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121.
The Sierra de La Laguna, in the southern part of the state of Baja California Sur, Mexico, was decreed a biosphere reserve
in 1994. It harbors relict plant communities with a high level of endemism. Floristic research has established that the two
main communities, the oak–pine woodland (OPW) and the dry tropical forest (TDF), harbor 43 and 33 endemic species, respectively.
We segregate the reserve surface into seven major habitats and each endemic species were assigned a hierarchic value into
an abundance-distribution scale, useful as dominance ranking. Using such scale, we found that almost half of the species are
in the lowest rank (low abundance and aggregate distribution). Using GIS, we obtain the surface for each habitat, where mountainsides
represent the largest in the reserve area. We obtained an index of importance from the proportion of endemic species by habitat
and its surface occupied, founding that, in spite of the small and fragmented nature of the upland riparian habitats, they
have the greatest index value. Consequently, for future conservation plans in the reserve, these habitats should be considered
remarkable sites, meriting better attention to ensure endemic plant preservation. Also, since 86% of the endemic species appear
in one, two or three habitats, a high dependency on particular environmental conditions is suggested for them. 相似文献
122.
Jelena Blaženčić Branka Stevanović Živojin Blaženčić Vladimir Stevanović 《Biodiversity and Conservation》2006,15(11):3445-3457
The occurrence of 47 species of all the six recent genera of charophytes recorded in the Balkans is demonstrated. Our aim
was to assess their status of threat according to the 2001 IUCN threat categories and criteria, in order to establish the
Red List of Charophytes distributed in the Balkan Peninsula. The List underlines the diversity of charophytes in this part
of the world and provides guidelines for its conservation. Analysis of the charophytes in each category of threat has provided
a general overview of their distribution, species richness, population features and possibility of survival throughout the
various regions of the Balkan Peninsula. 相似文献
123.
一类具有常数迁入且总入口在变化的SIRI传染病模型的稳定性 总被引:3,自引:0,他引:3
讨论一类具有常数迁入率,染病类有病死且有效接触率依赖于总人数的SIRI传染病模型.给出了基本再生数σ的表达式.如果σ≤1,则疾病消除平衡点是全局稳定的;如果σ>1,则存在唯一的传染病平衡点且是局部渐近稳定的.对具有双线性传染率和标准传染率的相应模型,进一步证明了当σ>1时传染病平衡点的全局稳定性. 相似文献
124.
亚洲中部荒漠区的植物特有属 总被引:22,自引:0,他引:22
系统地列出了亚洲中部荒漠区的13个植物特有属或近特有属——绵刺属、沙冬青属、四合木属、百花篙属、革苞菊属、河西菊属、喀什菊属、紊篙属、连蕊芥属、钝基草属、合头黎属、戈壁黎属、霸王属。这些属都是单种属或双种属,分类上属于孤立的类群;演化系统上是既起源古老而又进化的类群;生态上都是早生植物,绝大多数是荒漠种,其生活型灌木和半灌木占优势;地理分布上阿拉善荒漠区是其特有属的分布中心;起源上是多元的。 相似文献
125.
Thresholds, equilibria, and their stability are found for SIQS and SIQR epidemiology models with three forms of the incidence. For most of these models, the endemic equilibrium is asymptotically stable, but for the SIQR model with the quarantine-adjusted incidence, the endemic equilibrium is an unstable spiral for some parameter values and periodic solutions arise by Hopf bifurcation. The Hopf bifurcation surface and stable periodic solutions are found numerically. 相似文献
126.
Elevational patterns of species richness and endemism for some important taxa in the Hengduan Mountains, southwestern China 总被引:1,自引:0,他引:1
Da-Cai Zhang Yong-Hong Zhang David E. Boufford Hang Sun 《Biodiversity and Conservation》2009,18(3):699-716
We describe the elevational patterns of species richness and endemism of some important taxa in the Hengduan Mountains, southwest
China. Species richness data came from publications, an online database, herbaria and field work. Species richness was estimated
by rarefaction and interpolation. The Hengduan Mountains region was divided into a southern and northern subregion, and all
species were assigned to four groups based on their distributional range within this region. The conditional autoregressive
model (CAR) was used to relate species richness and explanatory variables. The elevational patterns of total, endemic and
non-endemic species richness, at subregion and entire region scales, presented to be unimodal and peaked at similar elevations.
Area size was strongly related with species richness, and was more powerful in explaining variation in species richness in
the northern subregion than in the southern subregion. A single climatic variable (mean annual rainfall, potential evapotranspiration
or moisture index) showed a weak relationship with the elevational pattern of species richness. Area and climatic variables
together explained more than 67% of the variation in non-endemic richness, 53% in total richness, and 50% in endemic richness.
There were three patterns of endemism at the generic level with increasing elevation: namely endemism increased, decreased,
or peaked at middle elevations. All selected taxa have experienced rapid speciation and evolution within this region, which
plays an important role in the uniform elevational patterns of total, endemic and non-endemic richness, and in the multiform
elevational patterns of endemism.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
127.
Narrowly-ranging species frequently harbor less genetic variability relative to widespread relatives and face graver extinction
threats due to the heightened impacts of stochastic events on ecological and genetic diversity. In this study, we examined
the impact of historical and current threats to the maintenance of genetic variation in Lithophragma maximum (Saxifragaceae), a perennial herb endemic to San Clemente Island, California. This species exists as small populations confined
to canyons along 4 km of the southeast coastline of the island. In 15 populations analyzed with 10 microsatellite markers,
we identified an average of 2.05 alleles per locus and 58.7% polymorphic loci. Significant departures from Hardy–Weinberg
equilibrium existed in six populations; five of these exhibited heterozygote deficiency. Bayesian inference of genetic structure
indicated a significant amount of structure among populations and canyons and infrequent gene flow even over very short distances.
We also identified a significant and positive correlation between genetic and geographic distances, indicative of isolation
by distance. There was no evidence of recent bottlenecks in any of the sampled populations, but older bottlenecks were detected
in two populations. These results suggest that populations of L. maximum have historically been small and isolated, which is likely due to the rugged habitat in which this species occurs and limited
pollen and seed dispersal. Given the high degree of structure observed across populations, we suggest that conservation efforts
should focus on preserving populations in multiple canyons, maintaining large population sizes to preserve genetic variation,
and controlling the spread of invasive species in areas where L. maximum occurs. 相似文献
128.
一些东亚特有种子植物的化石历史及其植物地理学意义 总被引:9,自引:2,他引:7
特有植物在植物区系地理的研究中有重要意义,不同等级的特有植物往往成为不同等级植物区系分区的重要依据。查明东亚特有植物的地史渊源对于揭示东亚植物区的特征与性质,理解中国植物区系及东亚植物发生和演变都有重要的意义。本文在现有资料的基础上,分析总结了东亚植物区系中有化石记录的银杏科、杜仲科、连香树科、大血藤科和昆栏树科等5个特有科,水杉等21个特有属的化石历史。从这些特有植物化石历史的分析可以发现,东亚植物的特有植物从来源上可以分为北极-第三纪,北热带,和就地起源等3种类型,特有植物的来源表明东亚植物区系是一个来源复杂的植物区系。尽管各种特有类群的地质历史各不相同,但是都经历了从广布到分布区逐步缩小,最后形成特有的过程。大部分特有类群形成特有的时间是在上新世末到第四纪初。根据特有类群划分区系等级的原则,东亚现代植物区系最终形成的时间应该是上新世末到第四纪初。 相似文献
129.
Camellia rhytidophylla is endemic and endangered and with important economic species, from the ⅢD10d in eastern Asia, which distributed at 573-920m in evergreen broad leaved deciduous mixed forest in Luowang river valley of Kaiyang county. So, it is significant to study the population characteristics, conservation and utilization. We took general survey in suspected distribution area about 6 square kilometers, set up plots in dense area, and analyzed the population structure, development and spatial distribution pattern. The results showed that the structure of Crhytidophylla populations were increasing and the proportion of young tree in population was 4638%. The size of population density was young shrubs>middle aged shrubs>adult shrubs. The survival curve of population was Deevey Ⅲ model. There were 2 peaks in the Ⅰand Ⅳ age classes on the mortality rate curve and disappearance rate curve respectively, then there are 2 troughs in the Ⅲ and Ⅵ age classed at the same time. The spatial distribution pattern of Crhytidophylla significantly differentiated at different stages of development, the young individuals were aggregated at all spatial scales while the middle aged individuals were aggregated at small spatial scales and randomly distributed at larger scales. Differences in the distribution of adult individuals could be attributed to artificial disturbance and habitat heterogeneity. The spatial pattern was not close in different stages of development, which all showed negative or no correlation. The spatial distribution pattern of Crhytidophylla was the interaction of the factors, such as its biological characteristics,habitat heterogeneity, and artificial disturbance, etc.. The key factor limiting population development was low natural reproduction rate; the primary factors causing its endemic distribution mainly included habitat heterogeneity, topographical constraints on seed dispersal and artificial disturbance. 相似文献
130.