全文获取类型
收费全文 | 5635篇 |
免费 | 503篇 |
国内免费 | 486篇 |
出版年
2024年 | 15篇 |
2023年 | 73篇 |
2022年 | 96篇 |
2021年 | 152篇 |
2020年 | 218篇 |
2019年 | 226篇 |
2018年 | 169篇 |
2017年 | 191篇 |
2016年 | 209篇 |
2015年 | 218篇 |
2014年 | 241篇 |
2013年 | 349篇 |
2012年 | 203篇 |
2011年 | 254篇 |
2010年 | 199篇 |
2009年 | 287篇 |
2008年 | 292篇 |
2007年 | 284篇 |
2006年 | 274篇 |
2005年 | 234篇 |
2004年 | 191篇 |
2003年 | 177篇 |
2002年 | 183篇 |
2001年 | 167篇 |
2000年 | 154篇 |
1999年 | 153篇 |
1998年 | 122篇 |
1997年 | 115篇 |
1996年 | 98篇 |
1995年 | 98篇 |
1994年 | 108篇 |
1993年 | 84篇 |
1992年 | 92篇 |
1991年 | 72篇 |
1990年 | 80篇 |
1989年 | 57篇 |
1988年 | 65篇 |
1987年 | 55篇 |
1986年 | 51篇 |
1985年 | 70篇 |
1984年 | 46篇 |
1983年 | 25篇 |
1982年 | 48篇 |
1981年 | 26篇 |
1980年 | 30篇 |
1979年 | 21篇 |
1978年 | 18篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1973年 | 6篇 |
排序方式: 共有6624条查询结果,搜索用时 15 毫秒
921.
Immunizations of New Zealand White rabbits with specific macrophage migration inhibitory factor (MIF) tick peptide (PEP) produced circulating anti-tick PEP antibodies in the hosts. Antibody titers of greater than 1:5000 to tick MIF peptide were observed for crude sera from PEP-immunized rabbits. PEP- and BSA-vaccinated rabbits were infested with Amblyomma americanum adults. Feeding intervals, female weights, egg masses and percent egg hatch were measured for ticks feeding on control and immunized hosts. Feeding intervals were significantly lengthened to 13.3 days for PEP-vaccinated hosts compared to BSA-vaccinated controls at 12.4 days, while female engorgement weights and egg masses were unchanged. By immunizing hosts using specific tick PEP, we were able to alter the length of time the ticks fed on their hosts. 相似文献
922.
The viable propagule banks of a temporary stream were studied from sections with different agricultural history. Hatching
of zooplankton (copepods, rotifers and cladocerans) was recorded in the laboratory under controlled temperature and light
conditions from an agriculturally modified area with average hydroperiods of about a week per year and two semi-natural reference
areas with average hydroperiods of more than 3 weeks per year. We found significant differences in both taxon richness and
abundance of zooplankton hatching between areas, which were lower in the agriculturally modified section, compared to the
reference sections. Another factor likely to have influenced hatching in our experiment was conductivity, which differed between
the two reference sections and might have affected hatching at high conductivities. For restoration purposes, hydrological
reconnection of stream segments is important to facilitate dispersal from the high diversity upstream segments to the depleted
sites downstream. 相似文献
923.
Habitat influences on urban avian assemblages 总被引:4,自引:0,他引:4
Urbanization is increasing across the globe and there is growing interest in urban ecology and a recognition that developed areas may be important for conservation. We review the factors influencing urban avian assemblages, focusing on habitat type and anthropogenic resource provision, and analyse data from a common bird monitoring scheme to assess some of these issues. The review suggests that (1) local factors are more important than regional ones in determining the species richness of urban avian assemblages, raising the potential for the management of urban sites to deliver conservation; (2) habitat fragmentation frequently influences urban avian assemblages, with the effects of patch size being greater than those of isolation, and (3) urban bird assemblages appear to respond positively to increasing the structural complexity, species richness of woody vegetation and supplementary feeding, and negatively to human disturbance. Data from Britain's Breeding Bird Survey, combined with habitat data obtained from aerial photographs, were used to assess a number of these issues at the resolution of 1‐km squares. Green‐space constituted 45% of these squares, and domestic gardens contributed 50% of this green‐space, though their contribution to large continuous patches of green‐space was negligible. There was no significant positive correlation between the densities of individual species in urban areas and surrounding rural areas. Rural species richness declined with increasing latitude, but urban species richness was not correlated with latitude. This contrast contributes to slightly higher avian species richness in rural squares in Southern England than urban ones. Occupancy and abundance were strongly positively correlated in urban avian assemblages, and some indicator species of conservation concern occurred in few urban areas and at low densities. Such species will require conservation action to be precisely targeted within urban areas. Of the urban indicators of conservation concern, only the House Sparrow Passer domesticus and Common Starling Sturnus vulgaris were more abundant in urban than rural areas. Moreover, the densities of these two species were strongly and positively correlated, indicating that they may be limited by shared resources, such as nest‐sites or supplementary food. There was little evidence that high densities of nest‐predating corvids were associated with reduced densities of their prey species. Species richness and the densities of individual species frequently declined with an increasing number of buildings. Current trends for the densification of many British urban areas are thus likely to be detrimental for many bird species. 相似文献
924.
MARTIJN VAN DE POL KEES OOSTERBEEK ANNE L. RUTTEN BRUNO J. ENS JOOST M. TINBERGEN & SIMON VERHULST 《Ibis》2009,151(1):171-180
Molecular sexing of birds has been possible for over a decade, but for practical reasons many studies still use biometric data for sex discrimination. In some species, the sexes are easy to distinguish but sexual dimorphism is often more subtle, requiring the use of statistical analyses of biometric measurements to discriminate sexes. These models are usually parameterized and validated using data from a limited number of sites and years. However, the resulting discriminant functions are often applied to other populations and periods. A crucial, but usually untested, assumption is that sexual dimorphism does not vary in time and space. Here we illustrate the consequences of violation of this assumption in Eurasian Oystercatchers Haematopus ostralegus , a species for which most studies have relied on biometric sexing. Using biometric data from captures of known-sex birds, we show that sexual dimorphism varied substantially in time and even reversed in some months and years. Furthermore, some biometric traits used in sexing changed gradually over time, causing a reduction in sexual dimorphism. We show that the consequences of this variation on sex discrimination in Oystercatchers are subtle and easily overlooked, but can result in inaccurate and strongly male- or female-biased sex-ratio estimates. We recommend that biometric sexing should be avoided in Oystercatchers unless specific calibration for each month, year and area is carried out. This recommendation also applies to other species where biometric traits may depend on environmental conditions. We argue that this condition might apply to many bird species and therefore advise caution when interpreting results based on biometric sex discrimination. 相似文献
925.
Effect of meal size on excess post-exercise oxygen consumption in fishes with different locomotive and digestive performance 总被引:1,自引:0,他引:1
Shi-Jian Fu Ling-Qing Zeng Xiu-Ming Li Xu Pang Zhen-Dong Cao Jiang-Lan Peng Yu-Xiang Wang 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2009,179(4):509-517
Effects of feeding on pre-exercise VO2 and excess post-exercise oxygen consumption (EPOC) after exhaustive exercise were investigated in sedentary southern catfish,
active herbivorous grass carp, omnivorous crucian carp, and sluggish omnivorous darkbarbel catfish to test whether feeding
had different effects on EPOC and to compare EPOC in fishes with different ecological habits. For fasting fish, the pre-exercise
and peak post-exercise VO2 were higher and recovery rates were faster in crucian carp and grass carp compared to those of darkbarbel catfish and southern
catfish. EPOC magnitudes of grass carp and southern catfish were significantly larger than those of crucian carp and darkbarbel
catfish. Feeding had no significant effect on peak post-exercise VO2, recovery rate, and EPOC magnitude in grass carp. Both the pre-exercise and peak post-exercise VO2 increased with meal size, while the EPOC magnitude and duration decreased significantly in the larger meal size groups of
crucian carp and southern catfish. In darkbarbel catfish, both the pre-exercise and peak post-exercise VO2 increased with meal size, but the VO2 increment elicited by exercise was larger in feeding groups compared with the fasting group. These results suggest that (1)
the characteristics of the post-exercise VO2 profile, such as peak post-exercise VO2 and recovery rate, were closely related to the activity of fishes, whereas the EPOC magnitude was not and (2) the effects
of feeding on EPOC were more closely related to the postprandial increase in VO2. 相似文献
926.
It has recently been suggested that the expression of parasite virulence depends on host population density, such that infected
hosts have a higher sensitivity to density, and thus reach their carrying capacity earlier than uninfected hosts. In this
scenario, parasite-induced reduction in fitness (i.e., virulence) increases with host density. We tested this hypothesis experimentally,
using outdoor mesocosm populations of Daphnia magna infected by the microsporidian Octosporea bayeri. Contrary to the prediction, virulence was independent of host density. In a competition experiment with initial prevalence
of 50%, O. bayeri reduced the competitive ability of infected Daphnia within the asexual growth phase independent of initial host population density. In an additional experiment we set up populations
with 100% and 0% prevalence and followed their population dynamics over the whole season. Consistent with the competition
experiment, we found no difference in population dynamics within the asexual growth phase of the host, suggesting that infected
hosts are not more sensitive to density than uninfected hosts. The additional experiment, however, included more than the
initial growth phase as did the competition experiment. Eventually, after 100 days, 100% infected populations assumed a reduced
carrying capacity compared to uninfected populations. We identify and discuss three reasons for the discrepancy between our
experiment and the predictions. 相似文献
927.
黄足黄守瓜取食南瓜诱导葫芦素B含量的变化规律 总被引:1,自引:0,他引:1
采用高效液相色谱分析方法研究黄足黄守瓜Aulacophora femoralis chinensisWeise取食子叶期南瓜诱导葫芦素B含量的变化规律。结果表明:黄足黄守瓜采用划圈的取食方式,主要是为了阻止植株合成新的葫芦素B进入圈内,以达到安全取食的目的;黄足黄守瓜对同株南瓜为害程度不同的子叶间,葫芦素B含量无显著差异,说明同一植株黄足黄守瓜取食化学诱导是系统性的;危害程度不同的不同植株间,葫芦素B含量有显著差异,并表现出受害阈值(被害1圈),在一定范围内,植株被害得越严重,诱导反应越强烈;试验证明南瓜被诱导合成葫芦素B与光照无关,只与植株被害有关。 相似文献
928.
Paul A. Rees 《Zoo biology》2009,28(2):79-97
Activity budgets were studied in eight Asian elephants (Elephas maximus) at Chester Zoo (UK) for 35 days, between January and November 1999. Recordings were made between 10:00 and 16:00 hr (with most behavior frequencies calculated between 10:00 and 14:00 hr). The elephants exhibited variation in activity depending on their age, sex, the time of day and the time of year. Only the five adult cows exhibited stereotypic behavior, with frequencies ranging from 3.9 to 29.4% of all observations. These elephants exhibited individual, diurnal and seasonal variation in stereotypic behavior. This has implications for studies that use short sampling periods and may make comparisons of data collected at different times of the day or year invalid. The six adult elephants spent 27.4–41.4% of the time feeding (between 10:00 and 14:00 hr), 22.9–42.0% standing still, 6.1–19.2% walking and 3.9–9.6% dusting. The hypothesis that the frequency of stereotypic behavior in adult cow elephants was negatively correlated with the frequency of feeding behavior was tested and was found to be true. Stereotypic behavior increased in frequency toward the end of the day—while waiting to return to the elephant house for food—and elephants spent more time stereotyping during the winter months than during the summer months. Elephants were inactive (i.e. exhibited behaviors other than locomotion) for between 70.1 and 93.9% of the time. Creating more opportunities for elephants to exhibit foraging behavior and the introduction of greater unpredictability into management regimes, especially feeding times, may reduce the frequency of stereotypic behavior and increase general activity levels. Zoo Biol 28:79–97, 2009. © 2008 Wiley-Liss, Inc. 相似文献
929.
Majd N. Aljamali Vijay G. Ramakrishnan Hua Weng James S. Tucker John R. Sauer Richard C. Essenberg 《Archives of insect biochemistry and physiology》2009,71(4):236-253
A collection of EST clones from female tick Amblyomma americanum salivary glands was hybridized to RNA from different feeding stages of female tick salivary glands and from unfed or feeding adult male ticks. In the female ticks, the expression patterns changed dramatically upon starting feeding, then changed again towards the end of feeding. On beginning feeding, genes possibly involved in survival on the host increased in expression as did many housekeeping genes. As feeding progressed, some of the survival genes were downregulated, while others were upregulated. When the tick went into the rapid feeding phase, many of the survival genes were downregulated, while a number of transport‐associated genes and genes possibly involved in organ degeneration increased. In the males, the presence of females during feeding made a small difference, but feeding made a larger difference. Males showed clear differences from females in expression, as well. Protein synthesis genes were expressed more in all male groups than in the partially fed females, while the putative secreted genes involved in avoiding host defenses were expressed less. © 2009 Wiley Periodicals, Inc. 相似文献
930.
Karen A. Stockin Vicky Binedell † Nicky Wiseman ‡ Dianne H. Brunton § Mark B. Orams # 《Marine Mammal Science》2009,25(2):283-301
Here we present the first data describing the behavior of common dolphins ( Delphinus sp.) in the Hauraki Gulf, New Zealand. Activity budgets are used to assess the effects of diel, season, depth, sea surface temperature, group size, and composition on dolphin behavior. Additionally, the presence/absence of Bryde's whale ( Balaenoptera brydei ) and Australasian gannet ( Morus serrator ) is examined in relation to dolphin behavior. Behavioral data were collected from 686 independent dolphin groups during boat-based surveys conducted between February 2002 and January 2005. Foraging (46.7%) and social (7.2%) were the most and least frequently observed behaviors, respectively. Travel (28.9%), mill (9.5%), and rest (7.7%) accounted for the remainder of the activity budget. Behavior varied seasonally, with the highest proportion of foraging and resting groups observed during the spring and autumn, respectively. Behavior also varied with water depth, with foraging animals observed in the deepest and resting groups recorded in the shallowest regions of the Hauraki Gulf. A correlation between group size and behavior was evident, although behavior did not vary with the composition of dolphin groups. Resting, milling, and socializing animals were more frequently observed in smaller group sizes. Foraging behavior was prevalent in both small and large group sizes, suggesting foraging plasticity exists within this population. Behavior differed between single- and multispecies groups, with foraging more frequent in multispecies groups. Resting, milling, or socializing was rarely observed in the presence of any associated species, indicating the primary mechanism for association is likely prey related. 相似文献