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   2024篇
  免费   33篇
  2017年   11篇
  2016年   10篇
  2015年   14篇
  2014年   12篇
  2013年   48篇
  2012年   65篇
  2011年   73篇
  2010年   139篇
  2009年   136篇
  2008年   114篇
  2007年   156篇
  2006年   86篇
  2005年   94篇
  2004年   44篇
  2003年   26篇
  2002年   19篇
  2001年   18篇
  2000年   30篇
  1999年   33篇
  1998年   30篇
  1997年   56篇
  1996年   66篇
  1995年   28篇
  1994年   36篇
  1993年   42篇
  1992年   47篇
  1991年   40篇
  1990年   42篇
  1989年   40篇
  1988年   30篇
  1987年   38篇
  1986年   18篇
  1985年   32篇
  1984年   30篇
  1983年   25篇
  1982年   27篇
  1981年   26篇
  1980年   20篇
  1979年   23篇
  1978年   28篇
  1977年   15篇
  1976年   18篇
  1975年   16篇
  1974年   16篇
  1973年   13篇
  1972年   24篇
  1971年   26篇
  1969年   6篇
  1957年   13篇
  1950年   5篇
排序方式: 共有2057条查询结果,搜索用时 15 毫秒
181.
Thirty‐two polymorphic microsatellite loci (29 novel, 3 cross‐species) are characterized in the creek chub, Semotilus atromaculatus, a widespread and abundant North American minnow. Novel repeat‐containing sequences were isolated using a standard microsatellite enrichment procedure. A set of di‐, tri‐, and tetranucleotide markers with allele numbers ranging from two 16 are now available for estimating relatedness within and among populations in this ecologically important species.  相似文献   
182.
Arthropods (mainly insects) were collected from a forest site that contained at least six species of Ni hyperaccumulators. Whole body Ni analysis was performed for 12 arthropod taxa, two of which were studied at different life cycle stages. We found two Nitolerant insects. The pentatomid heteropteran Utana viridipuncta, feeding on fruits of the Ni hyperaccumulator Hybanthus austrocaledonicus, contained a mean of 2 600 μNi/g in nymphs and 750 μNi/g in adults. The tephritid fly Bactrocera psidii, feeding on pulp of Sebertia acuminata fruits that contained 6 900 μNi/g, contained 420 μNi/g as larvae that had evacuated their guts and significantly less (65 μNi/g) as adults. European honeybees (Apis mellifera) visiting flowers of the Ni hyperaccumulator H. austrocaledonicus contained significantly more Ni (8‐fold more) than those collected from flowers of Myodocarpus fraxinifolius, a non‐hyperaccumulator. Our results show that some insects feed on Ni hyperaccumulator plants and that their feeding mobilizes Ni into local food webs.  相似文献   
183.
Solitary and gregarious locusts differ in many traits, such as body color, morphometrics and behavior. With respect to behavior, solitary animals shun each other, while gregarious animals seek each other's company, hence their crowding behavior. General activity, depending on the temperature, occurs throughout the day but is much lower in solitary locusts. Solitary locusts occasionally fly by night while gregarious locusts fly regularly during daytime (swarming). In search of new assays to identify substances that control or modify aspects of (phase) behavior, we designed a simple activity assay, meant to complement existing behavioral measurement tools. The general activity is reflected in the number of wall hits, that is, the number of contacts between the locust and the vertical walls of a small arena. Using this single parameter we were able to quantify differences in total activity of both nymphs and adults of isolation-reared (solitary), regrouped- and crowdreared (gregarious) locusts under different conditions. Furthermore, we demonstrate that there are inter- and intra-phase dependent differences in activities of 5th instar nymphs afar injections of the three different adipokinetic hormones.  相似文献   
184.
The monk parakeet (Myiopsitta monachus) is a successful invasive species that does not exhibit life history traits typically associated with colonizing species (e.g., high reproductive rate or long‐distance dispersal capacity). To investigate this apparent paradox, we examined individual and population genetic patterns of microsatellite loci at one native and two invasive sites. More specifically, we aimed at evaluating the role of propagule pressure, sexual monogamy and long‐distance dispersal in monk parakeet invasion success. Our results indicate little loss of genetic variation at invasive sites relative to the native site. We also found strong evidence for sexual monogamy from patterns of relatedness within sites, and no definite cases of extra‐pair paternity in either the native site sample or the examined invasive site. Taken together, these patterns directly and indirectly suggest that high propagule pressure has contributed to monk parakeet invasion success. In addition, we found evidence for frequent long‐distance dispersal at an invasive site (~100 km) that sharply contrasted with previous estimates of smaller dispersal distance made in the native range (~2 km), suggesting long‐range dispersal also contributes to the species’ spread within the United States. Overall, these results add to a growing body of literature pointing to the important role of propagule pressure in determining, and thus predicting, invasion success, especially for species whose life history traits are not typically associated with invasiveness.  相似文献   
185.
Geographic range differences among species may result from differences in their physiological tolerances. In the intertidal zone, marine and terrestrial environments intersect to create a unique habitat, across which physiological tolerance strongly influences range. Traits to cope with environmental extremes are particularly important here because many species live near their physiological limits and environmental gradients can be steep. The snail Melampus bidentatus occurs in coastal salt marshes in the western Atlantic and the Gulf of Mexico. We used sequence data from one mitochondrial (COI) and two nuclear markers (histone H3 and a mitochondrial carrier protein, MCP) to identify three cryptic species within this broad‐ranging nominal species, two of which have partially overlapping geographic ranges. High genetic diversity, low population structure, and high levels of migration within these two overlapping species suggest that historical range limitations do not entirely explain their different ranges. To identify microhabitat differences between these two species, we modelled their distributions using data from both marine and terrestrial environments. Although temperature was the largest factor setting range limits, other environmental components explained features of the ranges that temperature alone could not. In particular, the interaction of precipitation and salinity likely sets physiological limits that lead to range differences between these two cryptic species. This suggests that the response to climatic change in these snails will be mediated by changes to multiple environmental factors, and not just to temperature alone.  相似文献   
186.
Abstract: Habitat selection among vertebrates entails decision making at a number of spatial scales. An understanding of factors influencing decisions at each of these scales is required for the effective management of wildlife populations. This study investigates the foraging ecology of a population of Glossy Black‐cockatoos in central New South Wales. We took advantage of the characteristic feeding sign produced by Glossy Black‐cockatoos to examine factors influencing habitat selection at multiple spatial scales. Birds preferred to forage at sites where food was abundant and avoided open sites where the predation risk may be greater. Their two food species, Allocasuarina diminuta and Allocasuarina gymnanthera, differed in profitability (kernel intake rate as measured by the ratio of seed weight to total seed and cone weight), as did trees within a species. Both species were utilized extensively, although foraging intensity was greater at sites where the more profitable species was present. In order to maximize their food intake, birds selected individual trees on the basis of cone abundance and profitability. Cones produced in the previous year were preferred.  相似文献   
187.
  • 1 Habitats associated with humans, and gardens in particular, may prove to be important for populations of certain mammal species. However, means for measuring change in these populations are lacking.
  • 2 This paper uses power analyses to examine the potential for mammal observation data, gathered through an extensive volunteer‐based survey of garden birds, to provide measures of population change within garden habitats at both national and regional levels.
  • 3 Analyses show that this survey protocol has sufficient power to detect a decline in presence of between 5% and 40% at the national level for 20 of the 24 species included in this study. These results demonstrate that data from volunteer surveys allow the annual monitoring of garden use by a range of mammal species within Britain, and highlight the wider potential of such schemes worldwide.
  相似文献   
188.
189.
Pyraloidea, one of the largest superfamilies of Lepidoptera, comprise more than 15 684 described species worldwide, including important pests, biological control agents and experimental models. Understanding of pyraloid phylogeny, the basis for a predictive classification, is currently provisional. We present the most detailed molecular estimate of relationships to date across the subfamilies of Pyraloidea, and assess its concordance with previous morphology‐based hypotheses. We sequenced up to five nuclear genes, totalling 6633 bp, in each of 42 pyraloids spanning both families and 18 of the 21 subfamilies, plus up to 14 additional genes, for a total of 14 826 bp, in 21 of those pyraloids plus all 24 outgroups. Maximum likelihood analyses yield trees that, within Pyraloidea, differ little among datasets and character treatments and are strongly supported at all levels of divergence (83% of nodes with bootstrap ≥80%). Subfamily relationships within Pyralidae, all very strongly supported (>90% bootstrap), differ only slightly from a previous morphological analysis, and can be summarized as Galleriinae + Chrysauginae (Phycitinae (Pyralinae + Epipaschiinae)). The main remaining uncertainty involves Chrysauginae, of which the poorly studied Australian genera may constitute the basal elements of Galleriinae + Chrysauginae or even of Pyralidae. In Crambidae the molecular phylogeny is also strongly supported, but conflicts with most previous hypotheses. Among the newly proposed groupings are a ‘wet‐habitat clade’ comprising Acentropinae + Schoenobiinae + Midilinae, and a provisional ‘mustard oil clade’ containing Glaphyriinae, Evergestinae and Noordinae, in which the majority of described larvae feed on Brassicales. Within this clade a previous synonymy of Dichogaminae with the Glaphyriinae is supported. Evergestinae syn. n. and Noordinae syn. n. are here newly synonymized with Glaphyriinae, which appear to be paraphyletic with respect to both. Pyraustinae and Spilomelinae as sampled here are each monophyletic but form a sister group pair. Wurthiinae n. syn. , comprising the single genus Niphopyralis Hampson, which lives in ant nests, are closely related to, apparently subordinate within, and here newly synonymized with, Spilomelinae syn. n.  相似文献   
190.
Male fiddler crabs are commonly recognized by the presence of a single massive claw used in a variety of contexts, including territorial defence, agonistic interactions, and courtship behaviour. The most common behavioural context involving these enlarged chelipeds is their use in waving displays, which are remarkably diverse among species. Although the waving display is one of the most obvious behavioural features of male fiddler crabs, little is known about their main evolutionary trends during the diversification of the genus. The present study employed phylogenetic comparative methods to investigate the evolution of waving behaviour in a sample of 19 species of Uca from Central and North America. Digital recordings were used to quantify the temporal dynamics of waving behaviour in each species. Multivariate ordination methods were used to assess whether different elements of the display showed distinct evolutionary dynamics, particularly with respect to body size and the environment where species are most commonly found. Most of the interspecific variation in displays involves differences in the overall waving velocity, with no correspondence to their local environments, nor their body size. Interestingly, despite the strong concentration of variance in the first two ordination axes, there was no statistically significant evidence for phylogenetic signals in their respective scores. These results suggest that the overall structure of waving displays is evolutionarily labile, at the same time as being concentrated in a few particular axes of variation, possibly indicating evolution along lines of least resistance. The approach employed in the present study highlights the utility of phylogenetic comparative methods for elucidating the evolution of complex behavioural characteristics, such as the waving display in male fiddler crabs. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 307–315.  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