全文获取类型
收费全文 | 1194篇 |
免费 | 103篇 |
国内免费 | 366篇 |
出版年
2024年 | 9篇 |
2023年 | 27篇 |
2022年 | 45篇 |
2021年 | 40篇 |
2020年 | 62篇 |
2019年 | 59篇 |
2018年 | 53篇 |
2017年 | 59篇 |
2016年 | 70篇 |
2015年 | 47篇 |
2014年 | 81篇 |
2013年 | 73篇 |
2012年 | 66篇 |
2011年 | 82篇 |
2010年 | 57篇 |
2009年 | 86篇 |
2008年 | 75篇 |
2007年 | 109篇 |
2006年 | 113篇 |
2005年 | 71篇 |
2004年 | 66篇 |
2003年 | 59篇 |
2002年 | 34篇 |
2001年 | 38篇 |
2000年 | 22篇 |
1999年 | 24篇 |
1998年 | 17篇 |
1997年 | 17篇 |
1996年 | 9篇 |
1995年 | 13篇 |
1994年 | 7篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 7篇 |
1989年 | 1篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 6篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1963年 | 1篇 |
排序方式: 共有1663条查询结果,搜索用时 31 毫秒
131.
《农业工程》2014,34(2):106-109
Nature reserve has been served as the important pathway for biodiversity conservation and carbon storage. Global climate change is an indisputable fact and impacted the biodiversity and nature reserve. How nature reserves adapt to climate change has drawn more and more concerns. This research conducted questionnaires of 68 national nature reserves from 24 provincial regions, and the questionnaires showed that all surveyed nature reserves experienced climate change, and 68.57%, 61.43% and 68.57% of nature reserves, respectively, considered warming temperature, precipitation change, and occurrence of extreme climate events as new threats to them. These new factors directly threat the distribution range and survival of endangered species, change of ecosystem function, enhance of pest and disease damages, and directed damage the infrastructures. However, most of the surveyed nature reserves did not consider the systematic monitoring the facts of climate change, and lack actions and strategies of initiative adaptation to climate change. At last, we proposed the strategies for nature reserves to adapt to climate change, including enhancing the monitoring on the impact of climate change, making scientific planning and designing for development of nature reserves, decreasing the pressure through sustainable development, and enhancing the scientific research and the investment to improve the ability of nature reserves to adapt to climate change. 相似文献
132.
Peter J. Blank 《The Journal of wildlife management》2013,77(1):68-74
The northern bobwhite (Colinus virginianus; hereafter bobwhite) has experienced substantial population declines in recent decades in the United States, and especially in Maryland and Delaware. The United States Department of Agriculture's Conservation Reserve Program (CRP) could provide additional habitat for bobwhites, leading to an increase in bobwhite abundance. I investigated if bobwhite abundance was related to the percent cover of CRP land and landscape attributes in local landscapes on Maryland's Eastern Shore and Delaware. Observers conducted bobwhite point transect surveys at 113 locations during the breeding seasons of 2006–2007, and I calculated landscape metrics for 500-m radius landscapes centered on each point transect location. Most CRP land in the study landscapes was planted to herbaceous vegetation. Bobwhite abundance was strongly positively associated with percent cover of CRP land in the landscape but was not strongly related to percent cover of agriculture or forest or to landscape patch density. These results suggest that the CRP has created additional habitat for bobwhites in Maryland and Delaware and that landscapes with greater proportions of herbaceous CRP practices support more bobwhites. © 2012 The Wildlife Society. 相似文献
133.
Geolocators reveal migratory connectivity between wintering and breeding areas of Golden‐winged Warblers 下载免费PDF全文
Jeffery L. Larkin Douglas Raybuck Amber Roth Liliana Chavarría‐Duriaux Georges Duriaux Moisés Siles Curtis Smalling 《Journal of Field Ornithology》2017,88(3):288-298
The conservation of migratory songbirds is often impeded by a lack of understanding of how populations in breeding and wintering areas are geographically linked (migratory connectivity). In recent years, light‐level geolocators have improved our understanding of migratory connectivity. Such information is valuable for evaluating how conservation efforts align between the breeding and non‐breeding areas of at‐risk species, and help to more effectively prioritize the allocation of conservation funding. Golden‐winged Warblers (Vermivora chrysoptera) are imperiled migratory songbirds, but the extent to which conservation efforts in their breeding and non‐breeding areas coincide with patterns of migratory connectivity are not well known. We used light‐level geolocators to evaluate the extent to which conservation actions targeting Golden‐winged Warblers in Nicaragua and in their breeding range in North America align with patterns of migratory connectivity. We recovered six of 22 geolocators that had been deployed on male Golden‐winged Warblers at the El Jaguar Reserve during the winter of 2015–2016. All six males migrated to breeding areas in the western Great Lakes region that includes eastern Minnesota, northern Wisconsin, southwestern Ontario, and Michigan's Upper Peninsula. All six males also had similar migration routes, with spring stopovers in southern Mexico, Guatemala, and Belize, a trans‐Gulf flight, and a stopover in the region of Louisiana, Arkansas, eastern Oklahoma, and Texas. Our results, in combination with those of previous studies, demonstrate strong migratory connectivity between portions of the breeding and winter distributions of Golden‐winged Warblers currently targeted for conservation. However, additional studies are needed to improve our understanding of the stopover ecology of Golden‐winged Warblers, especially in areas where they remain for extended periods of time. Finally, patterns of migratory connectivity revealed in our study should be used in combination with existing demographic parameters for Golden‐winged Warblers in the western Great Lakes and Nicaragua to help inform full life cycle population models for this imperiled songbird. 相似文献
134.
In order to better understand the relationship between reserve design and the species represented by such designs, we examined
the effectiveness of the Western Australian reserve system for conserving angiosperm diversity, and examined the characteristics
of those species conserved. We overlayed species distribution data for 14 plant lineages with the distribution of the reserve
system (8.5% of the State’s area) and identified the species that remained unprotected. We found that, depending upon the
method employed, between 174 (5.7%) and 570 (18.7%) of species were not included within the reserve system. Two main unprotected
regions were identified, one of which was also a centre of high diversity. Geographical range sizes of unprotected species
were six times smaller than those species that were protected, while species richness of small-ranged endemic species coincided
with general patterns of species richness. At the level of Western Australia’s bioregions we found that conservation effectiveness
was most dependent on characteristics of the reserve system rather than characteristics (size and positioning) of species
ranges. At this scale, the most effective way to conserve more species in Western Australia would be to conserve more land,
while conservation would be most successful in a uniformly dispersed reserve system. Our results highlight the fact that reserve
systems may take on two design approaches based on scale––at continental scales, reserves should be clustered around the hotspots
of endemic species, while within regions, an evenly distributed reserve system will most adequately sample species. 相似文献
135.
Serge Andréfouët Simon Van Wynsberge Cécile Fauvelot Andrew W. Bruckner Georges Remoissenet 《Molluscan research.》2014,34(4):277-284
The giant clam subfamily Tridacninae (family Cardiidae) is an important group of bivalve molluscs found throughout the Red Sea and Indo-Pacific, from East Africa to the Eastern Pacific biogeographic region. The Tridacna genus is currently revised with numerous cryptic species identified with molecular markers. New Tridacna records from the fringe of the known distribution areas are extremely useful to identify genetically unique species, geographic ranges, and to examine processes associated with species differentiation. While Tridacna maxima is abundant in French Polynesia (Central South Pacific Ocean) the larger fluted giant clam Tridacna squamosa was formerly reported only in the Austral Islands in the south. Following a recent survey that spanned 23 islands and atolls of the Society, Tuamotu and Gambier Archipelagos, the presence of T. squamosa between the Cook Islands and Pitcairn Islands is confirmed using both morphological and molecular information, suggesting a relic distribution across the Central Pacific Ocean. Tridacna squamosa is rare, but present throughout Tuamotu and Gambier. However, it remained undetected from the Society Islands, probably due to historical over-fishing. This species is valued by local inhabitants, and is sought after mainly as gifts and also for a limited local shell trade. The rarity of T. squamosa may call for conservation measures in the near future. 相似文献
136.
正黑叶猴(Trachypithecus francoisi)是国家一级重点保护野生动物,也是少数仅分布在喀斯特石山地区的灵长类动物(Rowe,1996),曾在我国南部分布广泛,数量较多,在20世纪以前,野生种群数量约有3 500只(张荣祖等,2002),其中广西的种群数量约为2 200-2 500只(刘万福和韦振逸,1995),贵州数量估计为946-1 094只(李明 相似文献
137.
138.
139.
为探讨茂兰喀斯特森林群落结构和优势种更新机制,对茂兰自然保护区常绿落叶阔叶混交林群落进行了调查。结果表明,群落中有木本植物37科58属70种,以樟科、蔷薇科、无患子科和漆树科为主。常绿树种41种,占总种数的58.6%;落叶树种29种,占总种数的41.4%,这体现了喀斯特常绿落叶阔叶混交林的群落学性质。群落的优势乔木树种有轮叶木姜子(Litseaverticillata)、青冈栎(Cyclobalanopsisglauca)、狭叶润楠(Machilusrehderi)和翅荚香槐(Cladrastisplatycarpa)等。灌木优势种有皱叶海桐(Pittosporumcrispulum)、裂果卫矛(Euonymusdielsianus)、革叶铁榄(Sinosideroxylonwightianum)和南天竹(Nandina domestica)等。优势种种群结构有5种类型:单峰型,如樱桃(Cerasus pseudocerasus)、南酸枣(Choerospondias axillaris)等阳性乔木,为林窗更新种,随林窗的郁闭将衰退消失;逆J字型,如轮叶木姜子、香港四照花(Dendrobenthamia hongkongensis)、皱叶海桐等耐荫性强的常绿阔叶树种,更新能力较强,为顶极群落优势种;间歇型,如青冈栎、狭叶润楠、齿叶黄皮(Clausena dunniana)等树种,群落中的个体多集中在小径级阶段,能长期存在于群落中;L字型,如桂皮(Cinnamomumtamala)、梓叶槭(Acer catalpifolium)、革叶铁榄等;单柱型,如石岩枫(Mallotus repandus)、南天竹等常绿灌木树种,个体集中于幼苗、幼树阶段,为顶极群落亚乔木层和灌木层的主要组成种。因此,这表明调查区域的植物群落处于相对稳定的顶极阶段。 相似文献
140.
Habitat restoration and native grass conservation: a case study of switchgrass (Panicum virgatum) 下载免费PDF全文
Hsiaochi Chang Helen M. Alexander Evans Mutegi Allison A. Snow 《Restoration Ecology》2018,26(3):506-515
Switchgrass (Panicum virgatum) has been planted extensively for habitat restoration across the United States, such as with the Conservation Reserve Program (CRP). However, genetic profiles of these populations have never been studied nor compared with those of remnant prairies or cultivars. In this study, we sampled 16 CRP and 17 prairie populations across eastern Kansas. We assessed ploidy levels of all populations and compared genetic diversity and structure of 10 prairies, 10 CRP areas, and 5 standard cultivars, using nine simple sequence repeat (SSR) DNA markers. All CRP and prairie populations were octaploid (8x), except two prairies with both hexaploid (6x) and octaploid (8x) individuals. Based on the results of SSR analyses, there were no significant differences between CRP and prairie populations in genetic diversity, and 94% of total variation was partitioned within populations. Similarities among prairie and CRP populations were also observed in Bayesian clustering algorithms and principal coordinate analysis, suggesting that they had similar genetic compositions. In addition, positive spatial autocorrelations were detected up to 42 and 46 km among prairie and among CRP populations, respectively. To conclude, the CRP and prairie populations shared similar genetic profiles. However, remnant prairies still harbored unique genotypes and a high level of genetic diversity, highlighting the importance of seed sources for restoration efforts, that is using local wild seeds or cultivars from the same geographical region. A popular tetraploid (4x) cultivar known as “Kanlow” was genetically distinct from the prairie populations and therefore is not recommended for habitat restoration in this region. 相似文献