全文获取类型
收费全文 | 3014篇 |
免费 | 42篇 |
国内免费 | 25篇 |
专业分类
安全科学 | 163篇 |
废物处理 | 154篇 |
环保管理 | 612篇 |
综合类 | 334篇 |
基础理论 | 725篇 |
环境理论 | 5篇 |
污染及防治 | 752篇 |
评价与监测 | 212篇 |
社会与环境 | 106篇 |
灾害及防治 | 18篇 |
出版年
2023年 | 21篇 |
2022年 | 37篇 |
2021年 | 26篇 |
2020年 | 21篇 |
2019年 | 31篇 |
2018年 | 66篇 |
2017年 | 67篇 |
2016年 | 89篇 |
2015年 | 58篇 |
2014年 | 84篇 |
2013年 | 280篇 |
2012年 | 143篇 |
2011年 | 177篇 |
2010年 | 120篇 |
2009年 | 122篇 |
2008年 | 154篇 |
2007年 | 187篇 |
2006年 | 152篇 |
2005年 | 105篇 |
2004年 | 104篇 |
2003年 | 116篇 |
2002年 | 89篇 |
2001年 | 67篇 |
2000年 | 46篇 |
1999年 | 51篇 |
1998年 | 38篇 |
1997年 | 43篇 |
1996年 | 43篇 |
1995年 | 40篇 |
1994年 | 41篇 |
1993年 | 34篇 |
1992年 | 37篇 |
1991年 | 26篇 |
1990年 | 29篇 |
1989年 | 19篇 |
1988年 | 13篇 |
1987年 | 20篇 |
1986年 | 25篇 |
1985年 | 17篇 |
1984年 | 18篇 |
1983年 | 28篇 |
1982年 | 28篇 |
1981年 | 25篇 |
1980年 | 19篇 |
1979年 | 16篇 |
1977年 | 11篇 |
1976年 | 9篇 |
1975年 | 9篇 |
1974年 | 10篇 |
1971年 | 11篇 |
排序方式: 共有3081条查询结果,搜索用时 882 毫秒
451.
Female choice on the basis of male traits has been described in an array of taxa but has rarely been demonstrated in reptiles. In the sand lizard (Lacerta agilis), and possibly in other non-territorial reptiles, a male's contribution to a female's fitness is restricted to his genes. In order to choose males of high genetic quality, females have to trade the fitness gain against the costs of active choice. In a Swedish population of sand lizards, long-lived males sired offspring with higher embryonic survival compared to offspring sired by short-lived males. In spite of this female sand lizards did not mate selectively with older and/or larger males. There appeared to be mo reliable cues to male longevity; age-specific male body size was highly variable. Furthermore, estimates of male nuptial coloration did not covary with ectoparasite load and, hence, females cannot use male coloration as a cue to heritable resistance to pathogenic parasite effects. When cues to male genetic quality are poor, or inaccurate, and males make no parental investment, we predict that female choice will be rare. Sand lizard females mating with many partners lay clutches with higher hatching success. Thus, females may obtain good genes for their young by multiple mating, thereby avoiding costs associated with mate choice. 相似文献
452.
453.
The control of water collection in honey bee colonies 总被引:1,自引:0,他引:1
A honey bee (Apis mellifera) colony adaptively controls the collection of water by its foragers, increasing it when high temperatures necesssitate evaporative cooling inside the hive and decreasing it when the danger of overheating passes. This study analyzes the mechanisms controlling water collection once it has begun, that is, how a colony's water collectors know whether to continue or stop their activity. M. Lindauer suggested that water collectors acquire information about their colony's need for more water by noting how easily they can unload their water to bees inside the hive. In support of this hypothesis, we found that a water collector's ease of unloading does indeed change when her colony's need for water changes. How does a water collector sense the ease of unloading? Multiple variables of the unloading experience change in relation to a colony's water need. Three time-based variables – initial search time, total search time, and delivery time – all change quite strongly. But what changes most strongly is the number of unloading rejections (refusals by receiver bees to take the water), suggesting that this is the primary index of ease of unloading. Why does a water collector's ease of unloading change when her colony's need for water changes? Evidently, what links these two variables is change in the number of water receivers. These are middle-aged bees that receive water just inside the hive entrance, then transport it deeper inside the hive, and finally smear it on the walls of cells or give it to other bees, or both. A colony increases the number of water receivers when its water need increases by having bees engaged in nectar reception and other tasks (and possibly also bees that are not working) switch to the task of water reception. Evidently the activation of additional water receivers does not strongly reduce the number of nectar receivers in a colony, since a colony can increase greatly its water collection without simultaneously decreasing its collection of rich nectar. This study provides a clear example of the way that the members of a social insect colony can use indirect indicators of their colony's labor needs to adaptively control the work that they perform. 相似文献
454.
455.
Trouts native to the American Southwest provide an excellent example of the plight of endangered fishes from this region. The native species, Apache trout and Gila trout ( Oncorhynchus apache and O. gilae , respectively) have faced drastic reduction in habitat and detrimental interactions with introduced species, resulting in a dramatic decrease in numbers and sizes of populations. We used biochemical methods to identify diagnostic markers for the estimation of genetic relatedness and analysis of hybridization among native trouts and introduced cutthroat and rainbow trouts ( O. clarki and O. mykiss , respectively). Restriction endonuclease analysis of mitochondrial DNA (mtDNA) indicated that Apache and Gila trout were very similar to each other, and more similar to rainbow trout than cutthroat. Diagnostic allozyme marker loci indicated that Apache trout hybridized extensively with rainbows in four populations and provided no evidence for reproductive isolation between the forms. Analysis of mtDNA, however, indicated that introduced haplotypes were rare in these same individuals, identifying a bias in the direction of gene exchange between species. The potential reproductive isolation and lack of information concerning population structure necessitate further study of Apache trout to determine the appropriate management strategy for this threatened species. This case demonstrates that extreme care must be exercised when considering elimination of any contaminated population lest the unique genetic identity of the native taxon be lost forever. 相似文献
456.
Summary Foraging theory depicts dietary choice as a function of prey quality and absolute abundance. Ecological processes, however, can depend on the way foragers respond to the relative abundances of available prey types; several models for frequency-dependent foraging adequately describe these responses. Our laboratory experiments with white-throated sparrows investigated preferential choice of two food rewards as we manipulated both reward quality and relative abundance. In any single experiment the two rewards provided the same mean food quantity, but the variances differed. Average energy budgets predicted risk-aversion, so that foraging preference should decrease as reward variance increases. We presented each two-reward pairing at availability ratios of 1:2, 1:1, and 2:1 for three consecutive days. By the third day risk-aversion exceeded preference for reward variance significantly. When relative abundances of the low and high variance rewards were not equal, the birds tended to prefer the rare over the common reward. This response began before the birds had thoroughly sampled the reward distributions. Preference for rarity apparently constrained the birds' economic response to reward variance levels. 相似文献
457.
458.
On the integration of individual foraging strategies with colony ergonomics in social insects: nectar-collection in honeybees 总被引:1,自引:0,他引:1
Summary We experimentally tested whether foraging strategies of nectar-collecting workers of the honeybee (Apis mellifera) vary with colony state. In particular, we tested the prediction that bees from small, fast growing colonies should adopt higher workloads than those from large, mature colonies. Queenright small colonies were set up by assembling 10 000 worker bees with approximately 4100 brood cells. Queenright large colonies contained 35 000 bees and some 14 500 brood cells. Thus, treatments differed in colony size but not in worker/brood ratios. Differences in workload were tested in the context of single foraging cycles. Individuals could forage on a patch of artificial flowers offering given quantities and qualities of nectar rewards. Workers of small colonies took significantly less nectar in an average foraging excursion (small: 40.1 ± 1.1 SE flowers; large: 44.8 ± 1.1), but spent significantly more time handling a flower (small: 7.3 ± 0.4 s ; large: 5.8 ± 0.4 s). When the energy budgets for an average foraging trip were calculated, individuals from all colonies showed a behavior close to maximization of net energetic efficiency (i.e., the ratio of net energetic gains to energetic costs). However, bees from small colonies, while incurring only marginally smaller costs, gained less net energy per foraging trip than those from large colonies, primarily as a result of prolonged handling times. The differences between treatments were largest during the initial phases of the experimental period when also colony development was maximally different. Our results are at variance with simple models that assume natural selection to have shaped behavior in a single foraging trip only so as to maximize colony growth.
Offprint requests to: P. Schmid-Hempel 相似文献
459.
The ontogeny of behaviour relevant to dispersal was studied in situ with reared pelagic larvae of three warm temperate, marine,
demersal fishes: Argyrosomus japonicus (Sciaenidae), Acanthopagrus australis and Pagrus auratus (both Sparidae). Larvae of 5–14 mm SL were released in the sea, and their swimming speed, depth and direction were observed
by divers. Behaviour differed among species, and to some extent, among locations. Swimming speed increased linearly at 0.4–2.0 cm s−1 per mm size, depending on species. The sciaenid was slower than the sparids by 2–6 cm s−1 at any size, but uniquely, it swam faster in a sheltered bay than in the ocean. Mean speeds were 4–10 body lengths s−1. At settlement size, mean speed was 5–10 cm s−1, and the best performing individuals swam up to twice the mean speed. In situ swimming speed was linearly correlated (R
2=0.72) with a laboratory measure of swimming speed (critical speed): the slope of the relationship was 0.32, but due to a
non-zero intercept, overall, in situ speed was 25% of critical speed. Ontogenetic vertical migrations of several metres were
found in all three species: the sciaenid and one sparid descended, whereas the other sparid ascended to the surface. Overall,
74–84% of individual larvae swam in a non-random way, and the frequency of directional individuals did not change ontogenetically.
Indications of ontogenetic change in orientated swimming (i.e. the direction of non-random swimming) were found in all three
species, with orientated swimming having developed in the sparids by about 8 mm. One sparid swam W (towards shore) when <10 mm,
and changed direction towards NE (parallel to shore) when >10 mm. These results are consistent with limited in situ observations
of settlement-stage wild larvae of the two sparids. In situ, larvae of these three species have swimming, depth determination
and orientation behaviour sufficiently well developed to substantially influence dispersal trajectories for most of their
pelagic period. 相似文献
460.
After a 40-year absence caused by pollution and eutrophication, burrowing mayflies (Hexagenia spp.) recolonized western Lake Erie in the mid 1990s as water quality improved. Mayflies are an important food resource for the economically valuable yellow perch fishery and are considered to be major indicator species of the ecological condition of the lake. Since their reappearance, however, mayfly populations have suffered occasional unexplained recruitment failures. In 2002, a failure of fall recruitment followed an unusually warm summer in which western Lake Erie became temporarily stratified, resulting in low dissolved oxygen levels near the lake floor. In the present study, we examined a possible link between Hexagenia recruitment and periods of intermittent stratification for the years 1997 2002. A simple model was developed using surface temperature, wind speed, and water column data from 2003 to predict stratification. The model was then used to detect episodes of stratification in past years for which water column data are unavailable. Low or undetectable mayfly recruitment occurred in 1997 and 2002, years in which there was frequent or extended stratification between June and September. Highest mayfly reproduction in 2000 corresponded to the fewest stratified periods. These results suggest that even relatively brief periods of stratification can result in loss of larval mayfly recruitment, probably through the effects of hypoxia. A trend toward increasing frequency of hot summers in the Great Lakes region could result in recurrent loss of mayfly larvae in western Lake Erie and other shallow areas in the Great Lakes. 相似文献