全文获取类型
收费全文 | 1827篇 |
免费 | 199篇 |
国内免费 | 528篇 |
专业分类
安全科学 | 146篇 |
废物处理 | 20篇 |
环保管理 | 127篇 |
综合类 | 969篇 |
基础理论 | 717篇 |
污染及防治 | 116篇 |
评价与监测 | 69篇 |
社会与环境 | 352篇 |
灾害及防治 | 38篇 |
出版年
2024年 | 6篇 |
2023年 | 30篇 |
2022年 | 58篇 |
2021年 | 74篇 |
2020年 | 66篇 |
2019年 | 71篇 |
2018年 | 68篇 |
2017年 | 100篇 |
2016年 | 114篇 |
2015年 | 102篇 |
2014年 | 102篇 |
2013年 | 194篇 |
2012年 | 122篇 |
2011年 | 119篇 |
2010年 | 124篇 |
2009年 | 120篇 |
2008年 | 133篇 |
2007年 | 162篇 |
2006年 | 117篇 |
2005年 | 106篇 |
2004年 | 92篇 |
2003年 | 59篇 |
2002年 | 65篇 |
2001年 | 49篇 |
2000年 | 63篇 |
1999年 | 32篇 |
1998年 | 28篇 |
1997年 | 20篇 |
1996年 | 25篇 |
1995年 | 32篇 |
1994年 | 19篇 |
1993年 | 26篇 |
1992年 | 7篇 |
1991年 | 15篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 3篇 |
排序方式: 共有2554条查询结果,搜索用时 218 毫秒
921.
Birth sex ratios in toque macaques and other mammals: integrating the effects of maternal condition and competition 总被引:2,自引:0,他引:2
Wolfgang P. J. Dittus 《Behavioral ecology and sociobiology》1998,44(3):149-160
Mammalian life histories suggest that maternal body condition and social dominance (a measure of resource-holding potential)
influence the physical and social development of offspring, and thereby their reproductive success. Predictably, a mother
should produce that sex of offspring which contributes most to her fitness (as measured by the number of her grandchildren)
and that she is best able to raise within the constraints imposed by her condition, social rank, and environment. Such combined
effects were investigated by monitoring variations in body condition (weight) and behavior of female toque macaques, Macaca sinica of Sri Lanka, in a changing forest environment over 18 years. Maternal rank, by itself, had no influence on offspring sex,
but did affect maternal body condition. The combined effects of rank and condition indicated the following: mothers in robust
condition bore more sons, whereas those in moderate condition bore more daughters, but both effects were expressed most strongly
among mothers of high rank. Where the consequences of low rank were felt most acutely, as shown by poor condition, mothers
underproduced daughters. Environmental quality directly influenced rank and condition interactions, and thus sex ratios. These
relationships, and data from other mammals suggest an empirically and theoretically consistent pattern of sex allocation in
mammals. New predictions integrate effects, proposed by Trivers and Willard, that are rooted in male mate competition, which
is universal among polygynous mammals, with those of local resource competition (and/or female reproductive competition),
which are not universal and differ in intensity between the socioecologies and local environments of different species.
Received: 30 May 1998 / Accepted after revision: 29 August 1998 相似文献
922.
Sexual dimorphism has been linked to parasitoid mating structure by several authors. In turn mating structure has an important
influence on predicted sex ratio optima. Here we test the relationship between sexual dimorphism and sex ratio using data
from 19 species of bethylid wasps. Using phylogenetically based comparative methods we confirm the findings of a previous
cross-species analysis that sex ratio (proportion of males) is strongly and negatively correlated with clutch size. Using
cross-species comparisons we show an additional positive correlation of sex ratio and relative male size, as predicted. The
relationship however is not significant when using phylogenetically based methods. The cross-species result is largely due
to differences between two bethylid sub-families: the Epyrinae have relatively large males and relatively high sex ratios,
whereas the Bethylinae have relatively small males and lower sex ratios. Our study illustrates the benefits and drawbacks
of using cross-species versus phylogenetically based comparisons.
Received: 13 May 1997 / Accepted after revision: 12 January 1998 相似文献
923.
924.
This study examines the impact of flooding on land of high conservation value located along part of the southern shoreline
of the River Clyde Estuary in western Scotland. This paper hypothesizes that, over the next 50 years, the frequency and extent
of coastal flooding will increase due to the gradual effect of global warming and the consequent rise in sea-level and increase
in storminess. It is argued that because of the great cost of constructing new flood defence systems it will not be possible
to protect all land areas to an equal extent from flooding. A means of ranking different land use will be necessary so that
society can make a rational judgement concerning which sections of coastline will be worth protecting. This study provides
a methodology that combines an objective ranking of conservation areas using non-economic indicators with a GIS model of flood
potential, and permits accurate forecasts of flood losses to conservation areas of different ecological value. The conservation
case study used in this paper proposes the use of an ecological weighting value based on five ecological variables each of
10 categories. Tables and maps identify the sites that have been highlighted as consisting of the most ‘valuable’ conservation
sites. The methodology makes extensive use of geographical information systems (GIS) to model the predicted areas of flooding
and to calculate conservation weighting values of the land areas. 相似文献
925.
中国适度人口研究的回顾与再认识 总被引:9,自引:0,他引:9
陈如勇 《中国人口.资源与环境》2000,(1)
本文从可持续发展理论出发 ,对适度人口理论的演变历程进行了回顾 ,分析了中国适度人口研究中存在的问题 ,提出了中国适度人口研究的未来。 相似文献
926.
S. M. Drobenkov 《Russian Journal of Ecology》2000,31(1):49-54
In Belarussian Polesye, characteristics of reproduction, population dynamics, and factors responsible for a decrease in the
abundance of the pond turtle were studied. The main factors limiting pond turtle reproduction in Polesye include the focal
distribution of its populations and limited areas of reproductive biotopes. The pond-turtle popuation itself is characterized
by a high potential for reproductive growth; in the current situation, the population growth is mainly limited by a high pressure
of oophagous predators and anthropogenic destruction of reproductive sites. 相似文献
927.
928.
Thallium in the Environment and Health Effects 总被引:10,自引:0,他引:10
George Kazantzis 《Environmental geochemistry and health》2000,22(4):275-280
Thallium is present in the natural environment in low concentration, being found most frequently in the sulphide ores of a number of heavy metals. Atmospheric emission and deposition from industrial sources has resulted in raised levels in the vicinity of mineral smelters, coal burning power plants, brick works and cement plants. In contaminated areas, raised levels are found in vegetables, fruit and in farm animals. Thallium is used industrially in small quantities, with uses in electronics, in the production of certain glasses and crystals and in medical diagnostics. It has in the past been commonly used as a rodenticide, but its use has now been banned in many countries. Thallium salts are now considered to be amongst the most toxic compounds known. With regard to population exposure, an epidemiological study in an area with high thallium concentrations in soil and garden vegetables centred on a cement plant, has found evidence of a dose response relationship between thallium concentration in urine and a number of non-specific subjective symptoms. Much further research is required to investigate the possible adverse health effects of thallium following population exposure. 相似文献
929.
Fluctuating asymmetry (FA) in limb size in female decorated field crickets (Gryllodes sigillatus) was associated with a reduction in the size of the spermatophore and the amount of sperm transferred by males and an increase
in the time taken to transfer a spermatophore following introduction of a female. There was a weaker negative relationship
between limb asymmetry in males and sperm number but no significant relationship between asymmetry in either sex and spermatophylax
size. In line with a previous study, female size did not appear to influence spermatophore production or mating decisions
by males. The results imply that developmental instability affects both gamete production and mating decisions among males,
although the relationships between spermatophore size, sperm number and asymmetry in females are unlikely to be the result
of males perceiving differences in female FA.
Received: 24 July 1999 / Received in revised form: 22 November 1999 / Accepted: 31 December 1999 相似文献
930.
不同演替阶段中黧蒴栲种群的大小结构与分布格局 总被引:16,自引:0,他引:16
在中亚热带常绿阔叶林,有代表性地选择三个黧蒴栲群落类型,即黧蒴栲纯林(常绿阔叶林皆伐地自然演替而成的黧蒴栲纯林,用生长锥对优势木进行随机抽查得其最大年龄为13a)、以黧蒴栲为优势种的混交林(经优势木随机抽测其最大年龄为24a)和黧蒴栲已不成为优势种的混交林(实为天然次生林,优势木随机抽测最大年龄为53a)。测定分析结果表明,黧蒴栲群落在自然演替的前期即幼林阶段的种群大小级结构呈典型的三角状增长型; 相似文献