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651.
Nutrient enrichment of habitats (eutrophication) is considered to be one of the main causes of plant diversity decline worldwide. Several experiments have shown a rapid loss of species in the first years after fertilization started. However, little is known about changes in species richness in the long term. Here, we use a 50-year-old field experiment with a range of fertilization treatments in grasslands that were mown twice each year in the center of The Netherlands. We show that species richness in all plots initially declined but started to recover after approximately 25 years of continued fertilization. This was also true for the heavily fertilized treatment (NPK). In NPK-fertilized plots, the decline was strongest and associated with a strong divergence of plant trait composition from the control, reflecting a shift to a plant community adapted to nutrient-rich conditions. During the subsequent period of increase in species richness, the trait composition remained stable. These results show that plant species richness can, at least partially, recover after an initial diversity decline caused by fertilization. 相似文献
652.
Theoretical approaches, such as the Lotka-Volterra framework, enable predictions about long term species coexistence based on stability criteria, but generally assume temporal constancy of system equations and parameters. In real world systems, temporal variability may interfere with the attainment of stable states. Managed grassland ecosystems in Northwestern Europe experience structural periodic fluctuations in environmental conditions: the seasons. In addition, periodic disturbances such as cutting are very common. Here we show, using a Lotka-Volterra system applied to grassland species with empirically derived parameters, that seasonal variability can result in a time dependent equilibrium and redirection of displacement processes.Parameter estimates differed between species and - in most cases - between the seasons. As a result, five of the fifteen tested species combinations had different outcomes of species interactions between seasons. This indicates that systems remain in dynamic transience over the year as the equilibrium changes and the species composition of the system follows the equilibrium without ever attaining it. The non-attainment of the steady state enables coexistence of species even if there is competitive exclusion in one of the seasons. For three of the fifteen species combinations, cutting frequency affected the long-term coexistence patterns. Cutting resets the biomass of competing species and favours during regrowth those species that have a high growth rate, which can alter species coexistence in comparison to a Lotka-Volterra model without cutting. The Lotka-Volterra framework with seasonally changing empirical parameters predicts coexistence as a possible outcome of systems that in component seasons are characterised by exclusion, and vice versa. 相似文献
653.
How the properties of ecosystems relate to spatial scale is a prominent topic in current ecosystem research. Despite this, spatially explicit models typically include only a limited range of spatial scales, mostly because of computing limitations. Here, we describe the use of graphics processors to efficiently solve spatially explicit ecological models at large spatial scale using the CUDA language extension. We explain this technique by implementing three classical models of spatial self-organization in ecology: a spiral-wave forming predator-prey model, a model of pattern formation in arid vegetation, and a model of disturbance in mussel beds on rocky shores. Using these models, we show that the solutions of models on large spatial grids can be obtained on graphics processors with up to two orders of magnitude reduction in simulation time relative to normal pc processors. This allows for efficient simulation of very large spatial grids, which is crucial for, for instance, the study of the effect of spatial heterogeneity on the formation of self-organized spatial patterns, thereby facilitating the comparison between theoretical results and empirical data. Finally, we show that large-scale spatial simulations are preferable over repetitions at smaller spatial scales in identifying the presence of scaling relations in spatially self-organized ecosystems. Hence, the study of scaling laws in ecology may benefit significantly from implementation of ecological models on graphics processors. 相似文献
654.
655.
Damien Denis Jean-Sébastien PierreJoan van Baaren Jacques J.M. van Alphen 《Ecological modelling》2011,222(9):1604-1613
Adult parasitoid females lay their eggs in or on host insects. Most species are incapable of de novo lipogenesis as adults, and lipids accumulated during the larval stage are allocated either to egg production or to adult survival. Lipid consumption increases with distance covered by the parasitoids and thus with the distance between available hosts within a habitat. Temperature should affect parasitoid fitness because it changes the constraint imposed by a limited reserve of lipids and because it influences behaviour. Climate change involves both an increase in average temperature and an increased frequency of extreme weather such as heat waves. We investigated how the predicted increase of temperature will affect parasitoid fitness and how this depends on habitat parameters (spatial distribution of hosts and lipid cost of habitat exploitation). We studied optimal behaviour and calculated fitness at different temperatures and in different habitats using a stochastic dynamic programming model for pro-ovigenic parasitoids (which mature all their eggs before becoming adult). We show that an increase in temperature decreases fitness of parasitoids adapted to lower temperatures. This decrease in fitness depends on habitat quality. In field conditions (assuming small costs of intra-patch foraging), the loss of fitness should be larger in habitats with high inter-patch distance and in habitats with a more aggregated distribution of hosts. The foraging behaviour of parasitoids is also affected; at higher temperature we show that intra-patch foraging becomes less efficient, and patch residence times are longer. 相似文献
656.
A fundamental question of sexual selection theory concerns the causes and consequences of reproductive skew among males. The
priority of access (PoA) model (Altmann, Ann NY Acad Sci 102:338–435, 1962) has been the most influential framework in primates living in permanent, mixed-sex groups, but to date it has only been
tested with the appropriate data on female synchrony in a handful of species. In this paper, we used mating data from one
large semi-free ranging group of Barbary macaques: (1) to provide the first test of the priority-of-access model in this species,
using mating data from 11 sexually active females (including six females that were implanted with a hormonal contraceptive
but who showed levels of sexual activity comparable to those of naturally cycling females) and (2) to determine the proximate
mechanism(s) underlying male mating skew. Our results show that the fit of the observed distribution of matings with sexually
attractive females to predictions of the PoA model was poor, with lower-ranking males mating more than expected. While our
work confirms that female mating synchrony sets an upper limit to monopolization by high-ranking individuals, other factors
are also important. Coalitionary activity was the main tactic used by males to lower mating skew in the study group. Coalitions
were expressed in a strongly age-related fashion and allowed subordinate, post-prime males to increase their mating success
by targeting more dominant, prime males. Conversely, females, while mating promiscuously with several males during a given
mating cycle, were more likely to initiate their consortships with prime males, thus reducing the overall effectiveness of
coalitions. We conclude that high-ranking Barbary macaque males have a limited ability to monopolize mating access, leading
to a modest mating skew among them. 相似文献
657.
Darren P. Croft Mathew Edenbrow Safi K. Darden Indar W. Ramnarine Cock van Oosterhout Joanne Cable 《Behavioral ecology and sociobiology》2011,65(12):2219-2227
Understanding how individuals modify their social interactions in response to infectious disease is of central importance
for our comprehension of how disease dynamics operate in real-world populations. Whilst a significant amount of theoretical
work has modelled disease transmission using network models, we have comparatively little understanding of how infectious
disease impacts on the social behaviour of individuals and how these effects scale up to the level of the population. We experimentally
manipulated the parasite load of female guppies (Poecilia reticulata) and introduced fish either infected with the ectoparasites Gyrodactylus spp. (experimental) or uninfected (control) into replicated semi-natural populations of eight size-matched female guppies.
We quantified the behaviour and social associations of both the introduced fish and the population fish. We found that infected
experimental fish spent less time associating with the population fish than the uninfected control fish. Using information
on which fish initiated shoal fission (splitting) events, our results demonstrate that the population fish actively avoided
infected experimental fish. We also found that the presence of an infected individual resulted in a continued decline in social
network clustering up to at least 24 h after the introduction of the infected fish, whereas in the control treatment, the
clustering coefficient showed an increase at this time point. These results demonstrate that the presence of a disease has
implications for both the social associations of infected individuals and for the social network structure of the population,
which we predict will have consequences for infectious disease transmission. 相似文献
658.
Roger H. Green Brian A. McArdle Robert van Woesik 《Environmental monitoring and assessment》2011,178(1-4):455-460
Contemporary coral reefs are forced to survive through and recover from disturbances at a variety of spatial and temporal scales. Understanding disturbances in the context of ecological processes may lead to accurate predictive models of population trajectories. Most coral-reef studies and monitoring programs examine state variables, which include the percentage coverage of major benthic organisms, but few studies examine the key ecological processes that drive the state variables. Here we outline a sampling strategy that captures both state and process variables, at a spatial scale of tens of kilometers. Specifically, we are interested in (1) examining spatial and temporal patterns in coral population size-frequency distributions, (2) determining major population processes, including rates of recruitment and mortality, and (3) examining relationships between processes and state variables. Our effective sampling units are randomly selected 75 × 25 m stations, spaced approximately 250?C500 m apart, representing a 103 m spatial scale. Stations are nested within sites, spaced approximately 2 km apart, representing a 104 m spatial scale. Three randomly selected 16 m2 quadrats placed in each station and marked for relocation are used to assess processes across time, while random belt-transects, re-randomized at each sampling event, are used to sample state variables. Both quadrats and belt-transects are effectively sub-samples from which we will derive estimates of means for each station at each sampling event. This nested sampling strategy allows us to determine critical stages in populations, examine population performance, and compare processes through disturbance events and across regions. 相似文献
659.
Recent evidence shows that the frequently proclaimed collapse of the traditional career model is actually not supported by job tenure data. This paper argues that the observed stability of job tenure might be explained by an increasing number of shamrock organizations. This organizational form has three types of workers: core employees, professional freelancers, and routine workers. In such an organization, two very different career models coexist. The organization largely determines the career of the core employee, whereas the individual essentially shapes that of the professional freelancer. This paper studies extensively the career of this second group: the professional freelancer, a growing phenomenon in many developed countries but not yet the focus of many career studies. We develop a freelance career success model on basis of the intelligent career framework augmented by insights from literature on entrepreneurship. Data are from a web survey with responses from about 1600 independent professionals in the Netherlands, in combination with 51 in‐depth interviews. We provide two main contributions. First, we report findings from the first large‐scale quantitative study into freelance career success. Second, this study enhances our understanding of the success of the modern career by building bridges between career and entrepreneurship literatures. We conclude that the external environment in which an individual freelancer operates is the most important factor determining career success. The study therefore suggests that more work needs to be performed on the relationship between the environment and individual career success. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
660.
Farm Level Adaptation to Climate Change: The Case of Farmer’s in the Ethiopian Highlands 总被引:1,自引:0,他引:1
In Ethiopia, climate change and associated risks are expected to have serious consequences for agriculture and food security. This in turn will seriously impact on the welfare of the people, particularly the rural farmers whose main livelihood depends on rain-fed agriculture. The level of impacts will mainly depend on the awareness and the level of adaptation in response to the changing climate. It is thus important to understand the role of the different factors that influence farmers’ adaptation to ensure the development of appropriate policy measures and the design of successful development projects. This study examines farmers’ perception of change in climatic attributes and the factors that influence farmers’ choice of adaptation measures to climate change and variability. The estimated results from the climate change adaptation models indicate that level of education, age and wealth of the head of the household; access to credit and agricultural services; information on climate, and temperature all influence farmers’ choices of adaptation. Moreover, lack of information on adaptation measures and lack of finance are seen as the main factors inhibiting adaptation to climate change. These conclusions were obtained with a Multinomial logit model, employing the results from a survey of 400 smallholder farmers in three districts in Tigray, northern Ethiopian. 相似文献