全文获取类型
收费全文 | 354篇 |
免费 | 23篇 |
出版年
2021年 | 4篇 |
2019年 | 6篇 |
2018年 | 9篇 |
2017年 | 10篇 |
2016年 | 11篇 |
2015年 | 15篇 |
2014年 | 14篇 |
2013年 | 19篇 |
2012年 | 19篇 |
2011年 | 18篇 |
2010年 | 16篇 |
2009年 | 10篇 |
2008年 | 15篇 |
2007年 | 16篇 |
2006年 | 19篇 |
2005年 | 18篇 |
2004年 | 12篇 |
2003年 | 6篇 |
2002年 | 9篇 |
2001年 | 12篇 |
2000年 | 14篇 |
1999年 | 6篇 |
1998年 | 7篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 6篇 |
1991年 | 3篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1969年 | 2篇 |
1968年 | 1篇 |
排序方式: 共有377条查询结果,搜索用时 137 毫秒
1.
Theoretical models of species coexistence between desert mammals have generally been based on a combination of food and microhabitat selection by granivorous rodents. Although these models are applicable in various deserts of the world, they cannot explain resource use by mammals in Neotropical deserts. The present study examines diet composition in a mammal assemblage in the Monte desert, Argentina. The results show that two main strategies are used by these mammals: medium‐sized species (hystricognath rodents: Dolichotis patagonum, Lagostomus maximus, Microcavia australis and Galea musteloides; and an exotic lagomorph: Lepus europaeus) are herbivores, whereas small‐sized species (a marsupial: Thylamys pusillus; and sigmodontine rodents: Graomys griseoflavus, Akodon molinae, Calomys musculinus, Eligmodontia typus) are omnivorous. Small mammals also show a tendency towards granivory (C. musculinus), insectivory (A. molinae and T. pusillus) and folivory (G. griseoflavus). 相似文献
2.
A simple procedure for studying the microanatomy of the kidney by light microscopy is described. Specimens are dried by the critical-point method and later impregnated with a mounting medium. The retention of air within the lumen of nephrons and collecting tubules permits their visualization. The method allows the observation of renal microanatomy without the distortions produced by microdissection or the loss of structural relationships among different kidney components. It also permits observation of the same specimen by both light and scanning electron microscopy. 相似文献
3.
Body proportions,microhabitat selection,and adaptive radiation of Liolaemus lizards in central Chile
Summary A biometric analysis of body proportions with presumably functional meaning for microhabitat selection was made on 12 species of Liolaemus lizards in central Chile. Characters studied were forelimb length, hindlimb length, tail length (all standardized by the corresponding snout-vent length), and the ratio forelimb/hindlimb length. It is shown that irrespective of terrestrial, saxicolous, or arboreal habits, Liolaemus species are remarkably similar in body proportions. The only exceptions are: L. lemniscatus, an open ground-dweller which exhibits significantly shorter limbs; and L. chiliensis and L. schroederi, both shrub-climbers which exhibit significantly longer tail. It is concluded that the adaptive radiation of Liolaemus lizards in central Chile has been accomplished mainly by diversification of activity time, food size, and microhabitat type. Morphological divergence in body proportions seems to have played an unimportant role. 相似文献
4.
Abstract: The dynamics of newly established elk (Cervus elaphus) populations can provide insights about maximum sustainable rates of reproduction, survival, and increase. However, data used to estimate rates of increase typically have been limited to counts and rarely have included complementary estimates of vital rates. Complexities of population dynamics cannot be understood without considering population processes as well as population states. We estimated pregnancy rates, survival rates, age ratios, and sex ratios for reintroduced elk at Theodore Roosevelt National Park, North Dakota, USA; combined vital rates in a population projection model; and compared model projections with observed elk numbers and population ratios. Pregnancy rates in January (early in the second trimester of pregnancy) averaged 54.1% (SE = 5.4%) for subadults and 91.0% (SE = 1.7%) for adults, and 91.6% of pregnancies resulted in recruitment at 8 months. Annual survival rates of adult females averaged 0.96 (95% CI = 0.94-0.98) with hunting included and 0.99 (95% CI = 0.97-0.99) with hunting excluded from calculations. Our fitted model explained 99.8% of past variation in population estimates and represents a useful new tool for short-term management planning. Although we found no evidence of temporal variation in vital rates, variation in population composition caused substantial variation in projected rates of increase (Λ = 1.20-1.36). Restoring documented hunter harvests and removals of elk by the National Park Service led to a potential rate of Λ = 1.26. Greater rates of increase substantiated elsewhere were within the expected range of chance variation, given our model and estimates of vital rates. Rates of increase realized by small elk populations are too variable to support inferences about habitat quality or density dependence. 相似文献
5.
Recurrent wildfires constitute a major selecting force in shaping the structure of plant communities. At the regional scale, fire favours phenotypic and phylogenetic clustering in Mediterranean woody plant communities. Nevertheless, the incidence of fire within a fire-prone region may present strong variations at the local, landscape scale. This study tests the prediction that woody communities on acid, nutrient-poor soils should exhibit more pronounced phenotypic and phylogenetic clustering patterns than woody communities on fertile soils, as a consequence of their higher flammability and, hence, presumably higher propensity to recurrent fire. Results confirm the predictions and show that habitat filtering driven by fire may be detected even in local communities from an already fire-filtered regional flora. They also provide a new perspective from which to consider a preponderant role of fire as a key evolutionary force in acid, infertile Mediterranean heathlands. 相似文献
6.
The resistance to experimental, highly frequent disturbance has been analysed in three congeneric, strong-resprouter species
(Erica australis, E. scoparia and E. arborea) that co-occur in heath-dominated communities of the northern side of the Strait of Gibraltar, southern Spain. To do so,
mature individuals of the three species from a long undisturbed location were clipped at the ground level every sixth month
during two years. The relationship between the resprouted biomass dry weight (as indicative of the resprouting vigour) and
the upper surface area of the lignotuber along the experiment was established separately for each species at each clipping
event by means of linear regressions analysis. The resprouting vigour of the three species was compared by means of independent
one-way ANOVAs within each clipping event. Resprouting vigour decreased after recurrent clippings in the three species. Nevertheless,
significant differences between species in this loss of resprouting vigour were detected, being E. scoparia the most resistant to the experimental, highly frequent clipping. It is concluded that experimental levels of recurrent disturbance
may help to find out differences in resilience within similar (taxonomically, morfologically and/or ecologically), strong-resprouter
plant species. Considering the history of forestry management in the nothern side of the Strait of Gibraltar, differences
in this regard between the three Erica species may contribute to explain their somewhat segregated ecological distribution in this region. 相似文献
7.
R. D. Robarts T. Zohary F. Ojeda C. C.E. Lovengreen R. D. Walmsley 《African Journal of Aquatic Science》2013,38(2):78-81
SUMMARY The spectral composition of the underwater light field was examined in two hypertrophic South African Impoundments (Hartbeespoort and Roodeplaat Dams) under a range of inorganic turbidities and chlorophyll α concentrations. The data indicated that inorganic turbidity and gilvin were dominant to chlorophyll in regulating underwater light attenuation during the present study. Under all conditions the wavelengths between 405 and 510 nm were attenuated more rapidly than near UV and the wavelengths above 510 nm and the 623 nm component penetrated deepest. Under low turbidities the 546 nm wavelength was the next most penetrating component, but its attenuation increased with increasing turbidity. This characteristic of the underwater light field may be important to the cyanobacteria which dominate in these hypertrophic lakes. 相似文献
8.
Ana Carolina Santacruz‐García Sandra Bravo Florencia del Corro Fernando Ojeda 《Austral ecology》2019,44(8):1416-1429
Recent changes to fire regimes in many regions of the world have led to renewed interest in plant flammability experiments to understand and predict the consequences of such changes. These experiments require the development of practical and standardised flammability testing protocols. The research aims were (i) to compare plant flammability assessments carried out using two different approaches, namely functional trait analysis and testing with a shoot‐level device; and (ii) to evaluate the effect of disturbances and seasonal variability on flammability. The study area was located in the Western Chaco region, Argentina, and 11 species were selected based on their representativeness in forests. We studied six functional traits related to flammability, growth habit and foliar persistence, in forests without disturbances over the three last decades as well as in disturbed forests. The seasonal variation of these functional traits was evaluated over two consecutive years. Functional trait flammability index (FI) and shoot‐level measurements followed standard protocols. Sixty per cent of the species measured presented a high to very high FI. The results of both assessment methods were significantly correlated. Both methods identified the same species as having medium flammability, but differed in regards to the most flammable species. Senegalia gilliesii was identified as the most flammable species when using functional trait analysis, whereas shoot‐level assessments found Larrea divaricata and Schinus johnstonii to be the most flammable. There were no disturbance effects on the FI but there was seasonal variation. Our results validate the use of functional traits as a predictive method of flammability testing and represent the first global effort comparing flammability obtained through functional trait analysis with empirical measurements. The significant correlation between both methods allows the selection of the one that is more appropriate for the size of the area to be evaluated and for the availability of technical resources. Abstract in Spanish is available with online material. 相似文献
9.
David C Zappulla Arindel SR Maharaj Jessica J Connelly Rebecca A Jockusch Rolf Sternglanz 《BMC molecular biology》2006,7(1):40-12
Background
By screening a plasmid library for proteins that could cause silencing when targeted to the HMR locus in Saccharomyces cerevisiae, we previously reported the identification of Rtt107/Esc4 based on its ability to establish silent chromatin. In this study we aimed to determine the mechanism of Rtt107/Esc4 targeted silencing and also learn more about its biological functions. 相似文献10.
Miriam YH Ueda Paulo G Alvarenga Juliana M Real Eloisa de Sá Moreira Aripuan? Watanabe Ana Maria Passos-Castilho Matheus Vescovi Yana Novis Vanderson Rocha Adriana Seber Jose SR Oliveira Celso A Rodrigues Celso FH Granato 《Memórias do Instituto Oswaldo Cruz》2015,110(4):461-467
Human herpesvirus 6 (HHV-6) may cause severe complications after haematopoietic stem
cell transplantation (HSCT). Monitoring this virus and providing precise, rapid and
early diagnosis of related clinical diseases, constitute essential measures to
improve outcomes. A prospective survey on the incidence and clinical features of
HHV-6 infections after HSCT has not yet been conducted in Brazilian patients and the
impact of this infection on HSCT outcome remains unclear. A rapid test based on
real-time quantitative polymerase chain reaction (qPCR) has been optimised to screen
and quantify clinical samples for HHV-6. The detection step was based on reaction
with TaqMan® hydrolysis probes. A set of previously described primers and
probes have been tested to evaluate efficiency, sensitivity and reproducibility. The
target efficiency range was 91.4% with linearity ranging from 10-106
copies/reaction and a limit of detection of five copies/reaction or 250 copies/mL of
plasma. The qPCR assay developed in the present study was simple, rapid and
sensitive, allowing the detection of a wide range of HHV-6 loads. In conclusion, this
test may be useful as a practical tool to help elucidate the clinical relevance of
HHV-6 infection and reactivation in different scenarios and to determine the need for
surveillance. 相似文献