全文获取类型
收费全文 | 16656篇 |
免费 | 148篇 |
国内免费 | 109篇 |
专业分类
安全科学 | 444篇 |
废物处理 | 672篇 |
环保管理 | 1981篇 |
综合类 | 2871篇 |
基础理论 | 4295篇 |
环境理论 | 11篇 |
污染及防治 | 4533篇 |
评价与监测 | 1114篇 |
社会与环境 | 890篇 |
灾害及防治 | 102篇 |
出版年
2023年 | 113篇 |
2022年 | 226篇 |
2021年 | 237篇 |
2020年 | 149篇 |
2019年 | 168篇 |
2018年 | 338篇 |
2017年 | 336篇 |
2016年 | 471篇 |
2015年 | 361篇 |
2014年 | 521篇 |
2013年 | 1255篇 |
2012年 | 636篇 |
2011年 | 815篇 |
2010年 | 614篇 |
2009年 | 657篇 |
2008年 | 817篇 |
2007年 | 786篇 |
2006年 | 673篇 |
2005年 | 593篇 |
2004年 | 508篇 |
2003年 | 501篇 |
2002年 | 459篇 |
2001年 | 538篇 |
2000年 | 376篇 |
1999年 | 251篇 |
1998年 | 156篇 |
1997年 | 177篇 |
1996年 | 179篇 |
1995年 | 218篇 |
1994年 | 220篇 |
1993年 | 168篇 |
1992年 | 150篇 |
1991年 | 187篇 |
1990年 | 180篇 |
1989年 | 169篇 |
1988年 | 126篇 |
1987年 | 122篇 |
1986年 | 137篇 |
1985年 | 103篇 |
1984年 | 120篇 |
1983年 | 120篇 |
1982年 | 131篇 |
1981年 | 116篇 |
1980年 | 106篇 |
1979年 | 117篇 |
1978年 | 79篇 |
1977年 | 80篇 |
1975年 | 83篇 |
1973年 | 76篇 |
1972年 | 73篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Male willow warblers have song repertoires which vary in complexity along several dimensions. We examined whether female choice,
as measured by date of pairing, was based on these song characteristics in 4 different years. Pairing date was negatively
correlated with song repertoire size in 1 year, and with song versatility in another year, but there was no consistent effect
of any song characteristic on pairing throughout the years or in the pooled sample. The variable that best explained how soon
a male pairs is male arrival date (only males that had settled territories before the first female arrived were considered
in the analysis). This correlation is consistently significant in all years. This is most parsimoniously interpreted as females
choosing some habitat characteristic in the same way that males do. A small percentage of males (8.3%) attracted and paired
with a second female. The likelihood of becoming polygynous was not explained by any measured song characteristic, but it
was related to arrival date: early males were more likely to pair with two females. Males with large repertoires fledged more
young in their primary nests, and there was a trend for the offspring of these males to have a greater probability of being
recruited into the population. In conclusion, the results show that in most years there is no sexual selection by female preference
on song characteristics, although the data on reproductive success is consistent with the idea of repertoire size being an
indicator of male quality.
Received: 4 June 1999 / Received in revised form: 1 December 1999 / Accepted: 31 December 1999 相似文献
102.
In the meerkat (Suricata suricatta), a cooperative mongoose, pups follow potential feeders while the group is foraging and emit incessant calls when soliciting
food from them. In contrast to a ’stationary’ brood of chicks, in which nestlings are fed at a fixed location, meerkat pups
are ’mobile’ and become spread out. The question arises whether meerkat pups that experience different constraints to those
facing chicks have evolved similar begging strategies. This paper describes the vocalisations that meerkat pups emit in the
context of begging and investigates the influence of these calls on food allocation by older group members and on the behaviour
of littermates. Meerkat pups use two types of calls when soliciting food from a potential feeder. The most common is a ’repeat’
call, which pups emit continuously when following an older forager over several hours a day. In addition, when a potential
feeder finds a prey item, the pups next to it emit a bout of calls with increased calling rate, amplitude and fundamental
frequency, termed ’high-pitched’ calls. Observations, together with playback experiments, showed that more prey was allocated
to pups that called longer and more intensely. The pup closest to a feeder was almost always fed. The probability of emitting
high-pitched calls did not depend on the time since a pup had received food, and the change from repeat to high-pitched calls
occurred suddenly. The main function of the high-pitched call, therefore, does not appear to be to signal a pup’s hunger state.
More likely, the two calls, in the context of begging, may be an adaptation to energetic constraints in a mobile feeding system.
Pups, which are dispersed during foraging, may emit repeat calls over long periods to prevent potential feeders from eating
all the prey themselves. At the moment a potential feeder finds prey, pups may give the more intense high-pitched calls to
direct feeders to bring the food item to them and not to a littermate. Therefore, unlike the stationary feeding system where
chicks emit one type of begging call when the feeder approaches the nest, meerkats, with a mobile feeding system, have evolved
two discrete types of vocalisations in the context of begging.
Received: 22 November 1999 / Revised: 1 July 2000 / Accepted: 17 July 2000 相似文献
103.
Demographic Forecasting in Koala Conservation 总被引:1,自引:0,他引:1
Angela M. Penn William B. Sherwin †† Greg Gordon † Daniel Lunney ‡ Alistair Melzer § and Robert C. Lacy 《Conservation biology》2000,14(3):629-638
Abstract: The koala currently needs conservation intervention. There is clear evidence of decline in many populations, but the existence of other stable or expanding populations offers the possibility of a variety of creative solutions to their conservation problems. The 1998 National Koala Conservation Strategy emphasizes the need to obtain demographic information and to use this information to assess management options for koalas. We need accurate diagnoses of the status of koala populations and forecasts of their demographic future with and without particular management actions. In a qualitative fashion, this process has been undertaken many times on a local and national scale. Quantitative demographic forecasting tools are increasingly available, and koala management could benefit from their application both at the scale of individual populations and more broadly. There is already a considerable body of suitable data on the dispersal, effects of normal and catastrophic environmental variation on reproduction and survival, and on the effects of habitat change. Demographic forecasting, however, is hampered because the full suite of information is rarely available from a single population. In two Queensland populations, retrospective population viability analyses provided forecasts that were in agreement with observed population trends. Work is needed to determine whether data from one population can be applied to other populations. Models can then be developed to make projections at a multipopulation level on the basis of local population dynamics and dispersal. Certain koala populations, because of their long history of study, offer the opportunity to test demographic models retrospectively. These tests will not only aid in fine-tuning the models for koala biology and data but will also assist with the more general process of validating the models. 相似文献
104.
Phylogenetic analyses of the left domain of the mitochondrial DNA control-region sequence have been used to examine the relationships
among species of the genus Merluccius (Rafinesque, 1810), and to compare these with hypotheses based on morphological, meristic and allozyme characters. Analysis
of aligned sequences revealed that transition bias was much lower than in mammalian mtDNA, and that nucleotide composition
of control-region sequences was biased toward A and T. We have roughly calibrated a molecular clock for the genus, based on
the rise of the Isthmus of Panamá, which is believed to have created a barrier to dispersal between marine species of the
Atlantic and Pacific Oceans. Our mtDNA-based phylogeny was highly congruent with allozyme-based phylogenies, but poorly so
with a previously described phylogeny based on morphology. Specifically, our phylogeny resolved two well-supported principal
clades, one of American (west Atlantic and east Pacific) species and the other of Euro–African (east Atlantic) species. This
suggests an evolutionary history during which the ancestral lineage of Merluccius was divided between two geographic regions, with subsequent dispersal and vicariant events resulting in the evolution and
distribution of extant taxa. However, the relationships between some taxa within the American clade could not be resolved.
We suggest that this is consistent with an hypothesis of a rapid origin and radiation of these taxa.
Received: 12 December 1998 / Accepted: 15 October 1999 相似文献
105.
Maternal investment in rhesus macaques (Macaca mulatta): reproductive costs and consequences of raising sons 总被引:1,自引:0,他引:1
Maternal investment in offspring is expected to vary according to offspring sex when the reproductive success of the progeny
is a function of differential levels of parental expenditure. We conducted a longitudinal investigation of rhesus macaques
to determine whether variation in male progeny production, measured with both DNA fingerprinting and short tandem repeat marker
typing, could be traced back to patterns of maternal investment. Males weigh significantly more than females at birth, despite
an absence of sex differences in gestation length. Size dimorphism increases during infancy, with maternal rank associated
with son’s, but not daughter’s, weight at the end of the period of maternal investment. Son’s, but not daughter’s, weight
at 1 year of age is significantly correlated with adult weight, and male, but not female, weight accounts for a portion of
the variance in reproductive success. Variance in annual offspring output was three- to fourfold higher in males than in females.
We suggest that energetic costs of rearing sons could be buffered by fetal delivery of testosterone to the mother, which is
aromatized to estrogen and fosters fat accumulation during gestation. We conclude that maternal investment is only slightly
greater in sons than in daughters, with mothers endowing sons with extra resources because son, but not daughter, mass has
ramifications for offspring sirehood. However, male reproductive tactics supersede maternal investment patterns as fundamental
regulators of male fitness.
Received: 23 July 1999 / Received in revised form: 23 February 2000 / Accepted: 13 March 2000 相似文献
106.
R. Wilson G. Peters J. Regel D. Grémillet K. Pütz M. Kierspel H. Weimerskirch J. Cooper 《Marine Biology》1998,130(4):559-566
Stomach temperature loggers have proved useful for the study of feeding activity in free-living seabirds, but their usage
has been restricted because they are frequently dislodged and lost during the seabirds' regular regurgitation of indigestible
prey remains. In the present study we examine the incidence of spontaneous regurgitation (pellet production) in free-living
seabirds, consider the effect this has on the likely retrieval of stomach temperature loggers and present a structural modification
of the logger housings which leads to a much lower incidence of regurgitation. Systems were tested on albatrosses, cormorants,
gannets and penguins.
Received: 15 July 1997 / Accepted: 31 October 1997 相似文献
107.
J. B. Lewis 《Marine Biology》1998,130(4):651-662
Dipolydora armata (Langerhans, 1880) is a small (4 to 5 mm) spionid polychaete found burrowing in the calcareous hydrozoan Millepora complanata Lamarck, 1816, on coral reefs at Barbados, West Indies. It excavates complex networks of interconnecting burrows and forms
aggregations of worms in cavities within branches of the coral. Adult worms have a mixed feeding mode (suspension feeding
and deposit feeding). Size–frequency distributions of worms in branch samples suggest that they mature in a single year and
that reproduction occurs throughout the year. Burrow openings on the surface of the coral develop distinctive, erect spines
caused by combined growth of worm tubes and host tissue. Millepore zooids were absent in the vicinity of tube openings and
on spines, and thus the potential feeding surface of the coral will be reduced in heavily colonized branches. Burrows and
openings were densest at the bases of millepore branches where weakening of the skeleton would be expected to occur. The absence
of openings near the branch tips suggests difficulty in larval settlement there, amongst stinging zooids. Reproduction␣and
larval development of the worms were examined, and a sequence of larval stages from one to 20 segments and a juvenile stage
of 22 segments are described. Eggs are deposited in brood sacs attached to the burrow wall, and the larvae feed upon nurse
eggs (adelphophagy). The presence of larvae and juveniles occurring free in the burrows suggests that larval development may
be completed within the host coral as an alternative or in addition to a planktonic larval phase. Lack of provisional larval
setae, early development of adult capillary setae, production of special spermatophores and a protracted breeding cycle in
D. armata are all traits which would favour complete development within the host skeleton.
Received: 6 March 1997 / Accepted: 25 October 1997 相似文献
108.
Peter Weindler Frank Böhme Varis Liepa Wolfgang Wiltschko 《Behavioral ecology and sociobiology》1998,42(4):289-294
To assess the role of celestial rotation during daytime in the development of the magnetic compass course, pied flycatchers
(Ficedula hypoleuca Pallas, Muscicapidae) were handraised in Latvia under various celestial and magnetic conditions. Tests were performed during
autumn migration in the local geomagnetic field (50 000 nT, 73° inclination) in the absence of celestial cues. A group of
birds that had never seen the sky showed a bimodal preference for the migratory southwest-northeast axis, whereas a second
group that had been exposed to the natural sky from sunrise to sunset in the local geomagnetic field showed a unimodal preference
for the seasonally appropriate southwesterly direction. A third group that had also been exposed to the daytime sky, but in
the absence of magnetic compass information, also oriented bimodally along a southwest-northeast axis. These findings demonstrate
that observing celestial rotation during daytime enables birds to choose the right end of the migratory axis for autumn migration
at the Latvian test location. This transformation of axial behavior into appropriate migratory orientation, however, requires
the birds to have simultaneous access to information on both celestial rotation and the geomagnetic field.
Received: 19 September 1997 / Accepted after revision: 22 November 1997 相似文献
109.
K. Håkan Olsén J. Torbjörn Järvi Ian Mayer Erik Petersson Frederieke Kroon 《Chemoecology》1998,8(1):9-17
Summary. In the present experiment the behaviour and endocrine status of males of the brown trout, Salmo trutta L., (Salmoniformes: Salmonidae) were studied when males were kept in a stream tank with a nest digging female. Groups of
mature adult males and precocious intact or anosmic male parr were placed with the nesting female so that the group resembled
a natural spawning situation with big anadromous fish acting as dominant males and precocious parr acting as “sneakers”. A
control experiment was also run with only males without a female present. In intact parr there were significant positive correlations
between the per cent of the total observation time spent with a female, milt volume, and plasma concentration of 17α, 20β-dihydroxy-4-pregnen-3-one.
Anosmic parr had significantly lower volumes of strippable milt and gonadal steroid hormone levels compared with intact parr.
However, no differences were found in the control experiment. Significantly fewer anosmic parr attended and courted the nesting
female and those anosmic fish that attended the female had significantly lower plasma levels of gonadal hormones. Intact parr
also displayed a greater number of agonistic acts against other parr without any difference in fighting ability. No differences
in aggression occured in the control experiment. In adult males together with a female, post-experimental gonadal steroid
hormone levels were higher than pre-experimental levels. Positive correlations between aggression and androgen hormone levels
were observed in adult males. No differences in plasma hormone levels were observed between adult males and intact precocious
males. The results show that olfactory occlusion results in low steroid hormone levels and milt volumes in precocious males
placed in a spawning situation. The courting behaviour was also affected by anosmia. Odours from the nesting female may have
caused the enhanced plasma hormone levels and stimulated the males to attend the female.
Received 15 May 1997; accepted 29 June 1997. 相似文献
110.
Restricted adaptive cluster sampling 总被引:4,自引:0,他引:4
Adaptive cluster sampling can be a useful design for sampling rare and patchy populations. With this design the initial sample size is fixed but the size of the final sample (and total sampling effort) cannot be predicted prior to sampling. For some populations the final sample size can be quite variable depending on the level of patchiness. Restricted adaptive cluster sampling is a proposed modification where a limit is placed on the sample size prior to sampling and quadrats are selected sequentially for the initial sample size. As a result there is less variation in the final sample size and the total sampling effort can be predicted with some certainty, which is impor- tant for many ecological studies. Estimates of density are biased with the restricted design but under some circumstances the bias can be estimated well by bootstrapping. © Rapid Science 1998 相似文献