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   3308篇
  免费   498篇
  国内免费   770篇
  2024年   55篇
  2023年   133篇
  2022年   73篇
  2021年   149篇
  2020年   223篇
  2019年   314篇
  2018年   197篇
  2017年   162篇
  2016年   174篇
  2015年   137篇
  2014年   166篇
  2013年   164篇
  2012年   147篇
  2011年   198篇
  2010年   142篇
  2009年   214篇
  2008年   214篇
  2007年   223篇
  2006年   188篇
  2005年   196篇
  2004年   164篇
  2003年   149篇
  2002年   154篇
  2001年   126篇
  2000年   92篇
  1999年   60篇
  1998年   40篇
  1997年   32篇
  1996年   44篇
  1995年   28篇
  1994年   28篇
  1993年   29篇
  1992年   39篇
  1991年   16篇
  1990年   25篇
  1989年   9篇
  1988年   7篇
  1987年   12篇
  1986年   8篇
  1985年   8篇
  1984年   3篇
  1983年   3篇
  1982年   13篇
  1981年   2篇
  1980年   4篇
  1979年   4篇
  1978年   2篇
  1977年   2篇
  1976年   2篇
  1968年   1篇
排序方式: 共有4576条查询结果,搜索用时 15 毫秒
981.
The Reeuwijk Lakes (The Netherlands) present a typical example of eutrophication in the lower Rhine catchment area. In 1986 restoration of these lakes started by reducing the external P-loading. Two lakes, Lake Elfhoeven and Lake Nieuwenbroek, differing in P-load and residence time were selected for monitoring water quality parameters before (1983–1985) and after (1986–1987) these restoration measures. Reduction of the external P-loading did not result in lower P-concentrations in both lakes. In contrast, P and N increased. This may have been caused by an increase in diffuse discharges. However, seasonal cycles of P and N point to a strong internal loading of nutrients. The concentrations of chlorophyll a and carotene decreased, indicating a lower phytoplankton biomass. However, as C-phycocyanine concentrations increased the relative abundance of cyanobacteria became higher. Seston concentrations and zooplankton densities did not change. Transparency in the lakes slightly decreased after P-reduction and is far too low for the development of any vegetation of submerged waterplants. The typical differences between both lakes remained after restoration measures. The inverse relationship between the concentrations of chlorophyll a and total phosphorus at the two sampled stations remained constant. The differences in phytoplankton composition and the zooplankton biomass give a plausible explanation for this inverse relationship, between the two stations. Restoring the lakes after four decades of P-loading can presumably, not simply be done by lowering the external P-load alone. Supplementary in-lake measures may accelerate the restoration process.  相似文献   
982.
Synopsis We conducted underwater fish assemblage surveys, benthic quinaldine sampling, and ichthyoplankton collections over high and low relief reefs at Palos Verdes Point, California from 1974 through 1981. Seventy-three species (adults and larvae) were collected or observed at Palos Verdes. Of the 24 most abundant species, 8 showed significant abundance trends correlated with changes in sea surface temperatures; four increasing, and four decreasing. Two taxa occurred predominantly in the kelp canopy, while seven others were distributed at least 20% of the time between the canopy and the bottom. There was little significant change in the Palos Verdes benthic fish assemblage with development of the kelp bed. Only the kelp bass, Paralabrax clathratus, showed a significant increase in abundance that correlated with the kelp bed formation. Similarly, we did not note an increase in juvenile fish during this period. In general, we suggest that the presence or absence of kelp has little effect on the abundance of most fish species in a high relief environment. Analysis of our data and that of other studies indicate that kelp bed fish assemblages within the Southern California Bight vary in a south-north direction. The Palos Verdes Point kelp bed appears to be more productive than those nearer the limit of the Bight, but was consistently lower in both fish abundance and diversity then the nearby breakwater at King Harbor. This difference has been maintained even though changes in the species structure at King Harbor has altered sufficiently since 1976 for the two formerly distinct assemblages to be significantly concordant.  相似文献   
983.
Synopsis The hypothesis that Sacramento suckers, Catostomus occidentalis, compete with rainbow trout, Salmo gairdneri, for space in streams was examined by measuring microhabitat utilization of both species in three California streams. Two streams were similar in most respects except one contained only trout and one contained trout and a large population of suckers. The third stream, formed by the union of the first two, contained trout and a small population of suckers. The species overlapped in five of the six microhabitat variables measured: maximum depth, mean water column velocity, focal point velocity, surface water velocity, and substrate type. However, the species had strong vertical segregation; there was little overlap between species in focal point depth. Mean focal point velocities were also significantly different. Suckers roamed over and generally remained in contact with the bottom while trout held position in the water column. Microhabitat utilization by trout in the stream without suckers was similar to in the stream with a higher sucker density. Differences in microhabitat utilization by trout between the third stream and the other two was attributed to the larger size of the third stream. Both sucker and trout showed a similar within-species segregation of size classes - fish under 50 mm in length sought shallow water. Size-specific trends indicated ontogenic shifts in resource utilization which reduced overlap within species. These results suggest that competition for space between trout and suckers was not a major factor regulating microhabitat utilization of trout, although the possibility that larger suckers may displace small trout needs further study.  相似文献   
984.
任海 《广西植物》2023,43(8):1516-1523
《昆明-蒙特利尔全球生物多样性框架》提出要高质量保护和恢复各30%的土地,最大化地实现保护生物多样性和缓解气候变化的目标,而演替理论和植被恢复可以为实现30%的保护和恢复目标服务。演替理论是植被生态学中的核心理论,演替是指在一个地点上由一群不同物种组成的生命体的结构或组成随时间而变化的过程; 植被恢复是以植物种植、配置为主,恢复或重建植物群落或天然更新恢复植物群落的过程,植被恢复是生态系统结构和功能从简单到复杂、从低级向高级变化的过程,最终目的是建立健康稳定的植物群落。演替是植被恢复的基础,植被恢复被视为对演替过程的操纵,以达到恢复受损植被生态系统的目标。演替理论可以指导植被恢复,而植被恢复对演替理论的发展有益。演替按裸地性质可以分为原生演替和次生演替,有研究建议将恢复过程视为第三演替,这将有助于理解通过人为干预促进植被恢复成功的管理选择,特别是通过强调退化生态系统中的环境和生物遗存的管理选择。此外,该文还提出了植被恢复理论和演替理论未来可能重点关注的科学和技术问题。  相似文献   
985.
In 1997, a plan to restore Elk (Cervus elaphus) to Ontario was approved by the provincial government. The objective of the Ontario elk restoration program, a multipartnered collaboration, was to restore a species that had been extirpated from the province during the 1800s. During 1998–2001, 460 elk were acquired from Elk Island National Park, Alberta, for release in four areas of Ontario. As greater than 90% of the elk were radio collared, monitoring provided detailed information on the dynamics of the four populations. Comprehensive research projects using graduate students were implemented to determine the environmental impact of releasing elk in Ontario. Those studies are in progress or have been completed and include the effect of wolf predation on restored elk, white‐tailed deer and elk resource overlap, the development of genetic profiles for elk, and solutions for elk/human conflicts. Mortality of the released elk averaged 41% (190/460) during 1998–2004 with annual mortality generally declining over time in each release area. The primary causes of elk mortality included wolf predation (25% of mortalities), illegal shooting (13%), stress‐related emaciation (13%) (partially due to the stress of relocation), bacterial infections (7%), and collisions with vehicles (6%). Productivity has been high in one of the release areas with 24–65% of the cows being observed with calves during late winter surveys. However, productivity has been low in two of the northern release areas due to a variety of factors including wolf predation. In some areas, dispersion of elk appeared to be related to the length of time animals were kept in pens prior to release. The precalving population estimate for Ontario in March 2004 was 375–440 elk. A comprehensive program review was conducted in 2003/2004 that included recommendations relating to the future management of elk in Ontario.  相似文献   
986.
Summary   The purpose of this comment paper is to highlight the need for more research that aims to develop strategies and methods for establishing and maintaining native grasses in degraded pastures. Clues to finding the best strategies are linked to the vigorous debate concerning the relationship between biodiversity and ecosystem function, which has resonated in ecological research for more than 20 years. Studies aimed at investigating the operational validity of the 'sampling effect' hypothesis vs. the 'niche complementarity' hypothesis could provide valuable direction on how to successfully re-establish healthy and stable native pasture communities, including the number of species and functional groups needed. Once strategies have been developed, then practical methods on grazing management and sowing are also needed. In the long run, research initiatives in this direction could act to benefit the conservation of native grass diversity, while at the same time, sustain important ecological (and economic) functions such as production, nutrient cycling and resistance to invasion.  相似文献   
987.
Understanding the degree to which species assemblages naturally vary over time will be critically important when assessing whether direct management effects or contingency is responsible for species gain or loss. In this study, we tested three predictions related to short‐term variation in prairie moth communities: (1) communities would only exhibit significant temporal variation in newly restored sites (1–3 years old); (2) prairie size and age would positively influence community reassembly, with larger, older restorations sampling a greater proportion of the regional species pool; and (3) older restorations (7–10 years old) would have yet to converge on the community composition of prairie remnants. Moths were sampled from 13 Tallgrass prairie restorations and remnants in central Iowa in 2004–2005. Repeated measures analysis of variance revealed significant effects of sampling year on moth species richness and abundance as well as on the richness of two functional groups, but difference among prairie types was only observed in 2005. Rarefaction analysis revealed that older restorations and prairie remnants supported higher species richness compared to recently planted sites, and nonmetric, multidimensional scaling ordination indicated that restorations older than 7 years were clearly converging on the species composition of remnants. These results suggest that moth communities in restorations and remnants are highly variable in time but that as restorations age, they appear to reaccumulate moth species found in prairie remnants. The long‐term persistence of a particular species assemblage within a given site, however, might be a difficult endpoint to attain in central Iowa prairies because of significant annual variation in species occurrence.  相似文献   
988.
989.
Non‐axenic operation of a 400 L trickle bed reactor inoculated with the thermophile Caldicellulosiruptor saccharolyticus, yielded 2.8 mol H2/mol hexose converted. The reactor was fed with a complex medium with sucrose as the main substrate, continuously flushed with nitrogen gas, and operated at 73°C. The volumetric productivity was 22 mmol H2/(L filterbed h). Acetic acid and lactic acid were the main by‐products in the liquid phase. Production of lactic acid occurred when hydrogen partial pressure was elevated above 2% and during suboptimal fermentation conditions that also resulted in the presence of mono‐ and disaccharides in the effluent. Methane production was negligible. The microbial community was analyzed at two different time points during operation. Initially, other species related to members of the genera Thermoanaerobacterium and Caldicellulosiruptor were present in the reactor. However, these were out‐competed by C. saccharolyticus during a period when sucrose was completely used and no saccharides were discharged with the effluent. In general, the use of pure cultures in non‐sterile industrial applications is known to be less useful because of contamination. However, our results show that the applied fermentation conditions resulted in a culture of a single dominant organism with excellent hydrogen production characteristics. Biotechnol. Bioeng. 2009;102: 1361–1367. © 2008 Wiley Periodicals, Inc.  相似文献   
990.
Avian population dynamics are influenced by the availability of spatiotemporally variable prey resources, but the conditions producing abundant and accessible prey are not always clear. In the Florida Everglades, wading birds nest in the dry season when receding water levels concentrate prey and facilitate improved foraging efficiency. White Ibis (Eudocimus albus) feed extensively on crayfish in sloughs, and previous studies have demonstrated that crayfish move downgradient from higher elevation, heavily vegetated ridge habitats into adjacent less‐vegetated sloughs when ridges are almost dry. Most White Ibis foraging is thought to occur in sloughs with relatively shallow water (< 19 cm), but crayfish move and their densities peak when water in sloughs is deeper (~ 21–32 cm). We conducted an observational study of White Ibis foraging in drying wetlands to determine if White Ibis restricted their foraging to shallow water or if they foraged in relatively deep water when crayfish were migrating. In a series of large drying wetlands, we used time‐lapse imagery to quantify White Ibis foraging activity over 61 d from February to April 2017 and we also quantified crayfish biomass density in sloughs. Crayfish biomass density peaked when ridges were almost dry. Most White Ibis foraging occurred over 2–3 d when ridges were almost dry and water in sloughs averaged ≥ 29 cm deep. White Ibis selected slough edges for foraging, suggesting that they were capturing crayfish migrating between habitats. Our results point to a new mechanism of prey exploitation driven by inter‐habitat prey flux when ridge habitat dries. Although the results of previous studies suggest that White Ibis will not forage on fish in deeper water (> 25 cm), we found that White Ibis will forage on crayfish in water at those depths. Maintenance of habitat elevational differences and hydro‐patterns that promote crayfish production will be necessary to promote this predator–prey interaction in the ridge‐slough landscape of the Everglades.  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