全文获取类型
收费全文 | 15437篇 |
免费 | 86篇 |
国内免费 | 121篇 |
专业分类
安全科学 | 344篇 |
废物处理 | 702篇 |
环保管理 | 1783篇 |
综合类 | 3288篇 |
基础理论 | 3923篇 |
环境理论 | 5篇 |
污染及防治 | 3786篇 |
评价与监测 | 954篇 |
社会与环境 | 795篇 |
灾害及防治 | 64篇 |
出版年
2022年 | 95篇 |
2021年 | 108篇 |
2018年 | 264篇 |
2017年 | 257篇 |
2016年 | 410篇 |
2015年 | 230篇 |
2014年 | 271篇 |
2013年 | 899篇 |
2012年 | 728篇 |
2011年 | 670篇 |
2010年 | 397篇 |
2009年 | 462篇 |
2008年 | 530篇 |
2007年 | 600篇 |
2006年 | 532篇 |
2005年 | 770篇 |
2004年 | 907篇 |
2003年 | 789篇 |
2002年 | 416篇 |
2001年 | 515篇 |
2000年 | 380篇 |
1999年 | 261篇 |
1998年 | 138篇 |
1997年 | 161篇 |
1996年 | 134篇 |
1995年 | 192篇 |
1994年 | 178篇 |
1993年 | 148篇 |
1992年 | 152篇 |
1991年 | 189篇 |
1990年 | 170篇 |
1989年 | 152篇 |
1988年 | 156篇 |
1987年 | 132篇 |
1986年 | 94篇 |
1985年 | 117篇 |
1984年 | 143篇 |
1983年 | 140篇 |
1982年 | 151篇 |
1981年 | 107篇 |
1980年 | 101篇 |
1979年 | 122篇 |
1978年 | 101篇 |
1977年 | 97篇 |
1976年 | 88篇 |
1975年 | 99篇 |
1974年 | 116篇 |
1972年 | 82篇 |
1967年 | 90篇 |
1965年 | 86篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
581.
Maternal investment in rhesus macaques (Macaca mulatta): reproductive costs and consequences of raising sons 总被引:1,自引:0,他引:1
Maternal investment in offspring is expected to vary according to offspring sex when the reproductive success of the progeny
is a function of differential levels of parental expenditure. We conducted a longitudinal investigation of rhesus macaques
to determine whether variation in male progeny production, measured with both DNA fingerprinting and short tandem repeat marker
typing, could be traced back to patterns of maternal investment. Males weigh significantly more than females at birth, despite
an absence of sex differences in gestation length. Size dimorphism increases during infancy, with maternal rank associated
with son’s, but not daughter’s, weight at the end of the period of maternal investment. Son’s, but not daughter’s, weight
at 1 year of age is significantly correlated with adult weight, and male, but not female, weight accounts for a portion of
the variance in reproductive success. Variance in annual offspring output was three- to fourfold higher in males than in females.
We suggest that energetic costs of rearing sons could be buffered by fetal delivery of testosterone to the mother, which is
aromatized to estrogen and fosters fat accumulation during gestation. We conclude that maternal investment is only slightly
greater in sons than in daughters, with mothers endowing sons with extra resources because son, but not daughter, mass has
ramifications for offspring sirehood. However, male reproductive tactics supersede maternal investment patterns as fundamental
regulators of male fitness.
Received: 23 July 1999 / Received in revised form: 23 February 2000 / Accepted: 13 March 2000 相似文献
582.
Conservation in a Region of Political Instability: Bwindi Impenetrable Forest, Uganda 总被引:1,自引:0,他引:1
A. Hamilton A. Cunningham † D. Byarugaba ‡ and F. Kayanja‡ 《Conservation biology》2000,14(6):1722-1725
Abstract: Bwindi Impenetrable is the most important forest in Uganda for conservation of biodiversity. It contains over half the world's mountain gorillas. It is surrounded by densely populated agricultural land and lies within a region of political instability. Gazetted as a forest reserve in 1932, little forest now remains outside its boundaries. Transformation of nearby communal swampland to farmland, much owned by a few individuals, shows the probable fate of Bwindi if it had not been declared a protected area. Widespread illegal logging and other activities were among reasons why the status of the forest was raised to national park in 1991. This resulted in local resentment, fed by inadequate consultation and concern about the local people's loss of access to resources. Fires were set in the forest and threats made against the gorillas. Three schemes to provide benefits from the existence of the forest to communities and involve them in park management were then instituted: agreements allowing controlled harvesting of resources in the park, receipt of some revenue from tourism, and establishment of a trust fund partly for community development. Tension between people and park has been reduced. This case demonstrates the importance of protected areas and community involvement in such circumstances. Community support is especially critical, as here, when resources available to park managers are limited and political instability endemic. 相似文献
583.
Abstract: Estimating the risk of extinction for populations of endangered species is an important component of conservation biology. These estimates must be made from data that contain both environmental noise in the year-to-year transitions in population size (so-called "process error"), random errors in sampling, and possible biases in sampling ( both forms of observation errors). To determine how much faith to place in estimated extinction rates, it is important to know how sensitive they are to observation error. We used three simple, commonly employed models of population dynamics to generate simulated population time series. We then combined random observation error or systematic biases with those data, fit models to the time series data, and observed how close the extinction dynamics of the fitted models compared with the dynamics of the underlying models. We found that systematic biases in sampling rarely affected estimates of extinction risk. We also found that even moderate levels of random observation error do not significantly affect extinction estimates except over a small range of process errors, corresponding to the region where extinction risk is most uncertain. With more substantial sampling error, estimates of extinction risk degraded rapidly. Field census techniques for a variety of taxa often involve observation errors within ±32% of actual population sizes. For typical time series used in conservation, therefore, we often may not need to be overly concerned about observation errors as an extra source of imperfection in our estimated extinction rates. 相似文献
584.
585.
After introducing soft defence techniques as an alternative to hard defence techniques, the need is emphasized to consider
the coastal area as an integral system. By recalling the main driving factors for coastal management: conflict resolution,
resilience and sustainability, we logically arrive at the concepts of ecological engineering and ecotechnology, which are
increasingly acknowledged as possible solutions to achieve sustainable use of coastal space as a resource. In this context,
we refer to the principles of self design and of ecosystem conservation.
In order to deal with real situations we are in need of fundamental ‘tools’ for the application of the soft intervention technology
approach. We therefore introduce the concept of physiographic units and develop an initial elaboration for a coastal stretch
and for coastal wetlands. The latter deserve more attention because of the already established practices of ecotechnology,
at least as far as water and soil quality are concerned, but certainly also concerning morphology, especially in the future.
We conclude by briefly discussing how activities undertaken in two research projects currently being conducted under the framework
of the Marine Science and Technology Program of the Commission of the European Communities are expected to contribute to the
concepts introduced here. 相似文献
586.
587.
588.
The theory of space accessibility analysis, space syntax, has been considered for capturing trip-makers’ route-choice decisions with an understanding that, while selecting routes, trip-makers tend to follow forward-facing fields traditionally represented by axial lines. But the idea of treating axial lines as a set of roadway segments representing an urban texture has remained under scrutiny when distinct procedures have been tested without much success. Nor has the axial-line theory seemed to gain much popularity within the orthodox transportation planning community. Studies show that a long axial line is found gaining configurational centrality, in general, for being well connected to its incident lines. Fragmenting a long axial line results in weakening it (in centrality terms), but also in generating a locally oriented model. Recent investigations, dealing with this problem, have resulted in the emergence of the unit-segment theory, which has turned out to be more sensitive to the local structures located within a large urban texture than the axial counterpart for the purpose of modeling vehicular movement networks without using origin–destination trip-counts. Essentially, the question now is: Is the unit-segment analysis also capable of generating syntax configurations that are equally sensitive to large and complex urban textures? What should be the methodology of such an analysis? This paper throws deeper insights into these questions by producing theoretical and empirical research findings and seeks to develop a common ground for both transportation planning and space syntax communities to work together for gaining a better understanding of how urban configurations relate to urban vehicular movement networks. 相似文献
589.
590.
Analysis of uncertainty consideration in environmental assessment: an empirical study of Canadian EA practice 总被引:1,自引:0,他引:1
Juliette Lees Jill A.E. Gunn Bram F. Noble 《Journal of Environmental Planning and Management》2016,59(11):2024-2044
Identifying and communicating uncertainty is core to effective environmental assessment (EA). This study evaluates the extent to which uncertainties are considered and addressed in Canadian EA practice. We reviewed the environmental protection plans, follow-up programs, and panel reports (where applicable) of 12 EAs between 1995 and 2012. The types of uncertainties and levels of disclosure varied greatly. When uncertainties were acknowledged, practitioners adopted five different approaches to address them. However, uncertainties were never discussed or addressed in depth. We found a lack of suitable terminology and consistency in how uncertainties are disclosed, reflecting the need for explicit guidance, and we present recommendations for improvement. Canadian Environmental Impact Statements are not as transparent with respect to uncertainties as they should be, and uncertainties in EA need to be better considered and communicated. 相似文献