全文获取类型
收费全文 | 239篇 |
免费 | 11篇 |
国内免费 | 36篇 |
出版年
2023年 | 1篇 |
2022年 | 1篇 |
2021年 | 5篇 |
2020年 | 5篇 |
2019年 | 2篇 |
2018年 | 4篇 |
2017年 | 3篇 |
2016年 | 9篇 |
2015年 | 7篇 |
2014年 | 6篇 |
2013年 | 26篇 |
2012年 | 3篇 |
2011年 | 10篇 |
2010年 | 11篇 |
2009年 | 9篇 |
2008年 | 15篇 |
2007年 | 30篇 |
2006年 | 18篇 |
2005年 | 15篇 |
2004年 | 13篇 |
2003年 | 13篇 |
2002年 | 11篇 |
2001年 | 8篇 |
2000年 | 8篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有286条查询结果,搜索用时 15 毫秒
281.
This study concerns the formation, taphonomy, and preservation of human footprints in microbial mats of present-day tidal-flat environments. Due to differences in water content and nature of the microbial mats and the underlying sediment, a wide range of footprint morphologies was produced by the same trackmaker. Most true tracks are subjected to modification due to taphonomic processes, leading to modified true tracks. In addition to formation of biolaminites, microbial mats play a major role in the preservation of footprints on tidal flats. A footprint may be consolidated by desiccation or lithification of the mat, or by ongoing growth of the mat. The latter process may lead to the formation of overtracks. Among consolidated or (partially) lithified footprints found on present-day tidal flats, poorly defined true tracks, modified true tracks, and overtracks were most frequently encountered while unmodified and well-defined true tracks are rather rare. We suggest that modified true tracks and overtracks make up an important percentage of fossil footprints and that they may be as common as undertracks. However, making unambiguous distinctions between poorly defined true tracks, modified true tracks, undertracks, and overtracks in the fossil record will remain a difficult task, which necessitates systematic excavation of footprints combined with careful analysis of the encasing sediment. 相似文献
282.
Salinity and shade preferences result in ovipositional differences between sympatric tiger beetle species 总被引:2,自引:0,他引:2
W. Wyatt Hoback Douglas A. Golick Tina M. Svatos Stephen M. Spomer Leon G. Higley 《Ecological Entomology》2000,25(2):180-187
1. Adult tiger beetles of the genus Cicindela often co‐occur within a habitat but larvae do not. Larvae are sedentary and form usually permanent burrows at the site of oviposition where they require 1–3 years for development. 2. To test niche partitioning based on ovipositional preference, the behaviour of two sympatric salt marsh tiger beetles, Cicindela circumpicta and C. togata (Coleoptera: Cicindelidae), were examined. 3. In laboratory studies, female C. circumpicta and C. togata distinguished between experimental salinities, with the former preferring 4 parts per thousand (ppt) and the latter preferring 12 ppt. In the field, C. circumpicta larvae were associated with lower salinities (1–3 ppt) and vegetation along the edges of salt flats while C. togata larvae were found on open salt flats often near halophytes (average salinity = 7.8 ppt). 4. In the field, females chose sites for oviposition in response to shade but not vertical landmarks. In a direct test, 53 of 56 new larval burrows occurred in shaded treatments, three in nonshaded controls. 5. Compared with nonshaded controls, shade increased survival of eggs that were collected in the laboratory and placed in the field. For C. circumpicta eggs, 78% placed in shaded treatments hatched, while significantly fewer (22%) hatched in exposed treatments. For C. togata, 43% of eggs placed in the shade hatched, while no eggs placed in exposed treatments hatched. 6. These results support the hypothesis of niche partitioning between C. circumpicta and C. togata based on ovipositional choice and resulting larval habitat. 相似文献
283.
On a moderately protected intertidal sand flat in west Kyushu, Japan, most of the population of the cirolanid isopodEurydice nipponica Bruce & Jones (89%–100%) was found in the zones occupied by the thalassinidean ghost shrimpCallianassa japonica Ortmann in July, August and December 1980.C. japonica later extensively expanded its habitat, and in July 1984, when almost the whole sand flat had been densely populated byC. japonica, the range of the distribution ofE. nipponica was the same as that ofC. japonica with a density about 10 times greater than in July 1980. The occurrence ofE. nipponica has previously been recorded from several exposed sandy beaches as well as their adjacent subtidal areas of well oxygenated
sands along the coast of Kyushu, but not on more protected shores like the present sand flat. It is suggested thatC. japonica, through its bioturbating activities, produces sediment characteristics approximating those of the exposed sandy beaches
which are the preferred habitat ofE. nipponica. It is supposed thatE. nipponica is a facultative commensal ofC. japonica. In a field experiment to excludeC. japonica and to detect its positive influence onE. nipponica, the densities ofE. nipponica were found to be lower in the experimental plots than in the intact plots. Statistically, however, the difference was only
weakly significant, and the possible reasons for this are discussed. 相似文献
284.
Christian Buschbaum 《Helgoland Marine Research》2002,56(1):37-43
Balanids are the numerically dominant epibionts on mussel beds in the Wadden Sea. Near the island of Sylt (German Bight,
North Sea), Semibalanus
balanoides dominated intertidally and Balanus
crenatus subtidally. Field experiments were conducted to test the effects of predation on the density of barnacle recruits. Subtidally,
predator exclusion resulted in significantly increased abundances of B. crenatus, while predator exclusion had no significant effects on the density of S. balanoides intertidally. It is suggested that recruitment of B. crenatus to subtidal mussel beds is strongly affected by adult shore crabs (Carcinus maenas) and juvenile starfish (Asterias rubens), whereas recruits of S. balanoides in the intertidal zone are mainly influenced by grazing and bulldozing of the very abundant periwinkle Littorina littorea, which is rare subtidally. Thus, not only do the barnacle species differ between intertidal and subtidal mussel beds, but
the biotic control factors do so as well.
Electronic Publication 相似文献
285.
Karsten Reise 《Hydrobiologia》1984,118(1):73-81
Small-scale sediment disturbances on a sandy tidal flat in the North Sea changed the pattern of abundances in free-living platyhelminths (Turbellaria) and small Polychaeta, but not the species composition. Patches of 0.25 m2 were made anoxic, were regularly raked at the surface, or the upper layer of sediment was inverted. Recolonization after temporary anoxia proceeded very slowly. Raking caused diatom-feeding platyhelminths and the polychaetes to emigrate. Turning the upper 5 cm of the sediment upside down was harmful to both taxa, but platyhelminths recolonized quickly. Background densities were temporarily surpassed, first by predators, then by diatom feeders. Coherent responses of trophic guilds in platyhelminths do not suggest that release from competition is involved but indicate that such disturbances alter the availability of food resources and the organisms immigrate accordingly. 相似文献
286.
Summary The dolomitic Wojciechowice Formation distinctly differs from the remaining, mainly shaly Middle Devonian succession in northern
part of the Holy Cross Mountains (Central Poland). The upper Member of the Formation (Crystalline Dolostone Mb.), in greater
part dolomitized but also containing limestone beds, is composed of shallowing-upward cyclothems well exposed in Skaly quarry
in the Bodzentyn syncline.
The lower parts of the cyclothems, interpreted as subtidal facies, contain fossils characteristic for restricted environments.
They are grouped into two assemblages. The first, with brachiopods (largeBornhardtina andEmanuella), massive stromatoporoids, and subordinate gastropods and amphiporoids is related to a deeper subtidal environment, while
the second (mainly amphiporoids, gastropods, ostracodes and calcareous algae) is shallower subtidal. Towards the top of succession
the fossil content radically decreases. The upper parts of cyclothems are composed mainly of different types of laminites.
In these parts of the section, interpreted as intertidal/supratidal units, stromatolites, desiccation polygons, intraformational
breccias, and common bioturbations are present. The whole succession was deposited in a low-energy environment, only intermittently
affected by high-energy events. For their most spectacular example of this, aBornhardtina-coquinite, a tempestitic origin is proposed.
The interval with cyclic sedimentation studied correlates with the dolomitized lower “Unit I” of the Stromatoporoid-Coral
Kowala Formation from the southern part of the Holy Cross Mountains, which exhibits sabkha-type cyclicity. The differences
in development of cyclothems in both regions resemble outer and inner part of an extensive platform, and correspond well with
basic trends of the Lower-Middle Devonian transgression in the Holy Cross Mountains. The general succession of formations
deposited during this process coincides with transgressive events on Johnson's eustatic curve for the Devonian. 相似文献