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501.
When a community loses species through fragmentation, its total food consumption may drop. Compensatory responses of remaining species, whereby survivors assume roles of extinct competitors, may reduce the impact of species loss through numerical or functional responses. We measured compensatory responses in two remaining antbird species on Barro Colorado Island, Panama, four decades after the loss of their dominant competitor, the Ocellated Antbird, Phaenostictus mcleannani. We compared current abundances and behavior of these two species on Barro Colorado to those reported before the island lost Ocellated Antbirds, and to those in a nearby mainland population where all three species still exist as a space-for-time substitution. The smaller, more subordinate Spotted Antbird, Hylophylax naevioides, responded far more strongly than the larger Bicolored Antbird, Gymnopithys leucaspis, which is functionally more like the Ocellated Antbird. Islandwide density of Spotted Antbirds has more than doubled since the loss of Ocellated Antbirds. Moreover, Spotted Antbirds now spend so much more of their time following ant swarms that their metabolic biomass at these swarms has more than tripled since Ocellated Antbirds disappeared. These responses in Spotted Antbirds were apparently delayed by >20 years. Bicolored Antbirds have not increased substantially in islandwide density or metabolic biomass at ant swarms. We hypothesize that behavioral flexibility, as shown by Spotted Antbirds on Barro Colorado Island, is a major factor governing the extent to which fragmented ecosystems can buffer the impacts of species loss. 相似文献
502.
The Manawatu floods of 2004 have had significant, long-lasting social consequences. This paper draws on findings from a series of detailed surveys of 39 farm households directly affected by the floods and 17 individuals directly involved in managing the flood recovery programme. The nature of the impact on rural families highlights how the 'hollowing out' of rural New Zealand has changed the capacity of rural communities to respond to natural hazards and increased their sense of isolation. In addition, the floods exposed the vulnerability of rural communities. This is shown to have implications for policies designed to build resilience and improve responses to adverse events, including the need to support local, community initiatives on self-reliance and mutual support. Approaches to manage better long-term flood risks should be designed within a context of ongoing rural decline that has compromised the health of both individuals and communities. 相似文献
503.
Long Ngo Louise M. Ryan Maura Mezzetti Frédéric Y. Bois Thomas J. Smith 《Environmental and Ecological Statistics》2011,18(1):131-146
In a study of 133 volunteer subjects, demographic, physiologic and pharmacokinetic data through exposure to 1,3-Butadiene
(BD) were collected in order to estimate the percentage of BD concentration metabolized at steady state, and to determine
whether this percentage varies across gender, racial, and age groups. During the 20 min of continuous exposure to 2 parts
per million (ppm) of BD, five measurements of exhaled concentration were made on each subject. In the following 40 min washout
period, another five measurements were collected. A Bayesian hierarchical compartmental physiologically-based pharmacokinetic
model (PKPB) was used. Using prior information on the model parameters, Markov Chain Monte Carlo (MCMC) simulation was conducted
to obtain posterior distributions. The overall estimate of the mean percent of BD metabolized at steady state was 12.7% (95%
credible interval: 7.7–17.8%). There was no significant difference in gender with males having a mean of 13.5%, and females
12.3%. Among the racial groups, Hispanic (13.9%), White (13.0%), Asian (12.1%), and Black (10.9%), the significant difference
came from the difference between Black and Hispanic with a 95% credible interval from −5.63 to −0.30%. Those older than 30 years
had a mean of 12.2% versus 12.9% for the younger group; although this was not a statistically significant difference. Given
a constant inhalation input of 2 ppm, at steady state, the overall mean exhaled concentration was estimated to be 1.75ppm
(95% credible interval: 1.64–1.84). An equivalent parameter, first-order metabolic rate constant, was also estimated and found
to be consistent with the percent of BD metabolized at steady state across gender, race, and age strata. 相似文献
504.
The theory of storage, as related to commodities, makes two predictions involving the quantity of the commodity held in inventory. When inventory is low (i.e. a situation of scarcity), spot prices will exceed futures prices, and spot price volatility will exceed futures price volatility. Conversely, during periods of no scarcity, both spot prices and spot price volatility will remain relatively subdued. We test these predictions for the six base metals traded on the London Metal Exchange (aluminium, copper, lead, nickel, tin and zinc), and find strong validation for the theory. Including Chinese inventories reported by the Shanghai Futures Exchange strengthens the relationship further. We also introduce the concepts of excess volatility, inventory-implied spot price and inventory-implied spot volatility and illustrate some applications. 相似文献
505.
This paper provides a conceptual overview of economists′ attempts to learn about the effects of taxes on extractive resources. The emphasis is on research methods and techniques, with no attempt to provide a comprehensive tabulation of previous empirical results or policy conclusions regarding preferred tax instruments or systems. We argue, in fact, that the nature of such conclusions largely depends on, and is limited by, the researcher′s choice of modeling framework. Many alternative frameworks and approaches have been developed in the literature. Our goal is to describe the differences among them and to note their strengths and limitations. 相似文献
506.
This study examined the cooperative effects of diffusion and reaction on the degradation of a polymer. The work was intended
to describe bulk degradation processes, where the polymer volume remains nearly constant. Test coupons made from poly (2-ethyl
2-oxazoline) were immersed in water for controlled durations. The coupons were weighed before and after immersion in the wet
and dried condition. The coupons were observed to gain weight for short immersion durations. At longer durations the weight
change became negative as the solvent reacted with and dissolved the polymer. An average solvent concentration was found from
the weight measurements. Fick’s law was used to express the increase in concentration while a first order reaction law was
used to represent the decrease in concentration of the solvent. The competing changes in concentration were combined from
which the net solvent concentration at any given time was obtained. The model was compared with the experimental measurements
using coupons of various thicknesses. The model was found to describe the observed polymer degradation in water including
thickness effects. 相似文献
507.
Ann C. Smith 《环境质量管理》1991,1(1):3-8
Environmental management is moving away from the traditional use of specialists to currect end-of-the-pipe problems and instead is moving back in the process to reduce the production of these waste materials. The author shows how integrating environmental responsiblity all the way down the production line might work in a practical application, including a ten-step formula for putting a total quality environmental program in place. 相似文献
508.
509.
Clonal and solitary anemones (Anthopleura) of western North America: population genetics and systematics 总被引:2,自引:0,他引:2
Francis (1979) proposed that clonal and solitary forms of the anemone Anthopleura elegantissima are actually two species. In 1984 and 1985, samples from two to six California populations of all known forms and species of California Anthopleura were analyzed electrophoretically to determine their taxonomic relationships. Data from 14 enzymes and 18 loci, 17 of them polymorphic, show that the two forms of A. elegantissima are virtually identical electrophoretically, and there is no evidence of reduced gene flow between them. We conclude there are three species of Anthopleura in California: A. elegantissima (Brandt, 1835), A. xanthogrammica (Brandt, 1835) and A. artemisia (Dana, 1848). Genetic variation in the two species capable of asexual reproduction, A. elegantissima and A. artemisia, is extremely high, approximately 2.5 times that of the strictly sexual A. xanthogrammica. 相似文献
510.
The purpose of this paper is to respond to criticism of the previous representations of Sandler and Smith of the numeraire in an intertemporal analysis of Pareto efficiency conditions. This reexamination suggests that the role of the numeraire is shown to be less clear-cut than Cabe implies. Moreover, the analysis indicates that the earlier conclusions are not crucially dependent on the numeraire assumption. 相似文献