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101.
The introduction of some crustacean species has produced alterations of freshwater environments and declines of native species worldwide. The red swamp crayfish, Procambarus clarkii Girard, was introduced in the Southwest Iberian Peninsula in the 1970’s, producing severe impacts on rice agriculture and on native biota such as macrophytes, gastropods, native crayfish and amphibians. We studied the distribution of P. clarkii in two areas of SW Iberian Peninsula: the Sado River basin (SW Portugal), an area colonized by this species around 1990, and the Parque Natural del Entorno de Doñana (SW Spain), colonized soon after its introduction in the Iberian Peninsula, in the 1970’s. Our main goal was to determine which factors limit crayfish distribution, which could help to identify the most effective management practices to contain its spread. Procambarus clarkii was found in most types of water bodies, including small and shallow ones. Distance to a crayfish source was the single predictor variable explaining crayfish occurrence in most types of habitats and in both areas. The only exception was for the Sado permanent stream points, where crayfish presence was negatively affected by an interaction between elevation and flow velocity. Other habitat characteristics have apparently little or no importance for its successful colonization. Moreover, this study indicated that overland dispersal is apparently a frequent phenomenon in this species. Our findings can be used to determine which habitats are most likely to be colonized by the crayfish and to develop practical measures which may limit its spread and minimize its impacts. 相似文献
102.
Peeter Nõges Wouter Van de Bund Ana Cristina Cardoso Anna-Stiina Heiskanen 《Hydrobiologia》2007,584(1):373-379
In most cases the negative impacts of climate change to aquatic ecosystems cannot be mitigated by measures in the river basin
management. Ignoring climate change by the Water Framework Directive may have strong implications for the typology and quality
assessment systems used for water bodies. As a result of climate change, water bodies, especially those located near the type
boundaries may change their type. Compared to typology characteristics, water quality parameters are even more labile and
may be easily affected by climate change. The paper exemplifies that the anticipated deterioration of water quality within
the time frame relevant for WFD implementation may be large enough to endanger the fulfillment of the set water quality objectives.
The review of the river basin characterization every six years, as required by the WFD, might also include re-evaluation of
reference conditions according to the changes observed at pristine reference sites. As a consequence, the restoration targets
(i.e., the good ecological status) would also need to be evaluated periodically. 相似文献
103.
104.
C.M. Ghersa S.B. Perelman S.E. Burkart R.J.C. León 《Biodiversity and Conservation》2007,16(6):1575-1592
The Flooding Pampa natural grassland has an intricate pattern of plant communities, related to small topographic differences
that determine important changes in soil characteristics. Despite limitations imposed by soil properties and periodic waterlogging,
opportunistic tilling is carried out to plant pastures. There is little information on how pasture planting may affect the
structure of the grassland communities. In order to document changes caused by cultural activities on structural and functional
characteristics of plant communities in this landscape, we made field surveys in grasslands and very old pastures (grassland
communities recovered through secondary succession) using transects located across existing topographic gradients.
The patchy structure of this landscape was revealed by the multivariate analysis, by means of which four plant communities
could be identified in the natural grassland. Species composition of these communities differed from that of the corresponding
old pastures. They lost an important number of exclusive species, but also gained species: some new to the landscape and many
already present in other environments.
Pasture planting reduced the rate of species replacements along the gradient and produced changes in patchiness, but had no
effect on the species–area curve at the landscape scale. Neither did we find differences in total number of species, average
number of species/site and proportion of functional types. The new grassland created by opportunistic pasture planting has
developed into a structural gradient in which important differences occurred in the lower waterlogged-prone stands, whereas
the sites of the other communities experienced less structural changes. 相似文献
105.
Tracking the effects of one century of habitat loss
and fragmentation on calcareous grassland
butterfly communities 总被引:1,自引:0,他引:1
Emmanuelle Polus Sofie Vandewoestijne Julie Choutt Michel Baguette 《Biodiversity and Conservation》2007,16(12):3423-3436
Habitat loss and fragmentation are known to reduce patch sizes and increase their isolation, consequently leading to modifications
in species richness and community structure. Calcareous grasslands are among the richest ecosystems in Europe for insect species.
About 10% (1,150 ha) of the total area of a calcareous ridge region (Calestienne, Belgium) and its butterfly community was
analysed over a timeframe of about 100 years. Since 1905 to present day (2005), the Calestienne region has undergone both
calcareous grassland loss and fragmentation: not only did calcareous grassland size decrease and isolation increase, but also,
the number of calcareous grassland patches within the landscape increased until 1965, and subsequently decreased, clearly
reflecting the effects of fragmentation. These processes have had a profound effect on the butterfly community: extinction
and rarefaction affected significantly more often specialist species, which means that generalist species are more and more
overrepresented. This ecological drift, i.e. the replacement of specialists by generalists in species assemblages is likely
to be a general effect of habitat loss and fragmentation on natural communities. 相似文献
106.
Although plant quality can indirectly increase the performance of the third trophic level by bottom-up cascading effects,
the mechanisms of this indirect effect are still unclear. In this study the carbon–nitrogen stoichiometry in a tri-trophic
system consisting of the willow, a leaf beetle, and a predatory ladybird beetle were examined to determine the mechanisms
of the bottom-up cascading effect. The bottom-up cascade is initiated by increasing leaf nitrogen, because of artificial cutting
of willow trees. The relative growth rate (RGR) of the leaf beetle increased when fed on cut willow leaves, because of the
high leaf nitrogen in the cut willows. Ladybird beetle RGR also increased when fed on leaf beetles fed on cut willow leaves.
The increased RGR of the ladybirds cannot be explained by the quality of the prey, however, because leaf beetle nitrogen was
not affected by host plant quality. Thus, the carbon–nitrogen stoichiometry could not be a mechanism of the bottom-up cascade
through multiple trophic levels. 相似文献
107.
Cludia Turon Joaquim Comas Jaume Alemany Ulises Corts Manel Poch 《Ecological Engineering》2007,30(4):362-372
This paper describes the development and operation of an Environmental Decision Support System (EDSS) to improve the operation and maintenance of horizontal subsurface-flow constructed wetlands (EDSS-maintenance). Constructed wetlands (CWs) allow wastewater treatment in a sustainable manner since they involve low energy consumption, low construction and functioning costs and low environmental impact. However, operation and maintenance activities are essential to guarantee reliability in CWs performance. The definition of operation and maintenance protocols depends on several quantitative and qualitative aspects such as wastewater treatment plant configuration, CW design, influent characteristics, sensitivity of the receiving media, etc. Bearing this in mind and considering the limited technical knowledge about CWs, the need for a new tool to support CW performance is clear. In this sense, EDSSs offer a new approach because they can tackle problems of complex and uncertain systems. The EDSS-maintenance provides operation and maintenance manuals specifically defined for every CW. To achieve it, the required knowledge was implemented within a rule-based system, which forms the backbone of the EDSS. Several features presented in this paper demonstrate how the EDSS-maintenance provides a proper platform to support the necessary collaborative work in the ecological engineering problem of horizontal subsurface flow CWs operation and maintenance. 相似文献
108.
Testing the growth rate vs. geochemical hypothesis for latitudinal variation in plant nutrients 总被引:2,自引:0,他引:2
Two hypotheses have been proposed to explain increases in plant nitrogen (N) and phosphorus (P) concentrations with latitude: (i) geochemical limitation to P availability in the tropics and (ii) temperature driven variation in growth rate, where greater growth rates (requiring greater nutrient levels) are needed to complete growth and reproduction within shorter growing seasons in temperate than tropical climates. These two hypotheses were assessed in one forest type, intertidal mangroves, using fertilized plots at sites between latitudes 36º S and 27º N. The N and P concentrations in mangrove leaf tissue increased with latitude, but there were no trends in N : P ratios. Growth rates of trees, adjusted for average minimum temperature showed a significant increase with latitude supporting the Growth Rate Hypothesis. However, support for the Geochemical Hypothesis was also strong; both photosynthetic P use efficiency and nutrient resorption efficiency decreased with increasing latitude, indicating that P was less limiting to metabolism at the higher latitudes. Our study supports the hypothesis that historically low P availability in the tropics has been an important selective pressure shaping the evolution of plant traits. 相似文献
109.
Disease mapping and spatial regression with count data 总被引:3,自引:0,他引:3
Wakefield J 《Biostatistics (Oxford, England)》2007,8(2):158-183
In this paper, we provide critical reviews of methods suggested for the analysis of aggregate count data in the context of disease mapping and spatial regression. We introduce a new method for picking prior distributions, and propose a number of refinements of previously used models. We also consider ecological bias, mutual standardization, and choice of both spatial model and prior specification. We analyze male lip cancer incidence data collected in Scotland over the period 1975-1980, and outline a number of problems with previous analyses of these data. In disease mapping studies, hierarchical models can provide robust estimation of area-level risk parameters, though care is required in the choice of covariate model, and it is important to assess the sensitivity of estimates to the spatial model chosen, and to the prior specifications on the variance parameters. Spatial ecological regression is a far more hazardous enterprise for two reasons. First, there is always the possibility of ecological bias, and this can only be alleviated by the inclusion of individual-level data. For the Scottish data, we show that the previously used mean model has limited interpretation from an individual perspective. Second, when residual spatial dependence is modeled, and if the exposure has spatial structure, then estimates of exposure association parameters will change when compared with those obtained from the independence across space model, and the data alone cannot choose the form and extent of spatial correlation that is appropriate. 相似文献
110.
Disturbance persistence in managed grasslands: shifts in aboveground community structure and the weed seed bank 总被引:2,自引:0,他引:2
The length of time and form in which disturbances persist in systems depends on the intensity and frequency of disturbance
and on the abilities of resident species to recover from such events. In grazed grasslands, trampling by large mammalian herbivores
can periodically facilitate weed establishment by exposing patches of bare ground but whether an intense soil disturbance
event results in a temporary increase in weed abundance or a persistent weed problem remains unclear. In May 2002, cattle
trampling following heavy rain caused severe damage to nine-month old, rotationally grazed, cool-season pastures (Midwest
USA). In September 2002, we compared the aboveground composition of paddocks (i.e., fenced pasture sections) that were heavily
disturbed to those that received no damage. Relative to undisturbed paddocks, forage species relative cover was 17% lower
in disturbed paddocks, and weed species and bare ground relative cover was 61% and 100% higher, respectively. By September
2004, paddock types did not differ in all aboveground community components. However, the abundance and species richness of
weed seeds in the soil seed bank averaged respectively 82% and 30% higher in disturbed paddocks between 2003 and 2004. These
findings indicate that a spatially extensive, intense soil disturbance event may soon become undetectable in components of
aboveground pasture structure but can persist as an augmented weed seed bank. Because of high weed seed bank longevity, disturbances
to formerly disturbed pastures would likely result in higher weed recruitment, with more species represented, than in those
which lack previous disturbance. Disturbance history may thus be a useful predictor of weed community composition following
subsequent disturbance. Based on empirical data supporting this proposition, we recommend that grassland managers explicitly
incorporate disturbance history into dynamic management planning and do not rely exclusively on aboveground characters to
evaluate the invasion status or colonization potential of an area by undesirable plants. We emphasize that the ecological
legacies of past soil disturbance events cannot only influence the contemporary patterns and processes of grasslands, but
importantly, affect their compositional trajectories following subsequent perturbation. 相似文献