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801.
After a formerly grazed salt marsh was released from cattle grazing, changes in plant species composition were monitored for
20 yr, using vegetation maps and permanent plots. Three areas, differing in age and nutrient status were compared. The number
of plant species and plant communities decreased.Elymus athericus (Elytrigia pungens) became dominant in most plant communities after 5–20 yr on the oldest and most productive salt marsh. In younger areas it
took more time forE. athericus to become dominant. At least 7 cm of clay seemed to be a prerequisite for this plant species to increase in dominance. The
results from monitoring over decades are discussed in view of the knowledge on succession over centuries as derived from a
chronosequence. 相似文献
802.
Eelgrass (Zostera marina L.) in the Dutch Wadden Sea historically covered an area varying from 65–150 km2 in the eulittoral as well as the sublittoral zones. At present, this area comprises less than 1 km2 eulittoral eelgrass stands, with an associated decrease in habitat diversity. The causes for this decline are presumably
connected with the ‘wasting disease’ and the closure of the former Zuiderzee in the early 1930s resulting in increased tidal
range and increased currents. After a slight recovery of the eelgrass populations on the intertidal flats a definite decline
started in the early 1970s, possibly connected to increased turbidity. The present water quality and turbidity do not negatively
influence eelgrass growth up to a depth of at least 0.6m below Mean Sea Level. Based on mesocosm experiments and field experiments
it is concluded that re-establishment of eelgrass should be possible in sheltered bays and on unexposed tidal flats. The most
suitable depths for a reintroduction are those between 0 and 20–40 cm below mean sea level. 相似文献
803.
Jan van der Blom 《Behavioral ecology and sociobiology》1991,29(5):341-346
Summary If a honeybee (Apis mellifera L.) colony loses its queen and remains queenless, a small percentage of the workers will develop into egg-layers and subsequently will produce males. The process of differentiation into laying and non-laying workers is accompanied by a great deal of aggression within the colony. In this study, I tried to establish the relationship between the potential to become egg-layers and behavioural differences at the individual level. To eight observation hives, I introduced 200 individually marked workers of similar age and observed their behavioural differentiation during a queenless period. Shortly after the first egg-laying worker appeared, the marked workers were dissected in order to determine their level of ovarian activation. The future laying-workers seemed to be slightly more involved in the rearing of new queens than other workers. As reported by other authors, aggression was mostly directed towards workers with activated ovaries. Only a very small number of aggressive workers were aggressive; on an average, slightly fewer of these marked workers had activated ovaries. Aggression resulted in physical damage in only a small number of cases. The possible disadvantage of aggression for the bees under attack is discussed, as well as the possible benefit for the aggressors. Evidence is presented that the most detrimental effect of aggression for bees under attack is the fact that they lose considerable quantities of food through trophallaxis to other workers. Workers with activated ovaries withdrew inside empty cells significantly more often than other workers, possibly to avoid attacks. Aggressive workers were (almost) never seen to receive food from the bees they attacked. It is suggested that by preventing other workers from becoming egg-layers, aggressors increase their own chances of future reproduction. 相似文献
804.
J. van der Veer 《Marine Biology》1982,66(1):9-14
The fixation capacity of mixtures of acrolein, glutaraldehyde, and tannic acid, was investigated to arrive at a fixative for small and delicate marine plankton that is more universal than the current selective fixatives. A mixture containing 4% acrolein, 4% glutaraldehyde and 2% tannic acid added 1:1 to the cell suspension proved to be satisfactory. Mounting of the material in euparal before staining is recommended. Of several dyes tried, fuchsin acid proved suitable for observation of flagella and cell shapes. Delafield haematoxylin staining gave the best results for internal details. Fuchsin base staining, requiring a complex procedure, is recommended when it is necessary to search for flagellar hairs as found in Ochromonas spp. Schedules for the fixation, mounting and staining are given. 相似文献
805.
806.
We present a simple model of within-group leveling coalitions among male primates. The model assumes that the value of the coalition is the sum of the payoffs of its members, that the individuals payoff is monotonically decreasing with its rank and that coalitions do not cause rank changes. It predicts that mainly mid- to low rankers engage in leveling coalitions, and that most coalition partners are of adjacent ranks. These predictions agree reasonably well with observations in nature. The model also makes the novel predictions that leveling coalitions are found where male mating competition has only a moderate contest component, and that male dominance ranks will become poorly differentiated where leveling coalitions are frequent. Both these predictions are consistent with observations on groups of macaques and baboons. The model also may account for leveling coalitions among egalitarian human foragers, without making additional assumptions about special human capabilities.
相似文献
Carel P. van SchaikEmail: |
807.
A short outline is given of the floristic composition, structure and distribution of coastal dune vegetation found at Malindi
Bay, Kenya. The area was studied by air photo interpretation and field sampling to determine the relationship of plants to
aeolian features. TWINSPAN classification was used to distinguish geomorphological units on the basis of their species composition.
In this paper, an inventory and first quantitative analysis of vegetation distribution is presented. We identified 174 plant
species from 62 families in the sand dunes and several plant communities are distinguished based on the species content and
the connection with morphological units.Papilionaceae with 18 species andPoaceae with 17 species were the most represented families. A distinct zonal distribution of the plant communities was found.
The most important plant species are the pioneer vegetation consisting ofHalopyrum mucronatum, Ipomoea pescaprae andScaevola plumieri. The woody shrub species which have colonized the established primary dunes and hummock dunes areCordia somaliensis, Pluchea discoridis, Tephrosia purpurea (dunensis). Succulent herbs were identified in the dune slacks and salt marsh that are moist and damp environments. 相似文献
808.
Eddy van der Maarel Marijke van der Maarel-Versluys 《Journal of Coastal Conservation》1996,2(1):73-92
A comprehensive list of 1068 typical littoral plant species and subspecies has been composed. They are considered endemic
in a wide sense and are subdivided into widespread, transregional, regional and local endemics, the latter three categories
being considered as endemics s.s. For each taxon the distribution, habitat preference, endemic status and conservation status
are given. The list, which is available upon request, is summarized in a number of figures and tables, from which it appears
that 61% of all species are endemics s.s., that ca. 30% of all species are dune and beach species and another nearly 30% are
maritime rock species. Species of wet habitats are concentrated in northern and northwestern Europe, dune species in western
and southwestern Europe, western Mediterranean and Black Sea. The conservation status of most species is indicated; 37% is
considered threatened. It is concluded that the Bern Convention and the European Habitat Directive offer an entirely insufficient
framework for effective conservation action. It is suggested to take the present list as a starting point for a geographical/taxonomical/ecological
data base of European coastal endemics. 相似文献
809.
Serum concentrations of perfluorooctanesulfonate and other fluorochemicals in an elderly population from Seattle, Washington 总被引:14,自引:0,他引:14
Olsen GW Church TR Larson EB van Belle G Lundberg JK Hansen KJ Burris JM Mandel JH Zobel LR 《Chemosphere》2004,54(11):1599-1611
Perfluorooctanesulfonyl fluoride (POSF, C8F17SO2F) related-materials have been used as surfactants, paper and packaging treatments, and surface (e.g., carpet, textile, upholstery) protectants. A metabolite, perfluorooctanesulfonate (PFOS, C8F17SO3−), has been identified in the serum and liver of non-occupationally exposed humans and wildlife. Because of its persistence, an important question was whether elderly humans might have higher PFOS concentrations. From a prospective study designed to examine cognitive function in the Seattle (WA) metropolitan area, blood samples were collected from 238 dementia-free subjects (ages 65–96). High-pressure liquid chromatography-electrospray tandem mass spectrometry determined seven fluorochemicals: PFOS; N-ethyl perfluorooctanesulfonamidoacetate; N-methyl perfluorooctanesulfonamidoacetate; perfluorooctanesulfonamidoacetate; perfluorooctanesulfonamide; perfluorooctanoate; and perfluorohexanesulfonate. Serum PFOS concentrations ranged from less than the lower limit of quantitation (3.4 ppb) to 175.0 ppb (geometric mean 31.0 ppb; 95% CI 28.8–33.4). An estimate of the 95% tolerance limit was 84.1 ppb (upper 95% confidence limit 104.0 ppb). Serum PFOS concentrations were slightly lower among the most elderly. There were no significant differences by sex or years residence in Seattle. The distributions of the other fluorochemicals were approximately an order of magnitude lower. Similar to other reported findings of younger adults, the geometric mean serum PFOS concentration in non-occupational adult populations likely approximates 30–40 ppb with 95% of the population’s serum PFOS concentrations below 100 ppb. 相似文献
810.
Gudrun Fricke Jörg Hellhammer Andreas Klamt Jan Meerkamp van Embden Manfred Rose Birgit Sewekow Rolf Wittlinger 《Environmental Sciences Europe》1995,7(5):275-279
The substituent chlorine is generally considered to be an agent which reduces the reactivity of organic molecules with regard to the indirect photodegradation by OH radicals. A systematic study of selected, representative classes of compounds, however, reveals that the deactivating influence of chlorine on the OH radical reactivity is actually rather moderate. In individual cases, for instance, chlorine substitution can even lead to an increase in this reactivity. This study is based on both experimental data and on the molecular orbital calculations of OH radical reactivity. 相似文献