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   259篇
  免费   14篇
  国内免费   4篇
  2023年   1篇
  2022年   6篇
  2021年   20篇
  2020年   13篇
  2019年   14篇
  2018年   7篇
  2017年   5篇
  2016年   8篇
  2015年   6篇
  2014年   15篇
  2013年   37篇
  2012年   14篇
  2011年   11篇
  2010年   11篇
  2009年   13篇
  2008年   11篇
  2007年   8篇
  2006年   6篇
  2005年   12篇
  2004年   3篇
  2003年   7篇
  2002年   6篇
  2001年   3篇
  2000年   4篇
  1999年   4篇
  1998年   5篇
  1996年   1篇
  1994年   5篇
  1993年   2篇
  1991年   1篇
  1989年   3篇
  1988年   2篇
  1985年   3篇
  1984年   1篇
  1981年   1篇
  1979年   1篇
  1978年   2篇
  1977年   1篇
  1976年   1篇
  1975年   1篇
  1972年   2篇
排序方式: 共有277条查询结果,搜索用时 21 毫秒
1.
The primary structures of the alpha chains in hemoglobins from three stocks of mice with theHba w2,Hba w3, andHba w4 haplotypes were determined to establish whether the tentative alpha-chain assignments based on the results of isoelectric focusing patterns were correct. TheseHba haplotypes were identified in laboratory descendants of feral mice captured in different parts of the world. Hemoglobin from Centreville, Maryland,Mus musculus domesticus (Hba w2) contains equal amounts of alpha chains 1 and 3. Hemoglobin from CzechMus musculus musculus (Hba w4) contains equal amounts of alpha chains 3 and 4. Amino acid analysis of the alpha-globins of Skive DanishMus musculus musculus (Hba w3) establishes that its hemoglobin is comprised of about one-third alpha chain 2 as expected plus a greater amount of a unique alpha chain that has not been described previously. This unique alpha chain has glycine at position 25, isoleucine at position 62, and serine at position 68; it is called chain 7. It may represent an intermediate in the evolution of genes that code for chain 2 (which has glycine, valine, and serine at positions 25, 62, and 68, respectively) and chain 4 (which has valine, isoleucine, and serine at positions 25, 62, and 62, respectively).This research was sponsored jointly by the National Institutes of Environmental Health Sciences under Contract 1-ES-55078 and by the Office of Health and Environmental Research, U.S. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.  相似文献   
2.
Summary Quantification of specific allergens in household dust samples may provide important information for selecting appropriate allergen control methods, and monitoring efficacy and compliance. The purpose of this study was to investigate the source of variation in mite and cat allergen measurements associated with dust sample collection. Discrete and composite dust samples were collected on a filter using a special vacuum sampling device. Aqueous extracts of the dust samples were prepared thenDer p I,Der f I, andFel d I were quantitated by enzyme immunoassays (EIA). Mite and cat allergens were frequently detected in dust samples from human dwellings, and the amounts of these allergens varied significantly (p<0.01) among dwellings. The differences of allergen measurements among duplicate samples taken immediately and up to three weeks later appear to be much smaller than differences among houses and between rooms. Variation among dust samples taken from living rooms and bedrooms of the same dwelling suggest differences in allergen reservoirs. Composite samples formed by sampling specific objects within a room may provide a reliable estimate of allergen exposure in that room. Dust samples from discrete objects are useful to find and monitor specific reservoirs of mite and cat allergens.  相似文献   
3.
Studies on feral cat diet offer important ecological information and are the first step towards determining their impact upon endangered species. However, in comparing seasonal changes in diet with seasonal prey availability, the scarce amount of research into oceanic islands worldwide must be considered when deciding if a specific population is actually affected by cat predation. Cat diet was analysed on Santa Luzia (Cabo Verde Islands) since this invasive predator is considered one of the main threats to native endangered species that require conservation measures. These previous studies were carried out in different seasons, providing contrasting results, skinks being more preyed upon in the rainy season and mice in the driest periods. To check these different results, we focussed on how cat diet varied seasonally in response to changes in prey abundance. Saurians were the most important prey group, followed by mice, invertebrates and birds. No seasonal differences were, however, observed in the different prey groups consumed, saurians being the main prey in both seasons. All cases reflected their respective abundances. Results corroborate the generalist and opportunistic trophic ecology of feral cats, providing important information to assess their impact on prey populations and design future eradication programmes.  相似文献   
4.
Free-roaming equids (i.e., feral horses [Equus caballus] and burros [Equus asinus]) are widely distributed and locally abundant across the rangelands of the western United States. The 1971 Wild Free Roaming Horse and Burro Act (WFRHBA) gave the Bureau of Land Management (BLM) and United States Forest Service (USFS) the legal authority to manage these animals on designated public lands. To fulfill this responsibility, federal agencies established an Appropriate Management Level (AML), defined as the number of horses or burros that can be sustained on a given management unit under prevailing environmental conditions and land uses. Although the WFRHBA specifies that feral equids must be managed in ecological balance with other land uses, including conservation of native wildlife, population control measures such as gathers, contraception, and adoptions have failed to keep pace with intrinsic growth rates. Over 80% of federally managed herds currently exceed prescribed population levels, making the potential for competition between native ungulates and feral equids a growing concern among state wildlife agencies. Mule deer (Odocoileus hemionus), pronghorn (Antilocapra americana), elk (Cervus canadensis), and bighorn sheep (Ovis canadensis) are of ecological and economic value to the states where they occur, and all exhibit some degree of distributional, habitat, or dietary overlap with horses or burros. Notwithstanding the scale of the problem, to date there have been no range-wide assessments of competition potential among native and feral ungulates for space, forage, or water. To address this need, we compiled demographic, jurisdictional, and species occurrence data collected from 2010–2019 by federal and state agencies. We used these data to map the distributions of 4 native ungulate species across federal equid management units (FEMUS) in 10 western states (n = 174). We then made within-state rankings of the 50 units that were ≥2 times over AML and encompassed ≥3 native ungulates. Collectively, FEMUs covered approximately 225,000 km2, representing 18% of all BLM and USFS lands in affected states. Each FEMU supported ≥1 native ungulate and 14% contained all 4. The degree of overlap between native and feral species varied by state, ranging from <1% for mule deer in Montana, to 40% for bighorn sheep in Nevada. Oregon had the largest proportion of units that supported all 4 native ungulates (58%), whereas Montana and New Mexico had the fewest equids, but all populations were over target densities. Despite the perception that the problem of equid abundance is limited to the Great Basin states, high intrinsic growth rates and social constraints on management practices suggest all affected states should monitor range conditions and native ungulate demography in areas where forage and water resources are limited and expanding equid populations are a concern. © 2021 The Wildlife Society.  相似文献   
5.
Invasive wild pigs (Sus scrofa), also called feral swine or wild hogs, are recognized as among the most destructive invasive species in the world. Throughout the United States, invasive wild pigs have expanded rapidly over the past 40 years with populations now established in 38 states. Of the estimated 6.9 million wild pigs distributed throughout the United States, Texas supports approximately 40% of the population and similarly bears disproportionate ecological and economic costs. Genetic analyses are an effective tool for understanding invasion pathways and tracking dispersal of invasive species such as wild pigs and have been used recently in California and Florida, USA, which have similarly long-established populations and high densities of wild pigs. Our goals were to use molecular approaches to elucidate invasion and migration processes shaping wild pig populations throughout Texas, compare our results with patterns of genetic structure observed in California and Florida, and provide insights for effective management of this invasive species. We used a high-density single nucleotide polymorphism (SNP) array to evaluate population genetic structure. Genetic clusters of wild pigs throughout Texas demonstrate 2 distinct patterns: weakly resolved, spatially dispersed clusters and well-resolved, spatially localized clusters. The disparity in patterns of genetic structure suggests disparate processes are differentially shaping wild pig populations in various localities throughout the state. Our results differed from the patterns of genetic structure observed in California and Florida, which were characterized by localized genetic clusters. These differences suggest distinct biological and perhaps anthropogenic processes are shaping genetic structure in Texas. Further, these disparities demonstrate the need for location-specific management strategies for controlling wild pig populations and mitigating associated ecological and economic costs. © 2021 The Wildlife Society. This article has been contributed to by US Government employees and their work is in the public domain in the USA.  相似文献   
6.
The objective of this study was to evaluate the effects of diet on the feline stress response by measuring plasma and urinary cortisol. A study diet was developed with a unique combination of nutrients that supports the management of stressful situations. The specific formulation of the diet included alpha-casozepine, which is believed to have an anxiolytic effect, and tryptophan supplementation. Tryptophan is the precursor for the synthesis of the neurotransmitter serotonin. Twenty-one indoor cats were fed with the study diet (n = 10) or a control diet (n = 11) for 8 weeks, after which physiological responses were evaluated. The study diet significantly increased the ratio of plasma tryptophan to large neutral amino acids and decreased urinary cortisol concentrations after being consumed daily for 8 weeks, but there was no effect on plasma cortisol levels following a stressful event (veterinary examination and blood draw). Further studies, such as behavioral analyses, are needed to clarify the effects of the study diet.  相似文献   
7.
Feral cat control using aerial broadcasting of toxic baits continues to be used in the rangelands of Western Australia. The effectiveness of these operations has sometimes been compromised by different environmental factors that affect prey and cat numbers. This study demonstrates that the ratio of cats to their preferred prey (small mammals) can be used to predict the most effective time to bait. The regular baiting of three conservation sites offered an opportunity to study the relationship between feral cat abundance, the abundance of their prey and ingestion of toxic baits. Peron Peninsula on the mid‐west coast, Lorna Glen station in the northern Goldfields and the central Gibson Desert of Western Australia are sites where cat control using toxic baits has been routinely applied over the last 15 years. We postulated that bait ingestion by cats was linked to the availability of live prey. Small mammal abundance (capture rates in pit‐fall traps) and relative cat abundance (based on daily track counts) were assessed at these sites and the data used to produce a predator‐prey ratio index (PPRI). We used generalised linear mixed models to test the effect of prey abundance, prebaiting cat abundance and PPRI on baiting efficacy (BE). The best model for predicting efficacy of baiting contained only PPRI. This simple model was able to predict baiting success over the entire range of outcomes, from highly successful ( >75% cat reduction) to unsuccessful (0% cat reduction). The ability to predict feral cat BE in advance of planned toxic baiting operations will provide a valuable tool for wildlife managers involved in cat control.  相似文献   
8.
In North America, wild pigs (Sus scrofa; feral pigs, feral swine, wild boars) are a widespread exotic species capable of creating large-scale biotic and abiotic landscape perturbations. Quantification of wild pig environmental effects has been particularly problematic in northern climates, where they occur only recently as localized populations at low densities. Between 2016 and 2017, we assessed short-term (within ~2 yrs of disturbance) effects of a low-density wild pig population on forest features in the central Lower Peninsula of Michigan, USA. We identified 16 8-ha sites using global positioning system locations from 7 radio-collared wild pigs for sampling. Within each site, we conducted fine-scale assessments at 81 plots and quantified potential disturbance by wild pigs. We defined disturbance as exposure of overturned soil, often resulting from rooting behavior by wild pigs. We quantified ground cover of plants within paired 1-m2 frames at each plot, determined effects to tree regeneration using point-centered quarter sampling, and collected soil cores from each plot. We observed less percent ground cover of native herbaceous plants and lower species diversity, particularly for plants with a coefficient of conservatism ≥5, in plots disturbed by wild pigs. We did not observe an increase in colonization of exotic plants following disturbance, though the observed prevalence of exotic plants was low. Wild pigs did not select for tree species when rooting, and we did not detect any differences in regeneration of light- and heavy-seeded tree species between disturbed or undisturbed plots. Magnesium and ammonium content in soils were lower in disturbed plots, suggesting soil disturbance accelerated leaching of macronutrients, potentially altering nitrogen transformation. Our study suggested that disturbances by wild pigs, even at low densities, alters short-term native herbaceous plant diversity and soil chemistry. Thus, small-scale exclusion of wild pigs from vulnerable and rare plant communities may be warranted. © 2020 The Wildlife Society.  相似文献   
9.
Wild pigs (Sus scrofa; i.e., feral hogs, feral swine) are considered an invasive species in the United States. Where they occur, they damage agricultural crops and wildlife habitat. Wild pigs also depredate native wildlife, particularly ground-nesting bird species during nesting season. In areas inhabited by wild turkeys (Meleagris gallopavo), nest destruction caused by wild pigs may affect recruitment. There is debate whether wild pigs actively seek ground-nesting bird nests or depredate them opportunistically. To address this debate, in 2016 we examined the movements of wild pigs relative to artificial wild turkey nests (i.e., control [no artificial nests], moderate density [12.5–25 nests/km2], and high density [25–50 nests/km2]) throughout the nesting season (i.e., early, peak, and late) in south-central Texas, USA. We found no evidence that wild pigs learned to seek and depredate wild turkey nests relative to nest density or nesting periods. Despite wild pigs being important nest predators, depredation was not a functional response to a pulsed food resource and can only be associated with overlapping densities of wild pigs and nests. Protecting reproductive success of wild turkeys will require reducing wild pig densities in nesting habitat prior to nesting season. © 2019 The Wildlife Society.  相似文献   
10.
Eduardo Kohn 《Anthrozo?s》2013,26(2):203-206
ABSTRACT

Although the occurrence of cat-caretaking of free-roaming cats is widespread, particularly so in countries with a climate suitable for cats to reproduce year-round, our knowledge of this relationship is still incomplete. People who engage in daily activities of feeding and caring for groups of free-roaming cats (cat caretakers) are known to be devoted to their cats and invest considerable resources in their care, including neutering and veterinary care. These caretakers often encounter difficulties, such as resentment by neighbors and lack of cooperation or financing by the municipal veterinary services. Despite the fundamental understanding of these caretakers' high daily commitment, and sometimes strong bond with the cats, detailed knowledge is still lacking regarding the nature of this bond, the difficulties that ensue from this daily occupation, and the relationship between the two. The purpose of this study was thus to acquire a deeper understanding, by means of an in-depth interview with cat caretakers. The study has identified, for the first time, two distinct emotional approaches that accompany extensive caretaking for free-roaming cats: emotional attachment and emotional detachment. We show how these two different responses affect both social and financial aspects in the caretakers' lives, and report on the ways in which these individuals experience cat caretaking. Our findings provide a first systematic understanding of the relationship between the level of technical caretaking (feeding, medical care, etc.) and the level of emotional involvement, and reveal the ambivalence often inherent in human–animal relations in general and the caretaker–cat bond in particular. The understanding acquired here can be put into practice to reduce the emotional and technical difficulties experienced by cat caretakers, as well as to improve free-roaming cat management efforts and cat welfare. By increasing public and municipal awareness of the possible contribution of cat caretakers to cat management, and of the emotional and technical difficulties they experience, both the caretakers and other community members can benefit.  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