively larger ectoparasite loads. Higher parasite loads were also associated with individuals showing lower immunocompetence and larger changes in bib size. A new model for immunocompetence effects in sexual selection is introduced, integrating actions that the hypothalamopituitary axis exerts on gonads, adrenals and the thyroid gland. The ”integrated immunocompetence model” synthesizes both the ”handicap” (i.e. survival-decreasing) and ”badge of status” (i.e. survival- enhancing) models for evolution of secondary sexual traits. Received: 15 May 1999 / Received in revised form: 2 November 1999 / Accepted: 6 November 1999  相似文献   
942.
Previous studies have suggested that testosterone (T) profiles of male birds reflect a trade-off between mate attraction behaviours (requiring high T levels) and parental care activities (requiring low T levels). In this study, we experimentally elevated T levels of monogamous males in the facultatively polygynous European starling (Sturnus vulgaris), and compared mate attraction and paternal behaviour of T-treated males with those of controls (C-males). T-males significantly reduced their participation in incubation and fed nestlings significantly less often than C-males. Females paired to T-treated males did not compensate for their mate’s lower paternal effort. The observed reduction in a male’s investment in incubating the eggs was accompanied by an increased investment in typical female-attracting behaviours: T-males spent a significantly higher proportion of their time singing to attract additional females. They also occupied more additional nestboxes than C-males, although the differences just failed to be significant, and carried significantly more green nesting materials into an additional nestbox (a behaviour previously shown to serve a courtship function). T-males also behaved significantly more aggressively than C-males. During the nestling period, the frequency of mate-attracting behaviours by T-treated and control males no longer differed significantly. Despite the reduced paternal effort by T-males and the lack of compensation behaviour by females, hatching and breeding success did not differ significantly between T- and C-pairs. Received: 7 February 2000 / Revised: 10 August 2000 / Accepted: 3 September 2000  相似文献   
943.
An unusually long period of cold weather in May 1996 caused extensive mortality among insectivorous cliff swallows (Petrochelidon pyrrhonota) in the northern and central Great Plains. We analyzed how viability selection affected spring arrival time in a migratory Nebraska population by comparing capture histories of survivors with those of birds known to have died and by documenting how arrival time changed in the year following the selection event. Surviving birds had significantly later first-capture dates (an index of arrival time) in the years prior to selection than those that died; a significant selection differential suggested directional selection for birds that arrived later. Colony sites were occupied significantly later following the selection event, and the distribution of first-capture dates in the season after selection was significantly shifted toward later arrivals. Offspring of the survivors tended to arrive later than birds of the same age prior to the selection event. While major weather-caused mortality events of this magnitude are rare in the study area, spells of cold weather severe enough to cause limited mortality are frequent in April and early May. At least 25 probable mortality events of varying severity were identified in the last 50 years based on climatological data. Periodic weather-mediated selection against early arrival constrains the cliff swallow’s breeding season and may partly prevent directional selection for earlier nesting. Received: 19 October 1999 / Received in revised form: 15 January 2000 / Accepted: 24 January 2000  相似文献   
944.
One of the most interesting aspects of the mating system of pied flycatchers is the regular occurrence of polygyny. Here we present data on the reproductive success of polygynously paired pied flycatcher males compared to monogamous males based on paternity analyses through DNA fingerprinting. Males paired with two females suffered a higher loss in reproductive output per female compared to monogamous males due to (1) a greater proportion of unhatched eggs in their broods, (2) greater nestling mortality and (3) a greater probability of being cuckolded. Nevertheless, the number of fledglings was significantly greater for polygynous males. Based on the number of nestlings that returned for subsequent breeding seasons, however, the reproductive success of monogamous and polygynous males did not differ significantly. These data raise the question as to why males attempt polygyny. Received: 16 August 1999 / Received in revised form: 15 March 2000 / Accepted: 18 March 2000  相似文献   
945.
946.
947.
Abstract: Genebank collection databases can be used for ecogeographical studies under the assumption that the accessions are a geographically unbiased sample. We evaluated the representativeness of a collection of wild potatoes from Bolivia and defined and assessed four types of bias: species, species-area, hotspot, and infrastructure. Species bias is the sampling of some species more often than others. Species-area bias is a sampling that is disproportionate to the total area in which a species is found. Hotspot bias is the disproportionate sampling of areas with high levels of diversity. Infrastructure bias is the disproportionate sampling of areas near roads and towns. Each of these biases is present in the Bolivian wild potato collection. The infrastructure bias was strong: 60% of all wild potato accessions were collected within 2 km of a road, as opposed to 22%, if collections had been made randomly. This analysis can serve as a guide for future collecting trips. It can also provide baseline information for the application of genebank data in studies based on geographic information systems.  相似文献   
948.
Abstract: The consequences of rapid rainforest clearance on native avifauna are poorly understood. In Southeast Asia, Singapore, a newly developing country, has had 95% of its native lowland rainforest cleared. Most of the rainforest was lost in the mid- to late-nineteenth century. We compared avifauna checklists from 1923, 1949, and 1998 to determine the extent of extinctions between 1923 and 1998 in Singapore. Of 203 diurnal bird species, 65 were extirpated in Singapore in the past 75 years. Four of these species were nonforest- dependent species, whereas 61 (94%) were forest bird species dependent on the primary or old secondary forest to survive. Twenty-six forest bird species became extinct between 1923 and 1949, whereas 35 forest species disappeared after 1949. We compared the body lengths, feeding guilds, and vertical feeding zones between extinct and extant forest bird species to determine whether extinction patterns were dependent on these characteristics. Larger forest bird species went extinct between 1923 and 1949. Body sizes, however, did not affect the loss of forest bird species between 1949 and 1998. We observed high losses of insectivorous birds; the insectivore-carnivore and insectivore-granivore guilds lost> 80% of the species present in 1923. The highest losses were among birds that fed in the canopy. None of the forest bird species are currently common (>100 individuals/species) within Singapore. Our study shows that more than half the forest avifauna became locally extinct after extensive deforestation. Based on this fact, the countries within Southeast Asia should reconsider their heavy deforestation practices.  相似文献   
949.
Summary. The stems of many Macaranga ant-plants (Euphorbiaceae) are covered by epicuticular wax crystals rendering the surface very slippery for most insects. These wax blooms act as selective barriers protecting the symbiotic ant partners, which are specialized “wax-runners”, against the competition of other ants. Glaucous stems occur almost exclusively among the ant-plants of the genus Macaranga (). We analyzed the cuticular lipids of 16 Macaranga species by GC-MS and investigated the wax crystal morphology using SEM. Presence of crystalline wax blooms was strongly correlated with high concentrations (52%–88%) of triterpenoids. In contrast epicuticular waxes of glossy Macaranga surfaces contained only 0% to 36% of these dominant components. Therefore we conclude that triterpenoids are responsible for the formation of the thread-like Macaranga wax crystals. In all Macaranga ant-plants investigated, the principal components were epitaraxerol and taraxerone accompanied by smaller portions of taraxerol, β-amyrin and friedelin. Only in the case of the non-myrmecophytic M. tanarius did β-amyrin predominate. Moreover, we found that only in M. tanarius, the dense wax crystal lacework is torn into large mosaic-like pieces in the course of secondary stem diameter growth. Both chemical and macroscopic differences may contribute to a reduced slipperiness of M. tanarius stems and appear to be functionally important. The distribution of wax crystals and their composition amongst different sections of the genus suggests that glaucousness is a polyphyletic character within Macaranga. Received 7 October 1999; accepted 3 December 1999  相似文献   
950.
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