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   1108篇
  免费   133篇
  国内免费   173篇
  2024年   3篇
  2023年   16篇
  2022年   21篇
  2021年   26篇
  2020年   28篇
  2019年   45篇
  2018年   37篇
  2017年   49篇
  2016年   40篇
  2015年   39篇
  2014年   50篇
  2013年   114篇
  2012年   55篇
  2011年   46篇
  2010年   40篇
  2009年   89篇
  2008年   71篇
  2007年   82篇
  2006年   56篇
  2005年   63篇
  2004年   54篇
  2003年   35篇
  2002年   38篇
  2001年   25篇
  2000年   32篇
  1999年   35篇
  1998年   24篇
  1997年   21篇
  1996年   15篇
  1995年   23篇
  1994年   7篇
  1993年   20篇
  1992年   19篇
  1991年   6篇
  1990年   8篇
  1989年   10篇
  1988年   12篇
  1987年   10篇
  1986年   11篇
  1985年   7篇
  1984年   5篇
  1983年   6篇
  1982年   5篇
  1981年   9篇
  1979年   3篇
  1978年   2篇
  1975年   1篇
  1974年   1篇
排序方式: 共有1414条查询结果,搜索用时 15 毫秒
81.
Early Late Miocene coral assemblages from five outcrops of Gavdos Island, Southern Greece, are investigated with respect to their palaeoecological implications. Small patch reefs with Porites assemblage are a common feature of the low-diversity coral occurrences. The determined hermatypic colonies indicate a nearshore palaeoecosystem prevailing in a tropical to subtropical coastal sea at depths ranging from 5 to 50 m with an average temperature of 22–26 °C. Microfacially, the studied Scleractinian patch reefs are represented by Coral Framestones-Floatstones. The reefal facies has been affected by syndepositional processes (boring activities-micritization), as well as by post-depositional diagenesis in the meteoric realm (dissolution, cementation and intense pedogenesis). The palaeoecological and sedimentological analysis indicates a restricted to open-marine inner platform setting of moderate to high energy, possibly of ramp-type (inner-mid ramp). Coral reef growth took place mainly during stages of accommodation (i.e., transgressive episode, cycle 3.1 of Vail curve) and of relatively low siliciclastic input.  相似文献   
82.
记述了采自萨哈林岛1新种,Alopecosa mikhailovi sp.nov.,2♂♂,新种与solivaga种团近似.  相似文献   
83.
海南岛热带森林景观类型多样性   总被引:2,自引:0,他引:2  
王伯荪  彭少麟  郭泺  叶有华 《生态学报》2007,27(5):1690-1695
依据景观生态学原理,按地貌、气候、土壤、植被和土地利用方式的分异,以带、域、省、区、类型为5个基本单位及亚、组等为辅助单位,组建海南岛热带森林景观类型分类体系。把海南岛热带森林景观作为省级单位,它隶属于全球热带林景观带、亚洲(东方)热带林景观域、中国热带林景观亚域,其下划分为东部潮湿森林景观、西部半干旱森林景观、中南部山地森林景观、热带常绿针叶林景观、热带竹林景观和热带人工林景观6个森林景观区;以热带低地沟谷热带雨林为代表的11个森林景观亚区;以热带低地沟谷龙脑香森林景观为代表的26个森林景观类型组;以热带低地沟谷青皮林为代表的54个森林景观类型。海南岛热带森林景观类型分类体系较全面地表达了海南岛热带森林景观类型多样性。  相似文献   
84.
Over the past 30 years southern New England, USA has been invaded by several species of ascidians, including Botrylloides violaceus, Diplosoma listerianum, Styela clava, and Ascidiella aspersa. These species have become dominate in coastal embayments and marinas but are usually absent from more open water coastal areas. A colonial ascidian, Didemnum sp. has invaded southern New England during the past 10 years and we first observed this species in eastern Long Island Sound in 2000. It has become the dominant at several field sites while remaining in low abundance at others. We conducted an experiment at two places, a protected marina and an open coast site, to examine its ability to compete with the established fouling community. Small colonies of Didemnum were transplanted onto panels with communities that varied in age from one to four weeks old and these treatments along with controls with only Didemnum were exposed at both sites. In most treatments Didemnum became a dominant species in the communities at both sites but it reached higher abundances at the open coast site. Potential causes of the observed differences are predation on other species of ascidians at the open coast site reducing recruitment of these species and competition, lower tolerance for elevated temperatures at the marina site, or other environmental parameters that might affect growth rates.  相似文献   
85.
在深圳市内伶仃岛薇甘菊危害的不同群落生境中,设立9块样地81个小样方,用森草净(即70%嘧碘降水溶性粉剂)杀灭样地中的薇甘菊施量为0.0001~0.02 g•m-2,结果表明:各浓度的森草净杀灭效果均较好,杀灭率随着用药量的增加而提高;在坡地和溪谷生境中,森草净用药量分别为0.05~0.1 g•m-2、>0.2 g•m-2能较彻底地杀灭薇甘菊。应用HPLC法检测样地土壤中嘧磺隆残留量,溪谷土壤中嘧磺隆半衰期C=C0•e-0.083TT1/2=8.4,施药后37 d消解95.9%,坡地高浓度级半衰期C=C0•e-0.046TT1/2=15.1 d,施药后37 d消解85.0%,坡地低浓度级半衰期C=C0•e-0.090T, T1/2=7.7 d,施药后15 d消解742%。不同浓度森草净处理样地,施药后7、15、37 d均可检测到嘧磺隆,并且含量越来越小,但施药后68 d的土样,均未检测到嘧磺隆的存在。  相似文献   
86.
The loss of biotic integrity in ecosystems due to human pressure has been receiving much attention from the scientific community. The primary aim of this study is to understand how the increasing human pressure on natural forests in the Azorean archipelago (North Atlantic) is affecting their epigean arthropod communities and which biological parameters it affects most. An expert team did fieldwork covering most of the natural forests (mainly inside nature reserves) of the archipelago using standardized pitfall trapping. To build a multimetric index we tested a number of taxonomic and ecological parameters that can potentially be influenced by disturbance. Sixteen of these were found to be significantly influenced by disturbance in forests. We retained seven metrics due to both, desirable scalability properties and relatively low correlation between them. These included the percentages of endemic and predator species richness and also predator abundance, which are inversely related to disturbance; and the percentages of native and saprophagous species richness and introduced and herbivore abundance, which are positively related to disturbance. All seven metrics were combined in an Index of Biotic Integrity (IBI) value. We then proceeded to understand which potential disturbance factors are influencing the biotic integrity of communities and how such influence is felt. Five disturbance factors were found to influence the IBI, although in different ways: the size and fragmentation of reserves, the distance of sites to the reserve borders, the invasion by alien plants and the density of human paths at the sites. Given that only percentages of taxonomical or ecological characteristics were chosen as metrics, we tested and found the scalability of the IBI to be possible, allowing the comparison of sites with different collecting effort or even the comparison of reserves with different areas and numbers of collecting sites in each. Finally, we propose a novel graphical representation for multimetric indices like the IBI, one which allows retaining much of the information that is usually lost in multimetric indices.  相似文献   
87.
A novel landscape classification system was proposed in this study based on landscape ecological theory and the differentiation in climate, topography, soil, vegetation and land use mode. Five basic units (zone, tract, province, region and type) and two assistant units (sub and group) were used in this system. The tropical forest landscape in Hainan Island was regarded as a landscape province, belonging to global tropical forest landscape zone, Asiatic (oriental) tropical forest landscape tract and Chinese tropical forest landscape subtract. Based on the grade system of region, sub-region, type-group and type, this landscape province (Hainan Island) is divided into 6 landscape regions (east moist forest landscape, west semi-arid forest landscape, central-south mountainous forest landscape, tropical evergreen needle-leaved forest landscape, tropical bamboo landscape and tropical plantation landscape), 11 tropical forest landscape sub-regions represented by tropical lowland valley rain forest landscape, 26 tropical forest landscape-type groups represented by tropical lowland valley Dipterocarpaceae forest landscape, and 54 forest landscape types (FLT) represented by lowland valley Vatica mangachapoi forest. Generally, this classification system represents the landscape diversity of the tropical forest in Hainan Island. Further studies are needed to better understand the landscape diversity of Hainan Island.  相似文献   
88.
Non-native mammals are major drivers of ecosystem change and biodiversity loss; this is especially apparent on islands. However, techniques exist to remove non-native mammals, providing a powerful conservation tool. Conservation practitioners are now targeting larger islands for restoration. Leveraging existing and developing new techniques and technologies will prove critical to successful eradications on large islands. Using the removal of introduced goats (Capra hircus) from Santiago Island, Galápagos as a case study, we present a suite of Geographic Information System (GIS) tools that aid island conservation actions. GIS tools were incorporated into the three phases of the eradication campaign: planning, hunting, and monitoring. Further, these tools were adopted for three eradication techniques: ground-based hunting, aerial hunting by helicopter, and Judas goats. These geographic approaches provide a foundation for statistical, spatial, and economic analyses that should increase the capability and efficiency of removal campaigns. Given limited conservation funds and the dire status of many insular species, efficiently removing non-native mammals from islands is of paramount global conservation importance.  相似文献   
89.
In the Crete Island, late Messinian Lago-Mare facies are not well known. At present, the occurrence in Crete of the uppermost Messinian post evaporitic deposits is a matter of debate. According to several authors, the well-known late Messinian Lago-Mare facies does not occur in Crete. In this paper the preliminary results obtained from the biostratigraphical analysis of some sections sampled in the Messarà Plain will be shown. Nearby Faneromeni and Ano Akria villages, the Miocene/Pliocene boundary is well exposed. There, gypsum-bearing clay, laminated microcrystalline gypsum and gypsum-rudites characterize the evaporitic deposits of the Messinian stage. In these areas, above the Messinian evaporite, post-evaporitic fine-laminated polychrome clays, with intercalations of sandstones and conglomerates, have been found. In both the Faneromeni and Ano Akria area, the Pliocene grey clays and conglomerates rest unconformably on the uppermost Messinian post-evaporitic deposits. A 20 cm-spaced sampling has been performed in both the sections, for more than 100 samples collected. The results of the micropaleontological analysis performed on the Faneromeni and Ano Akria sections point to the occurrence of ostracod assemblages containing: Loxocauda limata (Schneider in Agalarova et al.), Loxocauda sp., Cytherura pyrama Schneider, Cyprideis anlavauxensis Carbonnel, Cyprideis agrigentina Decima, Amnicythere palimpsesta (Livental), Amnicythere propinqua (Livental), Amnicythere accicularia (Olteanu), Amnicythere costata (Olteanu), Amnicythere multituberculata (Livental), Amnicythere sp. D (Miculan in Bassetti et al.), Amnicythere sp. 2 Gliozzi and Grossi, Amnicythere sp., Euxinocythere (Maeotocythere) praebaquana (Livental in Agalarova et al.), Mediocytherideini indet., Pontoniella pontica (Agalarova), Camptocypria sp. 1 Gliozzi and Grossi, Caspiocypris sp., Zalanyiella venusta (Zalanyi), Tyrrhenocythere sp., Loxoconcha rhombovalis Pokorny, Loxoconcha eichwaldi Livental, Loxoconcha sp. A (Miculan in Bassetti et al.), Loxocorniculina djafarovi (Schneider in Suzin). In the analysed samples, reworked planktonic foraminifers and well-preserved charophyte gyrogonites have been also found. The ostracod assemblages found in the Messarà Plain belong to the Loxocorniculina djafarovi Zone (sensu Carbonnel, 1978), which characterizes the uppermost Messinian deposits of the whole Mediterranean Basin. At that time, the well-known Lago-Mare biofacies was also widespread on the Crete Island. The presence of Paratethyan ostracods in the post-evaporitic Messinian deposits of both Faneromeni and Ano Akria sections suggests that in the Crete Island the latest Messinian sedimentation took place in brackish water palaeoenvironments.  相似文献   
90.
陈艳  沈浪  应向阳 《广西植物》2007,27(4):555-559
在大陆青冈分布区东缘的大金山岛种群中按不同坡向取样,旨在揭示相对均一条件下青冈种子的形态变异及变异的来源,以从不同侧面研究环境对形态变异的作用。大金山岛种群青冈种子的长度、直径和重量分别为1.557±0.128cm、1.175±0.112cm、1.321±0.310g,处于青冈种子正常分布范围内。单颗种子的长度、直径符合正态分布,而重量则明显偏离正态分布。不同坡向青冈种子间差异很小,表明微地形的影响较小;种群变异的来源株内和株间约各占一半,说明种子表型具有一定的可塑性,主要由于种子处于植株不同部位的缘故。  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