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   113篇
  免费   3篇
  2017年   1篇
  2016年   1篇
  2015年   2篇
  2013年   2篇
  2012年   4篇
  2011年   2篇
  2010年   6篇
  2009年   5篇
  2008年   6篇
  2007年   6篇
  2006年   3篇
  2005年   1篇
  2004年   2篇
  2003年   3篇
  2002年   3篇
  2000年   1篇
  1999年   1篇
  1998年   2篇
  1997年   1篇
  1996年   2篇
  1995年   1篇
  1993年   2篇
  1992年   2篇
  1991年   1篇
  1990年   1篇
  1989年   3篇
  1988年   3篇
  1985年   1篇
  1980年   1篇
  1979年   1篇
  1972年   2篇
  1971年   1篇
  1960年   1篇
  1959年   1篇
  1958年   1篇
  1957年   4篇
  1956年   4篇
  1955年   3篇
  1954年   1篇
  1953年   11篇
  1952年   6篇
  1951年   5篇
  1950年   4篇
  1949年   1篇
  1948年   1篇
排序方式: 共有116条查询结果,搜索用时 15 毫秒
41.
42.
Abstract: Ecologically based management must incorporate components that consider how individuals associate temporally and spatially to environments that provide specific habitat requirements. Recent research has assessed how environments could be classified based on potential to provide deer (Odocoileus virginianus) habitat components. If habitat potential (HP; i.e., capability of habitat types to provide annual life requisites) classifications can be correlated to deer spatial structure and seasonal movement patterns, managers could better understand how spatial distribution of habitat components influences deer distribution. We analyzed home-range distribution and seasonal movement patterns from 45 adult (≥2 yr old) female deer radiocollared between 1999-2002, and deer habitat characteristics in northeastern Lower Peninsula, Michigan, USA, to investigate whether we can predict deer seasonal movement patterns based on the distribution of HP. We constructed logistic regression models that calculated the probability of deer migration given specific HP within seasonal home ranges of migratory and nonmigratory deer. Our results suggested that the probability of seasonal deer migrations relates to the juxtaposition (arrangement) of different habitat types that collectively provide all annual life requisites. We demonstrated that use of habitat-type classifications and HP models can track and predict deer movement patterns, which can facilitate establishment of management units and ecologically based deer management practices.  相似文献   
43.
Fat is the prime energy source for birds during prolonged exercise, but protein is also catabolized. Estimates of the amount of catabolizable fat and protein (termed fat and protein fuel) are therefore important for studying energetics of birds. As fat and protein fuel can only be measured by sacrificing individuals or by use of technically complex methods, scoring systems were invented to estimate fat and protein fuel of birds in the field. The visible subcutaneous fat deposits and the thickness of the flight muscles are each scored on an ordinal scale but these scales do not correspond linearly to fat and protein fuel within species, which is needed for analyses such as flight range estimates. We developed an anova ‐type model to estimate fat and protein fuel from fat scores (FS) and muscle scores (MS) along with total mass and a size measurement. Using data from 11 337 individuals of eight passerine species (Common Nightingale Luscinia megarhynchos, Eurasian Reed Warbler Acrocephalus scirpaceus, Melodious Warbler Hippolais polyglotta, Willow Warbler Phylloscopus trochilus, Orphean Warbler Sylvia hortensis, Garden Warbler Sylvia borin, Common Whitethroat Sylvia communis, Subalpine Warbler Sylvia cantillans) mist‐netted in Mauritania, West Africa, we tested for independence of FS and MS and for variation in the relationship between scores and associated mass in response to physiological state. FS, MS and third primary length (size) explained variation in body mass of all eight species analysed (R2: 0.56–0.77). The parameter estimates of the model showed that fat and protein fuel increased monotonically with increasing fat and muscle scores. In two species we found small differences in the estimates between physiological states (seasons). We evaluated our model by comparing the predicted body mass of birds with both FS and MS equal to 0 with the mean body mass of individuals mist‐netted with both scores equal to zero. The values were very close. The amount of fat extracted from dead Garden and Willow Warblers was within the range of predicted fat fuel derived from the model. We conclude that our model is a useful non‐invasive method to estimate simultaneously mean fat and protein fuel of small passerines and we provide recommendations on its use.  相似文献   
44.
A phylogenetic analysis of the leafhopper genus Apogonalia was conducted based on a matrix of 40 terminal taxa and 147 morphological characters. The analysis yielded 1391 equally most‐parsimonious trees, which do not support the monophyly of Apogonalia in the strict consensus. A successive weighting procedure yielded 62 trees in which the genus appeared as a monophyletic group. The strict consensus of these 62 trees is almost entirely dichotomous, showing only two polytomies. The test of phylogenetic integrity was applied for distinct variations of three species: A. germana, A. sanguinipes, and A. histrio. Only for the first species was the conjecture that its variations belong to the same entity corroborated. The best‐supported clade within Apogonalia, which has several synapomorphies and high branch support indices, comprises nine Antillean endemic species. This distributional pattern probably was originated by vicariance in the Late Cretaceous, when the Proto‐Antillean archipelago was pushed north‐eastward by the Caribbean Plate to become the modern Greater Antilles. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163 , 548–570.  相似文献   
45.
The oceanic water strider (or ocean‐skater) Halobates sericeus Eschscholtz has a disjunct distribution in the Pacific Ocean, with northern and southern populations widely separated by an equatorial zone. It is sensitive to sea surface conditions and, consequently, its distribution and population structure may provide an insight into environmental changes on the ocean surface on both recent and historical time scales. We assessed the genetic diversity and population structure of H. sericeus in the Pacific Ocean using three gene markers – cytochrome oxidase subunit I (COI), elongation factor 1α and internal transcribed spacer 1 (ITS‐1). These markers indicate that both populations are evolutionarily distinct with limited gene flow, having separated 20 000–50 000 years ago. This suggests that physical conditions and/or biotic interactions on the surface of the Pacific Ocean have provided significant barriers to gene flow since the late Pleistocene or earlier, creating biotic stability over large geographical and temporal scales in spite of a long history of global climate change. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 797–805.  相似文献   
46.
Family interrelationships among Anisoptera (dragonflies) are unresolved. Molecular markers applied thus far have not been particularly useful for resolving relationships at the family level. Previous morphological studies have depended heavily on characters of wing venation and articulation which are believed to display considerable degrees of homoplasy due to adaptations to different flight modes. Here, we present a comprehensive anatomical dataset of the head morphology of Anisoptera focusing on muscle organization and endoskeletal features covering nearly all families. The characters are illustrated in detail and incorporated into an updated morphological character matrix covering all parts of the dragonfly body. Phylogenetic analysis recovers all families as monophyletic clades except Corduliidae, Gomphidae as sister group to all remaining Anisoptera, and Austropetaliidae as sister group to Aeshnidae (=Aeshnoidea). The position of Petaluridae and Aeshnoidea to each other could not be resolved. Libelluloidea is monophyletic with Neopetalia and Cordulegastridae as first splits. Chlorogomphidae is sister to monophyletic [Synthemistidae + (‘Corduliidae’ + Libellulidae)]. In addition, we applied a recently published formal approach to detect concerted convergence in morphological data matrices and uncover possible homoplasies. Analyses show that especially head and thorax characters may harbour homoplasies. After exclusion of possible homoplastic characters, Gomphidae is corroborated as sister group to all remaining Anisoptera.  相似文献   
47.
Species delimitation requires an assessment of varied traits that can contribute to reproductive isolation, as well as of the permanence of evolutionary differentiation among closely related lineages. Integrative taxonomy, including the combination of genome‐wide molecular data with ecological data, offers an effective approach to this issue. We use genotyping‐by‐sequencing together with a review of ecological divergence to assess the traditionally recognized species status of three closely related members of the spruce budworm species complex, Choristoneura fumiferana (Clemens), C. occidentalis Freeman (=C. freemani Razowski) and C. biennis Freeman, each of which is a major defoliator of conifer forests. We sampled a broad region of overlap between these three taxa in Alberta and British Columbia (Canada) where potential for gene flow provides a strong test of the durability of divergence among lineages. A total of 2218 single nucleotide polymorphisms (SNPs) were assayed, and patterns of differentiation were evaluated under the biological, ecological, genotypic cluster and phylogenetic species concepts. Choristoneura fumiferana was genetically distinct with substantial barriers to genetic exchange with C. occidentalis and C. biennis. Conversely, divergence between C. occidentalis and C. biennis was limited to a small subset of outlier loci and was within the range observed within any one of the taxa. Considering both population genetic and ecological patterns of divergence, C. fumiferana should continue to be recognized as a distinct species, and C. biennis ( syn.n. ) should be treated as a subspecies (C. occidentalis biennis Freeman, 1967) of C. occidentalis, thereby automatically establishing the nominate name C. occidentalis occidentalis Freeman, 1967 for univoltine populations of this species.  相似文献   
48.
49.
Despite formal separation based on molecular and morphological evidence, the genera Prosthechea Knowles & Westc. and Encyclia Hook. have not been studied in terms of their vegetative anatomy. In this study we examine 16 Brazilian species of these genera. Additionally, one species of subtribe Laeliinae and another from subtribe Oncidiinae were studied in order to evaluate the taxonomic consistency of the anatomic characters observed. Except for Epidendrum crassifolium Lindl., all species possess a velamen differentiated into epivelamen and endovelamen. Endodermal cells with uniformly thickened walls, calcium oxalate raphides and cells with phi thickenings in the cortical region were observed in all species. Silica bodies, raphides and fibre bundles in the mesophyll were common in leaves of all species studied. By contrast, flavonoid crystals were observed in both roots and leaves of Prosthechea only. Encyclia is characterized by the presence of fibre bundles on the subepidermal layer, and a cuticle, which is clearly thicker than that of Prosthechea . These data strongly support the separation of Prosthechea and Encyclia .  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 143 , 293–303.  相似文献   
50.
Norway spruce (Picea abies [L.] Karst.) is a broadly distributed European conifer tree whose history has been intensively studied by means of fossil records to infer the location of full‐glacial refugia and the main routes of postglacial colonization. Here we use recently compiled fossil pollen data as a template to examine how past demographic events have influenced the species’ modern genetic diversity. Variation was assessed in the mitochondrial nad1 gene containing two minisatellite regions. Among the 369 populations (4876 trees) assayed, 28 mitochondrial variants were identified. The patterns of population subdivision superimposed on interpolated fossil pollen distributions indicate that survival in separate refugia and postglacial colonization has led to significant structuring of genetic variation in the southern range of the species. The populations in the northern range, on the other hand, showed a shallow genetic structure consistent with the fossil pollen data, suggesting that the vast northern range was colonized from a single refugium. Although the genetic diversity decreased away from the putative refugia, there were large differences between different colonization routes. In the Alps, the diversity decreased over short distances, probably as a result of population bottlenecks caused by the presence of competing tree species. In northern Europe, the diversity was maintained across large areas, corroborating fossil pollen data in suggesting that colonization took place at high population densities. The genetic diversity increased north of the Carpathians, probably as a result of admixture of expanding populations from two separate refugia.  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