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   424篇
  免费   5篇
  国内免费   3篇
安全科学   10篇
废物处理   16篇
环保管理   47篇
综合类   79篇
基础理论   115篇
污染及防治   122篇
评价与监测   22篇
社会与环境   20篇
灾害及防治   1篇
  2023年   2篇
  2022年   5篇
  2021年   5篇
  2020年   6篇
  2019年   3篇
  2018年   14篇
  2017年   13篇
  2016年   13篇
  2015年   16篇
  2014年   17篇
  2013年   33篇
  2012年   27篇
  2011年   34篇
  2010年   12篇
  2009年   25篇
  2008年   30篇
  2007年   17篇
  2006年   21篇
  2005年   13篇
  2004年   17篇
  2003年   20篇
  2002年   10篇
  2001年   11篇
  2000年   3篇
  1999年   5篇
  1998年   5篇
  1997年   1篇
  1996年   6篇
  1995年   4篇
  1994年   5篇
  1993年   1篇
  1992年   1篇
  1991年   1篇
  1990年   1篇
  1989年   2篇
  1988年   3篇
  1987年   2篇
  1986年   1篇
  1985年   3篇
  1984年   5篇
  1983年   4篇
  1982年   4篇
  1981年   1篇
  1979年   2篇
  1977年   3篇
  1975年   3篇
  1973年   1篇
  1972年   1篇
排序方式: 共有432条查询结果,搜索用时 31 毫秒
81.
Accelerated execution effects for lifting and lowering a 12-kg box using two footstep strategies associated with experienced workers were studied. Eight healthy male participants performed a normal and an accelerated execution of a lifting task and a lowering task, using a minimal feet displacement strategy (oblique-step) and a strategy with a step (crossed-step). It was hypothesized that the accelerated executions, as compared to the normal executions, would have a different effect on L5/S1 resultant moment, body posture, and other kinematic variables. A tridimensional dynamic rigid body model was used to compute L5/S1 resultant moments. Results showed that the accelerated condition did not reduce body asymmetry of posture, but it reduced the length of the path of the global center of gravity and the duration of the supporting phase of the box, and it did not significantly affect L5/S1 maximal resultant moments for lifting but increased them for lowering. These results indicate that the net work production for accelerated strategies might be smaller, which may represent an economy of energy. Furthermore, the results showed that the use of an accelerated strategy for lowering should be avoided.  相似文献   
82.
In social hymenoptera, the reproductive division of labor is often linked to differences in individual body size with the reproductive caste (the queen) being larger than the workers. Likewise, the reproductive potential may vary with size within the worker caste and could affect the evolution of worker size in social insects. Here, we tested the relationship between worker size and reproductive potential in the facultative parthenogenetic ant Cataglyphis cursor. Colonies are headed by a multiply mated queen, but workers can produce gynes (virgin queens) and workers by thelytokous parthenogenesis after the queen's death. We observed the behaviour of workers (n = 357) until the production of gynes (212 h over 3 months) in an orphaned colony (mated queen not present). The size of workers was measured, and their paternal lineage determined using six microsatellite markers, to control for an effect of patriline. Larger workers were more likely to reproduce and lay more eggs indicating that individual level selection could take place. However, paternal lineage had no effect on the reproductive potential and worker size. From the behavioural and genetic data, we also show for the first time in this species, evidence of aggressive interactions among workers and a potential for nepotism to occur in orphaned colonies, as the five gynes produced belonged to a single paternal lineage.  相似文献   
83.
84.
The high-frequency measurements of valve activity in bivalves (e.g., valvometry) over a long period of time and in various environmental conditions allow a very accurate study of their behaviors as well as a global analysis of possible perturbations due to the environment. Valvometry uses the bivalve's ability to close its shell when exposed to a contaminant or other abnormal environmental conditions as an alarm to indicate possible perturbations in the environment. The modeling of such high-frequency serial valvometry data is statistically challenging, and here, a nonparametric approach based on kernel estimation is proposed. This method has the advantage of summarizing complex data into a simple density profile obtained from each animal at every 24-h period to ultimately make inference about time effect and external conditions on this profile. The statistical properties of the estimator are presented. Through an application to a sample of 16 oysters living in the Bay of Arcachon (France), we demonstrate that this method can be used to first estimate the normal biological rhythms of permanently immersed oysters and second to detect perturbations of these rhythms due to changes in their environment. We anticipate that this approach could have an important contribution to the survey of aquatic systems.  相似文献   
85.
Forward selection of explanatory variables   总被引:6,自引:0,他引:6  
Blanchet FG  Legendre P  Borcard D 《Ecology》2008,89(9):2623-2632
This paper proposes a new way of using forward selection of explanatory variables in regression or canonical redundancy analysis. The classical forward selection method presents two problems: a highly inflated Type I error and an overestimation of the amount of explained variance. Correcting these problems will greatly improve the performance of this very useful method in ecological modeling. To prevent the first problem, we propose a two-step procedure. First, a global test using all explanatory variables is carried out. If, and only if, the global test is significant, one can proceed with forward selection. To prevent overestimation of the explained variance, the forward selection has to be carried out with two stopping criteria: (1) the usual alpha significance level and (2) the adjusted coefficient of multiple determination (Ra(2)) calculated using all explanatory variables. When forward selection identifies a variable that brings one or the other criterion over the fixed threshold, that variable is rejected, and the procedure is stopped. This improved method is validated by simulations involving univariate and multivariate response data. An ecological example is presented using data from the Bryce Canyon National Park, Utah, U.S.A.  相似文献   
86.
Spatially periodic vegetation patterns, forming gaps, bands, labyrinths, or spots, are characteristic of arid and semiarid landscapes. Self-organization models can explain this variety of structures within a unified conceptual framework. All these models are based on the interplay of positive and negative effects of plants on soil water, but they can be divided according to whether they assume the interactions to be mediated by water redistribution through runoff/diffusion or by plants' organs. We carried out a multi-proxy approach of the processes operating in a gapped pattern in southwest Niger dominated by a shrub species. Soil moisture within the root layer was monitored in time and space over one month of the rainy season. Soil water recharge displayed no spatial variation with respect to vegetation cover, but the stock half-life under cover was twice that of bare areas. A kernel of facilitation by the aboveground parts of shrubs was parameterized, and soil water half-life was significantly correlated to the cumulated facilitative effects of shrubs. The kernel range was found to be smaller than the canopy radius (81%). This effect of plants on soil water dynamics, probably through a reduction of evaporation by shading, is shown to be a better explanatory variable than potentially relevant soil and topography parameters. The root systems of five individuals of Combretum micranthum G. Don were excavated. Root density data were used as a proxy to parameterize a kernel function of interplant competition. The range of this kernel was larger than the canopy radius (125%). The facilitation-to-competition range ratio, reflecting the above-to-belowground ratio of plant lateral extent, was smaller than 1 (0.64), a result supporting models assuming that patterning may emerge from an adaptation of plant morphology to aridity and shallow soils by means of an extended lateral root system. Moreover, observed soil water gradients had directions opposite to those assumed by alternative mathematical models based on underground water diffusion. This study contributes to the growing awareness that combined facilitative and competitive plant interactions can induce landscape-scale patterns and shape the two-way feedback loops between environment and vegetation.  相似文献   
87.
Bize P  Devevey G  Monaghan P  Doligez B  Christe P 《Ecology》2008,89(9):2584-2593
Major life history traits, such as fecundity and survival, have been consistently demonstrated to covary positively in nature, some individuals having more resources than others to allocate to all aspects of their life history. Yet, little is known about which resources (or state variables) may account for such covariation. Reactive oxygen species (ROS) are natural by-products of metabolism and, when ROS production exceeds antioxidant defenses, organisms are exposed to oxidative stress that can have deleterious effects on their fecundity and survival. Using a wild, long-lived bird, the Alpine Swift (Apus melba), we examined whether individual red cell resistance to oxidative stress covaried with fecundity and survival. We found that males that survived to the next breeding season tended to be more resistant to oxidative stress, and females with higher resistance to oxidative stress laid larger clutches. Furthermore, the eggs of females with low resistance to oxidative stress were less likely to hatch than those of females with high resistance to oxidative stress. By swapping entire clutches at clutch completion, we then demonstrated that hatching failure was related to the production of low-quality eggs by females with low resistance to oxidative stress, rather than to inadequate parental care during incubation. Although male and female resistance to oxidative stress covaried with age, the relationships among oxidative stress, survival, and fecundity occurred independently of chronological age. Overall, our study suggests that oxidative stress may play a significant role in shaping fecundity and survival in the wild. It further suggests that the nature of the covariation between resistance to oxidative stress and life history traits is sex specific, high resistance to oxidative stress covarying primarily with fecundity in females and with survival in males.  相似文献   
88.
89.
Neutral models provide an alternative to niche-based assembly rules of ecological communities by assuming that communities’ properties are shaped by the stochastic interplay between ecological drift, migration and speciation. The recent and ongoing interest about neutral assumptions has produced many developments on the theoretical side, with nevertheless limited echoes in terms of analyses of real-world data. The present review paper aims to help bridge the widening gap between modellers and field ecologists through two objectives. First, to provide a multi-criteria typology of the main neutral models, including those from population genetics that have not yet been transposed to ecology, by considering how the fundamental processes of ecological drift, speciation and migration are modelled and, specifically, how space is taken into account. Second, to review methods recently proposed to estimate models parameters from field data, a point that should be mastered to allow for broader applications.  相似文献   
90.
Classical sampling methods often miss important components of coral reef biodiversity, notably organisms that remain sheltered within the coral matrix. Recent studies using sea kraits (sea snakes) as bio-indicators suggest that the guild of predators represented by anguilliform fish (Congridae, Muraenidae, Ophichthidae, henceforth “eels” for simplicity) were far more abundant and diverse than previously suspected. In the current study, eel diversity (similarity and species richness indices) estimated via sea snake sampling (SSS) was compared among six areas of one of the main oceanic biodiversity hotspot of the Pacific Ocean (southwest lagoon of New Caledonia). Based on the eel diversity in the snakes’ diet, the results obtained in six areas, in two snake species, and using different estimates (ANOSIM, Shannon index…) were consistent, suggesting that SSS provided robust information. Analyses also suggested subtle, albeit significant, differences in the eel assemblages among islets. Such spatial differences are discussed in light of local management practices. As SSS is easy to use, cost-effective, and provides the best picture of eel assemblages to date, it can be employed to monitor the eel assemblages in addition to the snakes themselves in many areas of the Indo-Pacific Ocean, thereby providing an index of the top predator biodiversity of many coral reefs.  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