全文获取类型
收费全文 | 475篇 |
免费 | 90篇 |
国内免费 | 2篇 |
出版年
2021年 | 4篇 |
2020年 | 4篇 |
2019年 | 4篇 |
2018年 | 16篇 |
2017年 | 12篇 |
2016年 | 12篇 |
2015年 | 17篇 |
2014年 | 13篇 |
2013年 | 15篇 |
2012年 | 25篇 |
2011年 | 29篇 |
2010年 | 21篇 |
2009年 | 18篇 |
2008年 | 13篇 |
2007年 | 27篇 |
2006年 | 22篇 |
2005年 | 15篇 |
2004年 | 10篇 |
2003年 | 10篇 |
2002年 | 8篇 |
2001年 | 17篇 |
2000年 | 15篇 |
1999年 | 16篇 |
1998年 | 6篇 |
1997年 | 10篇 |
1996年 | 7篇 |
1995年 | 8篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 12篇 |
1990年 | 6篇 |
1989年 | 13篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 10篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1982年 | 6篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1975年 | 8篇 |
1974年 | 5篇 |
1973年 | 5篇 |
1972年 | 4篇 |
1971年 | 5篇 |
1970年 | 5篇 |
排序方式: 共有567条查询结果,搜索用时 15 毫秒
101.
Michael D. Weiser Brian J. Enquist Brad Boyle Timothy J. Killeen Peter M. Jørgensen Gustavo Fonseca Michael D. Jennings rew J. Kerkhoff Thomas E. Lacher Jr Abel Monteagudo M. Percy Núñez Vargas Oliver L. Phillips Nathan G. Swenson Rodolfo Vásquez Martínez 《Global Ecology and Biogeography》2007,16(5):679-688
Aim Relationships between range size and species richness are contentious, yet they are key to testing the various hypotheses that attempt to explain latitudinal diversity gradients. Our goal is to utilize the largest data set yet compiled for New World woody plant biogeography to describe and assess these relationships between species richness and range size.
Location North and South America.
Methods We estimated the latitudinal extent of 12,980 species of woody plants (trees, shrubs, lianas). From these estimates we quantified latitudinal patterns of species richness and range size. We compared our observations with expectations derived from two null models.
Results Peak richness and the smallest- and largest-ranged species are generally found close to the equator. In contrast to prominent diversity hypotheses: (1) mean latitudinal extent of tropical species is greater than expected; (2) latitudinal extent appears to be decoupled from species richness across New World latitudes, with abrupt transitions across subtropical latitudes; and (3) mean latitudinal extents show equatorial and north temperate peaks and subtropical minima. Our results suggest that patterns of range size and richness appear to be influenced by three broadly overlapping biotic domains (biotic provinces) for New World woody plants.
Main conclusions Hypotheses that assume a direct relationship between range size and species richness may explain richness patterns within these domains, but cannot explain gradients in richness across the New World. 相似文献
Location North and South America.
Methods We estimated the latitudinal extent of 12,980 species of woody plants (trees, shrubs, lianas). From these estimates we quantified latitudinal patterns of species richness and range size. We compared our observations with expectations derived from two null models.
Results Peak richness and the smallest- and largest-ranged species are generally found close to the equator. In contrast to prominent diversity hypotheses: (1) mean latitudinal extent of tropical species is greater than expected; (2) latitudinal extent appears to be decoupled from species richness across New World latitudes, with abrupt transitions across subtropical latitudes; and (3) mean latitudinal extents show equatorial and north temperate peaks and subtropical minima. Our results suggest that patterns of range size and richness appear to be influenced by three broadly overlapping biotic domains (biotic provinces) for New World woody plants.
Main conclusions Hypotheses that assume a direct relationship between range size and species richness may explain richness patterns within these domains, but cannot explain gradients in richness across the New World. 相似文献
102.
Friedrich W. Keppeler Annette S. Engel Linda M. Hooper-Bùi Paola C. López-Duarte Charles W. Martin Jill A. Olin Katelyn J. Lamb Michael J. Polito Nancy N. Rabalais Brian J. Roberts Erick M. Swenson Olaf P. Jensen 《Restoration Ecology》2024,32(3):e14072
Advancing ecological restoration assessments requires a more detailed consideration of species interactions and ecosystem processes. Most restoration projects rely on a few metrics not always directly linked with ecological theory. Here, we used Odum's theory of ecosystem development to assess and compare the ecosystem structure and services of created marshes (4–6 years old) with preexisting, reference marshes in a brackish water region of the Mississippi River Delta. We built ecosystem models for created and reference marshes that integrated large datasets of stomach contents, stable isotopes, and taxa abundances. Despite strong resemblance in community structure, created marshes were at an earlier succession stage compared to the reference marshes, having lower biomass (including exploited species), higher biomass turnover and production, less dependence on detritus, lower material cycling, and less energy flowing through specialist pathways. Although preserving preexisting marshes should be a priority, created marshes may still be an important tool for the restoration of coastal areas and their ecosystem services. In addition, our results show that comparisons of species biodiversity alone may fail to capture essential differences in ecosystem processes between habitats, which reinforces the importance of ecosystem modeling approaches to assess restoration projects. 相似文献
103.
Dana L. Swenson Kyoung-Jin Kim Erich W. Six Steven Clegg 《Molecular & general genetics : MGG》1994,244(2):216-218
The gene fimU, located on a recombinant plasmid carrying the Salmonella typhimurium type 1 fimbrial gene cluster is closely related to the Escherichia coli tRNA gene argU. The fimU gene complements an E. coli argU mutant that is a P2 lysogen, thereby allowing the phage P4 to grow in this strain but preventing the growth of phage lambda. In addition, fimU was shown to be involved in fimbrial expression since transformants of the E. coli argU mutant could produce fimbriae only in the presence of fimU but not in its absence, whereas in an E. coli argU
+ strain fimbriation did not require the fimU gene. 相似文献
104.
105.
Forest tree neighborhoods are structured more by negative conspecific density dependence than by interactions among closely related species 下载免费PDF全文
Lei Chen Liza S. Comita S. Joseph Wright Nathan G. Swenson Jess K. Zimmerman Xiangcheng Mi Zhanqing Hao Wanhui Ye Stephen P. Hubbell W. John Kress Maria Uriarte Jill Thompson Christopher J. Nytch Xugao Wang Juyu Lian Keping Ma 《Ecography》2018,41(7):1114-1123
Interactions among neighbors influence the structure of communities of sessile organisms. Closely related species tend to share habitat and resource requirements and to interact with the same mutualists and natural enemies so that the strength of interspecific interactions tends to decrease with evolutionary divergence time. Nevertheless, the degree to which such phylogenetically related ecological interactions structure plant communities remains unclear. Using data from five large mapped forest plots combined with a DNA barcode mega‐phylogeny, we employed an individual‐based approach to assess the collective effects of focal tree size on neighborhood phylogenetic relatedness. Abundance‐weighted average divergence time for all neighbors (ADT_all) and for heterospecific neighbors only (ADT_hetero) were calculated for each individual of canopy tree species. Within local neighborhoods, we found phylogenetic composition changed with focal tree size. Specifically, significant increases in ADT_all with focal tree size were evident at all sites. In contrast, there was no significant change in ADT_hetero with tree size in four of the five sites for both sapling‐sized and all neighbors, even at the smallest neighbourhood scale (0–5 m), suggesting a limited role for phylogeny‐dependent interactions. However, there were inverse relationships between focal tree size and the proportion of heterospecific neighbors belonging to closely related species at some sites, providing evidence for negative phylogenetic density dependence. Overall, our results indicate that negative interaction with conspecifics had a much greater impact on neighborhood assemblages than interactions among closely related species and could contribute to community structure and diversity maintenance in different forest communities. 相似文献
106.
Lourens Poorter Masha T. van der Sande Eric J. M. M. Arets Nataly Ascarrunz Brian Enquist Bryan Finegan Juan Carlos Licona Miguel Martínez‐Ramos Lucas Mazzei Jorge A. Meave Rodrigo Muñoz Christopher J. Nytch Alexandre A. de Oliveira Eduardo A. Perez‐García Jamir Prado‐Junior Jorge Rodríguez‐Velazques Ademir Roberto Ruschel Beatriz Salgado‐Negret Ivan Schiavini Nathan G. Swenson Elkin A. Tenorio Jill Thompson Marisol Toledo Maria Uriarte Peter van der Hout Jess K. Zimmerman Marielos Peña‐Claros 《Global Ecology and Biogeography》2018,27(3):389-390
107.
The aim of this study was to determine the effect of matrix on the occurrence of hazel grouse (Bonasa bonasia) in habitat fragments. The study was conducted in two kinds of landscape: (1) an agricultural landscape, where the censused forest habitat fragments were surrounded by farmland, and (2) in an intensively managed forested landscape, where the censused habitat fragments were surrounded by nonhabitat coniferous forest. Occupied and unoccupied habitat fragments in the agricultural landscape differed significantly in distance to the nearest suitable continuous habitat, with hazel grouse occurring only in habitat fragments closer than 100 m from continuous forest. In the intensively managed forest landscape, the effect of isolation was less evident, but there might be a threshold around 2 km. Effects of isolation occurred over much shorter distances when the surrounding habitats consisted of farmland than when it was forested habitats. The size of the habitat fragments was important in both landscapes, with larger habitat fragments more often containing hazel grouse. 相似文献
108.
A large-subunit mitochondrial ribosomal DNA sequence translocated to the nuclear genome of two stone crabs (Menippe) 总被引:1,自引:0,他引:1
Two DNA sequences that appear to be homologous to large-subunit
mitochondrial ribosomal RNA genes have been identified in the stone crabs
Menippe mercenaria and M. adina. Amplification from whole genomic DNA by
polymerase chain reaction (PCR) with oligonucleotide primers based on
conserved portions of large-subunit mitochondrial rRNA genes consistently
amplified two products of similar length (565 and 567 bp). These products
differed at 3% of their nucleotide bases, and could be distinguished by a
HindIII site. Only one of these sequences (designated the A sequence) was
detected by PCR in purified mitochondrial DNA. The other (designated the B
sequence) hybridized to total genomic DNA at a level consistent with a
nuclear genome location. It is unlikely that the type B product would have
been recognized as a nuclear copy by examination of its sequence alone.
This is the first report of a mitochondrial gene sequence translocated into
the nuclear genome of a crustacean.
相似文献
109.
110.
The rotational relaxation times of nonpolymerizable skeletal and smooth muscle tropomyosin were measured by analysis of the decay of the zero-field birefringence at different temperatures and salt concentrations. Skeletal tropomyosin in solution is equally well modeled as a rigid rod or as a semiflexible rod with a persistence length of 150 nm. Smooth muscle tropomyosin does not fit the rigid rod model but is well approximated by a semiflexible rod model with a persistence length of 55 nm. The results indicate that smooth muscle tropomyosin is either a more flexible molecule than skeletal muscle tropomyosin or is a curved structure with an end-to-end length shorter than the coiled-coil contour length. Smooth muscle tropomyosin controls the actomyosin ATPase differently from skeletal muscle tropomyosin and it had been suggested that the reason is because it is more rigid; clearly, another explanation must be sought. 相似文献