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   68篇
  免费   4篇
  2017年   3篇
  2015年   1篇
  2014年   1篇
  2013年   1篇
  2012年   2篇
  2011年   4篇
  2010年   7篇
  2009年   8篇
  2008年   2篇
  2007年   2篇
  2006年   3篇
  2005年   3篇
  2004年   1篇
  2003年   1篇
  2001年   1篇
  2000年   1篇
  1999年   3篇
  1998年   2篇
  1997年   3篇
  1996年   1篇
  1992年   1篇
  1991年   1篇
  1990年   1篇
  1989年   2篇
  1988年   3篇
  1987年   3篇
  1984年   1篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1977年   1篇
  1976年   1篇
  1959年   2篇
  1955年   2篇
  1953年   1篇
排序方式: 共有72条查询结果,搜索用时 46 毫秒
31.
ABSTRACT Three polyclonal antibodies raised against Paraurostyla sp. cyst wall polypeptides of molecular weight 110,000 (p110), 66,000 (p66) and 52,000 (p52) have been obtained. The specificity of the antisera was tested by immunoblotting. Anti-p110 antibody detected five bands of 300, 170, 135, 110 and 40 kDa, respectively. Antiserum obtained against p66 recognized only this protein. Anti-p52 antiserum showed reaction for two different bands of 52 and 44 kDa, respectively. The precise localization of these proteins in the cyst wall was assessed by light microscope immunocytochemistry. Anti-p110 antiserum produced a strong positive reaction in both the ectocyst and endocyst. Both anti-p66 and anti-p52 antibodies recognized the ectocyst.  相似文献   
32.
In April 2008, 585 tortoises illegally imported into Italy from North Africa were examined for the presence of ticks. Of these, 221 tortoises (37.8%) were infested with a mean intensity of 3.9 ± 3.1 ticks (range 1–17 ticks). A total of 798 ticks (672 males, 125 females and one nymph) were collected and identified as Hyalomma aegyptium (L.) (Acari: Ixodidae). The overall male : female ratio observed was 5 : 1. The prevalence and mean intensity [±standard deviation (SD)] of ticks were higher among male (67.4%, 4.0 ± 3.2) than female (55.6%, 3.8 ± 3.1) tortoises, although this difference was not significant. By contrast, the prevalence and mean intensity of ticks were significantly higher on tortoises weighing >100 g (61.5%, 4.0 ± 3.2) compared with tortoises weighing <100 g (12.1%, 2.1 ± 1.1). Of the infested tortoises, 89.8% had ticks on their hind limbs, 21.0% on forelimbs, 18.6% on the tail and pre‐anal area, and 4.8% on the head; thus the hind limbs were evidently the preferred attachment site. The present report highlights the need to develop surveillance systems to prevent the introduction and spreading of exotic ticks and tick‐borne pathogens in Italy and other European countries.  相似文献   
33.
Ants were studied on Puerto Rico and 44 islands surrounding Puerto Rico. Habitat diversity was the best predictor of the number of species per island and the distributions of species followed a nested subset pattern. The number of extinctions per island was low, approximately 1–2 extinctions per island in a period of 18 years, and the rates of colonization seem to be greater than the extinction rates. Ant dynamics on these islands do not seem to support the basic MacArthur and Wilson model of island biogeography. The MacArthur and Wilson equilibrium is based on the notion that species are interchangeable, but some extinctions and colonizations can change the composition and number of species drastically.  相似文献   
34.
Abstract.  Variation in the mtDNA 16S ribosomal RNA gene in populations of Triatoma infestans (Klug) was surveyed. DNA sequence comparisons yielded 18 haplotypes among 130 individuals from 16 localities that represent a large proportion of the range of T. infestans in Argentina. The most common genotype in all populations was found in 76.9% of individuals and two other haplotypes were shared among different populations. The remaining 15 haplotypes were present exclusively in one of the populations, suggesting currently low levels of genetic exchange. Analysis of mtDNA 16S sequences uncovered substantial genetic variation among T. infestans populations. Haplotype and nucleotide diversities varied among populations, from 0% to 0.84% and 0% to 0.29%, respectively. It appears that this locus has a low mutation rate. Uncorrected pairwise differences of T. infestans haplotypes ranged from 0% to 1.2%. The molecular phylogeny supported the monophyly of T. infestans haplotypes and clustered two different pairs of haplotypes with a moderate degree of bootstrap support (∼ 60%). Mitochondrial DNA phylogeographic differentiation was not evident, suggesting a recent rapid spread of the species. Analysis of molecular variance showed hierarchical structure in the data. Considerably less variation was found among T. infestans populations from the northwest and northeast regions than among those belonging to the central area. Such a lack of variation may be indicative of one or more past population bottlenecks.  相似文献   
35.
Based on known data sets and maximum entropy distribution data of fern and lycopod species registered in the Yucatán Peninsula, track and parsimony analyses were undertaken to evaluate the contribution of these groups to the establishment of biogeographical relationships of the peninsula with other areas. The resulting generalized tracks clearly agree with the geological origin of the peninsula and the previously recognized relationship with the Greater Antilles is not supported for ferns and lycopods. Instead, a Central American generalized track connects the Yucatán Peninsula with south‐eastern México and Central America. Floristically, the peninsula harbours 66 species of ferns and lycopods. Seven are registered for the first time in the Yucatán Peninsula and one is a new species for México. These species do not follow the latitudinal pattern expected if ecological factors, such as humidity and rainfall, were the most important in determining their distributions. Groups of areas recognized with parsimony analysis of endemicity could not be defined as provinces as a result of the lack of endemic species. Nevertheless, a regionalization scheme based on maximum entropy distribution data and supported by track analyses is proposed. Two separate districts are recognized within the Yucatán Peninsula: arid/dry Yucatán in the north and El Petén (humid) in the south. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 775–786.  相似文献   
36.
  • 1 The vaquita Phocoena sinus is a small porpoise that is endemic to the northern Gulf of California, Mexico. It is the most critically endangered marine small cetacean in the world. The most precise estimate of global abundance based on a 1997 survey is 567 (95% CI 177–1073).
  • 2 Vaquitas mainly live north of 30°45′N and west of 114°20′W. Their ‘core area’ consists of about 2235 km2 centred around Rocas Consag, 40 km east of San Felipe, Baja California. Genetic analyses and population simulations suggest that the vaquita has always been rare, and that its extreme loss of genomic variability occurred over evolutionary time rather than being caused by human activities.
  • 3 Gill nets for fish and shrimp cause very high rates of by‐catch (entanglement) of vaquitas. Estimates of bycatch rates are from 1993–94 and refer to one of three main fishing ports: 84 per year (95% CI 14–155) using only data collected by observers and 39 per year (95% CI 14–93) using combined data from observers and interviews with fishermen. Boats from other ports may experience similar rates, and the total is probably well above what would be sustainable.
  • 4 Other less well‐characterized and longer‐term risk factors include the potential for disturbance by trawling to affect vaquita behaviour, and the uncertain effects of dam construction on the Colorado River and the resultant loss of freshwater input to the upper Gulf. However, entanglement is the clearest and most immediate concern.
  • 5 Progress towards reducing entanglement has been slow in spite of efforts to phase out gill nets in the vaquita’s core range, and the development of schemes involving compensation for fishermen. The Biosphere Reserve in the northern Gulf has fallen far short of its potential for vaquita conservation. On 29 December 2005, the Mexican Ministry of Environment declared a Vaquita Refuge that contains within its borders the positions of approximately 80% of verified vaquita sightings. In the same decree, the state governments of Sonora and Baja California were offered $1 million to compensate affected fishermen. The effectiveness of this major initiative remains to be seen.
  • 6 The vaquita’s survival does not depend on more or better science but on improved management. As a funding priority, implementation of conservation measures and evaluation of their effectiveness should come ahead of more surveys or improved estimation of by‐catch.
  相似文献   
37.
The biomass of tropical forests plays an important role in the global carbon cycle, both as a dynamic reservoir of carbon, and as a source of carbon dioxide to the atmosphere in areas undergoing deforestation. However, the absolute magnitude and environmental determinants of tropical forest biomass are still poorly understood. Here, we present a new synthesis and interpolation of the basal area and aboveground live biomass of old‐growth lowland tropical forests across South America, based on data from 227 forest plots, many previously unpublished. Forest biomass was analyzed in terms of two uncorrelated factors: basal area and mean wood density. Basal area is strongly affected by local landscape factors, but is relatively invariant at regional scale in moist tropical forests, and declines significantly at the dry periphery of the forest zone. Mean wood density is inversely correlated with forest dynamics, being lower in the dynamic forests of western Amazonia and high in the slow‐growing forests of eastern Amazonia. The combination of these two factors results in biomass being highest in the moderately seasonal, slow growing forests of central Amazonia and the Guyanas (up to 350 Mg dry weight ha?1) and declining to 200–250 Mg dry weight ha?1 at the western, southern and eastern margins. Overall, we estimate the total aboveground live biomass of intact Amazonian rainforests (area 5.76 × 106 km2 in 2000) to be 93±23 Pg C, taking into account lianas and small trees. Including dead biomass and belowground biomass would increase this value by approximately 10% and 21%, respectively, but the spatial variation of these additional terms still needs to be quantified.  相似文献   
38.
Rabal‐Garcés, R., Cuenca‐Bescós, G., Canudo, J.I. & de Torres, T. 2011: Was the European cave bear an occasional scavenger? Lethaia, Vol. 45, pp. 96–108. The cave bear Ursus spelaeus fossils remains are quite abundant in the Late Pleistocene site of Coro Tracito (Huesca, Spain). The site constitutes the highest mountain record of cave bears in the Iberian Peninsula. Being a monospecific locality, it permits the study of the biology and dietary habits of this species. The study of the limb bones established first, the mortality pattern of this population of Ursus spelaeus and, second, the alteration pattern due to carnivore tooth‐marks. Some authors have performed similar analyses in the same kind of skeletal elements in other cave bear localities all over Europe and, therefore it has been possible to compare our results with those from other sites. The tooth‐marks found in the bones of cave bears, especially in monospecific sites, have been attributed to a scavenging behaviour. In agreement with the authors, our analysis presented here supports the hypothesis of scavenging behaviour for cave bears. □Behaviour, Late Pleistocene, Spain, taphonomy, tooth‐marks, Ursus spelaeus.  相似文献   
39.
Flowering synchrony is essential for plant reproductive success, especially in the case of small‐sized populations of self‐incompatible species. Closely related to synchrony, flowering intensity influences pollinator attraction and pollinator movements. Thus, a high flowering intensity may increase pollinator attraction but, at the same time, may also increase the probability of geitonogamous pollinations. Depending on the mating system, the female fitness of plants in small populations may be affected by both the positive effects of higher flowering synchrony and pollinator attraction and the negative effects of geitonogamous pollinations induced by a high flowering intensity. It was hypothesized that different‐sized plants in a population would show contrasting flowering patterns, resulting in differences in pollinator behaviour. These influences could result in differences in mating and female reproductive success. This hypothesis was tested by studying the flowering pattern of Erodium paularense (Geraniaceae), a rare and endangered endemic of central Spain. The temporal distribution of flower production was explored throughout the reproductive season, and the probability of xenogamy and geitonogamy and their relationship to plant size and fitness components were calculated. The analysis of this partially self‐compatible species showed diverse flowering patterns related to different plant sizes. Small plants produced a larger number of seeds per fruit in spite of having lower values of flowering synchrony. By contrast, large plants produced a larger number of seeds from geitonogamous pollinations. The effect of different flower displays and outcrossing rates on seed set varied throughout the season in the different groups. Our findings highlight the relevance of individual plant size‐dependent phenology on female reproductive success and, in particular, on the relationship between flowering synchrony and fitness. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 156 , 227–236.  相似文献   
40.
Galium crespianum J.J. Rodr. is a perennial plant inhabiting cliffs and crevices on the Balearic Islands. Although it was described from the mountains of Mallorca, several populations from the islands of Eivissa and Formentera have been traditionally considered as belonging to the same species. A careful comparison of specimens from the western and eastern Balearics revealed several discriminant features in gross morphology (robustness and habit of the stems, presence or absence of hairs at the internodes and leaves, waxiness of stems and leaves, shape of the inflorescence, flower colour and scent) and stem anatomy. In addition, several mutations in the ribosomal ITS sequences distinguished western and eastern plants. It is proposed that the plants from Eivissa and Formentera previously subsumed under G. crespianum be recognized at the specific level as G. friedrichii N. Torres, L. Saez, Mus & Rossello nom. nov. The new species is tetraploid (2n = 44) and shares morphological features with G. crespianum or with several taxa in the G. glaucum complex from the south east Iberian peninsula, to which it is most probably related.  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