全文获取类型
收费全文 | 2429篇 |
免费 | 192篇 |
国内免费 | 187篇 |
出版年
2023年 | 39篇 |
2022年 | 37篇 |
2021年 | 75篇 |
2020年 | 80篇 |
2019年 | 102篇 |
2018年 | 105篇 |
2017年 | 85篇 |
2016年 | 89篇 |
2015年 | 81篇 |
2014年 | 94篇 |
2013年 | 137篇 |
2012年 | 88篇 |
2011年 | 97篇 |
2010年 | 77篇 |
2009年 | 93篇 |
2008年 | 118篇 |
2007年 | 113篇 |
2006年 | 120篇 |
2005年 | 103篇 |
2004年 | 89篇 |
2003年 | 74篇 |
2002年 | 77篇 |
2001年 | 69篇 |
2000年 | 62篇 |
1999年 | 72篇 |
1998年 | 49篇 |
1997年 | 42篇 |
1996年 | 45篇 |
1995年 | 37篇 |
1994年 | 43篇 |
1993年 | 33篇 |
1992年 | 27篇 |
1991年 | 36篇 |
1990年 | 25篇 |
1989年 | 38篇 |
1988年 | 34篇 |
1987年 | 37篇 |
1986年 | 20篇 |
1985年 | 21篇 |
1984年 | 21篇 |
1983年 | 12篇 |
1982年 | 20篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1979年 | 13篇 |
1978年 | 10篇 |
1977年 | 10篇 |
1976年 | 7篇 |
1973年 | 8篇 |
1972年 | 7篇 |
排序方式: 共有2808条查询结果,搜索用时 15 毫秒
901.
Erin P. Riley 《International journal of primatology》2007,28(1):107-133
I examined how Sulawesi Tonkean macaques (Macaca tonkeana) respond in terms of their diet and activity patterns to anthropogenic habitat alteration in Lore Lindu National Park, Sulawesi,
Indonesia. Anthropogenic habitat alteration comprised clearing of forest for agriculture and small-scale forest product collection.
I quantified the diet and activity of 2 groups (CH and Anca), occupying habitats with different levels of alteration, via
scan sampling. Tree abundance, key food specific density, and fruit production were greater in the minimally altered habitat
(CH), substantiating the characterization of the group’s habitat as higher quality. For the group in the heavily altered habitat
(Anca), alternative foods, e.g., insects, fungus, young and mature leaves, shoots, and stems, accounted for a significantly
greater proportion of the diet. Dietary diversity is significantly lower in the Anca group, with 52% of their diet being palm
fruits from Arenga pinnata. The activity patterns of the Anca group—more time foraging, less time moving, and more time resting than the CH group—reflect
the lower resource availability in their habitat and their reliance on more alternative food items, coupled with their extremely
small group size (6–9 individuals). The group may be at the optimal size in which foraging efficiency is maximized for the
habitat, a response, in conjunction with dietary and behavioral flexibility, to alteration of their habitat. The results are
contextualized with respect to the conservation value of human-modified landscapes. 相似文献
902.
BACKGROUND AND AIMS: Prior work has shown that above- and below-ground dry biomass across individual plants scale in a near isometric manner across phyletically and ecologically diverse species. Allometric theory predicts that a similar isometric scaling relationship should hold true across diverse forest-types, regardless of vegetational composition. METHODS: To test this hypothesis, two compendia for forest-level above- and below-ground dry biomass per hectare (M(A) and M(R), respectively) were examined to test the hypothesis that M(A) vs. M(R) scales isometrically and in the same manner as reported for data from individual plants. Model Type II regression protocols were used to compare the numerical values of M(A) vs. M(R) scaling exponents (i.e. slopes of log-log linear relationships) for the combined data sets (n =1534), each of the two data sets, and data sorted into a total of 17 data subsets for community- and biome-types as well as communities dominated by angiosperms or conifers. KEY RESULTS: Among the 20 regressions examined, 15 had scaling exponents that were indistinguishable from that reported for M(A) vs. M(R) across individual plants. The isometric hypothesis could not be strictly rejected on statistical grounds; four of these 15 exponents had broad 95% confidence intervals resulting from small sample sizes. Significant variation was observed in the y-intercepts of the 20 regression curves, because of absolute differences in M(A) or M(R). CONCLUSIONS: The allometries of forest- and individual plant-level M(A) vs. M(R) relationships share strikingly similar scaling exponents, but differ because of considerable variation in y-intercepts. These results support prior allometric theory and provide boundary conditions for the scaling of M(A) and M(R). 相似文献
903.
904.
中国种子植物属的地理成分分布格局及其与气候和地理的关系 总被引:2,自引:0,他引:2
基于覆盖了全中国各地理区的204个地区植物区系研究资料和这些地区的841个气象站资料,我们对中国种子植物属的地理成分分布格局及其与气候、经纬度分布的关系进行了研究,并结合这些分布格局探讨了中国植被分带和植物区系分区。结果如下:(1)除世界分布、栽培和入侵成分外,大部分中国种子植物属的地理成分的分布与地理相关密切;(2)热带分布属(泛热带分布、热带亚洲至热带美洲间断分布、旧世界热带分布、热带亚洲至热带大洋洲分布、热带亚洲至热带非洲分布及热带亚洲分布合计)占各地方植物区系的0.84%到94.38%,其最低值出现在中国西北部的新疆和青海地区,最高值出现在中国云南南部和海南;(3)热带分布属在〈北纬30°的地区占优势,除热带亚洲至热带美洲间断分布外,其它热带成分随纬度增加迅速减少;(4)温带分布属(北温带分布、东亚和北美间断分布、旧世界温带分布、温带亚洲分布、地中海区、西亚到中亚分布、中亚分布和东亚分布合计)占各地方植物区系的5.1%至98.83%,其最高值出现在中国西北部的新疆地区,最低值出现在中国云南南部和海南;(5)除东亚和北美间断分布、东亚分布和中国特有分布外,其它温带成分随纬度增加迅速增加;(6)在温带成分中,东亚和北美间断分布及东亚分布属主要出现在中国亚热带到暖温带地区,北温带分布、旧世界温带分布和温带亚洲分布属在中国北部占优势,而地中海区、西亚到中亚分布和中亚分布属则在中国西北部占优势;(7)除世界分布、东亚和北美间断分布、东亚分布和中国特有分布外,所有其他成分都显示了与气候因素(主要是气温和降雨量)密切相关,其中,北温带分布属与年均温和年降雨量最为密切相关。中国种子植物属的地理成分的分布格局与现行的中国植被分带和植物区系分区密切匹配。支持现行的中国植被分带和植物区系分区方案。 相似文献
905.
Chen X Wang Z Gu R Fu J Wang J Zhang Y Wang M Zhang J Jia J Wang G 《Plant cell reports》2007,26(9):1555-1565
By screening a genomic library of maize, a 2.2 kb 5′ flanking fragment of Zpu1 gene, encoding the pullulanase-type starch debranching enzyme, was isolated. Promoter fragments of various lengths, including
the full 5′ flanking sequence (−2267 to −1) (Z1), a 3′ deletion (−2267 to −513) (Z5) and three 5′ deletions extending to −1943
(Z2), −1143 (Z3) and −516 (Z4) upstream of the translational initiation codon (ATG), were fused to the GUS reporter gene and
introduced into tobacco. When these constructs were tested in transgenic tobacco plants, seed-preferred GUS activity was observed
in pZ1-transgenic lines. In pZ2-transgenic lines, the GUS activity was not only restricted to seeds, but was also detected
in calyxes, petals, stamens and mature leaves. At the same time, negligible GUS activity was detected in roots, stems, young
leaves, stigmas and ovaries from the transgenic tobacco plants, which had integrated the full isolated sequence of Zpu1 promoter or its deletions. Deletion analysis indicated that the promoter contained a putative positive cis-regulatory element and the proximal region (−516 to −1) was essential for directing the expression of gus reporter gene. Analysis of GUS activity during the fruit development and seed germination suggested that Zpu1 promoter is active both in starch anabolism and in starch catabolism, which is consistent with the function of the endogenous
gene in maize. GUS activity in leaves under light and darkness confirmed that Zpu1 promoter functions in the starch degradation of photosynthetic tissues in the dark phase of the diurnal cycle. 相似文献
906.
Casper J. Breuker Peter W. de Jong Kathleen Victoir Klaas Vrieling Paul M. Brakefield 《Evolutionary ecology》2007,21(1):13-26
In the Danish region of Kværkeby, a mutation in an, as yet, unknown single autosomal gene has resulted in a dominant resistance (R-) allele in the flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae: Alticinae). It enables the beetle to overcome the defences of Barbarea vulgaris ssp. arcuata (Opiz.) Simkovics G-type (Brassicaceae) and use it as a host plant. In this study, we investigated the pleiotropic effects associated with the presence of this particular R-allele in female P. nemorum. These females had the R-allele backcrossed into the genetic background of non-resistant beetles. The effects were investigated under both favourable and stressful conditions (cold shock). The presence of the R-allele in a non-resistant genetic background caused a very high mortality in resistant individuals during the early stages of development under both conditions, but it did not affect the adult life-history traits longevity, body size and fecundity, under both conditions. Regardless of temperature treatment, resistant females in general were found to lay significantly more eggs. Developmental stability, as measured by tibia length fluctuating asymmetry, was not correlated with overall developmental stress in this study. 相似文献
907.
The relative abundance of an ungulate community in hunted and non-hunted areas was studied in Calakmul, a tropical forest
in southern Mexico that includes a 723,815 ha Biosphere Reserve where no hunting is allowed and communal lands where hunting
activity does take place. Tapir, white-lipped peccary (WLP), collared peccary, two species of brocket deer and white-tailed
deer were the focal species. Relative abundance of ungulates was obtained by counting tracks along transects in three hunted
sites and in the southern core area of the Calakmul Biosphere Reserve, a enforced part of the protected area of 350,000 ha
where no hunting has been-permitted since 1989. A total of 1708 ungulates tracks were obtained along 206.1 km (n = 90 transects) divided among the four sites. Relative abundance of the species was estimated from the track encounter rate
(TER) calculated as the number of tracks per species encountered per km of transect. No significant differences in brocket
deer and collared peccary TER was found among hunted and non-hunted areas. TER for WLP was significant higher in the non-hunted
area, while TER for white-tailed deer and tapir was significant higher in hunted sites. Hunting activity affected the ungulate
species in Calakmul differently. Brocket deer, white-tailed deer and collared peccary are more resistant species toward the
hunting pressure than the WLP, who needs an immediate protection plan throughout Calakmul. Hunting areas are important habitat
refuges for tapir populations and they should be protected in these areas.
Resumen La abundancia relativa de seis especies de ungulados fue obtenida a través de conteos de huellas a lo largo de transectos en tres sitios con cacería y en la parte sur de la Reserva de la Biosfera de Calakmul, una área de alrededor de 350,000 ha donde la cacería ha sido prohibida desde 1989. Tapir, pecarí de labios blancos, pecarí de collar, dos especies de venados temazates y el venado cola blanca fueron las especies estudiadas. Un total de 1708 huellas de ungulados fueron obtenidos a lo largo de 206.1 km (90 transectos) divididos en los cuatro sitios. La abundancia relativa de las especies fue estimada por medio de la tasa de encuentro de huellas (TEH) calculado como el número de huellas encontradas por km de transecto recorrido. La TEH de los venados temazates y el pecarí de collar fue similar entre las áreas con cacería y sin cacería. La TEH para el pecarí de labios blancos fue significativamente mayor en el área sin cacería, mientras que la TEH del venado cola blanca y el tapir fueron significativamente mayor en los sitios con cacería. La actividad de cacería afecta a las especies de ungulados diferente en la región de Calakmul. Los venados temazates, el venado cola blanca y el pecarí de collar están resistiendo mejor la presión de cacería que el pecarí de labios blancos quien necesita un plan inmediato de protección en toda el área. La evidencia sugiere que las áreas con cacería en este estudio son importantes para las poblaciones de tapir, y que se debe prestar atención a su protección en dichas áreas.相似文献
908.
树干附生尖叶拟船叶藓的孢子体分布格局和败育研究 总被引:3,自引:0,他引:3
通过野外定位考察和实验室观察分析,结果显示:(1)贵州梵净山树干附生尖叶拟船叶藓的孢子体种群80%分布在树干的基部;(2)尖叶拟船叶藓的孢子体在其主茎和一到四次分枝上皆有分布,但在一次分枝上分布最多,并且一次分枝上的与主茎或其它各次分枝上分布的之间存在显著差异;(3)孢子体败育发生在胚胎发育时期、孢蒴膨大期或者孢子形成期等3个时期,孢子体败育率为7.3%,败育孢子体的平均生物量为正常成熟的43.3%.表明尖叶拟船叶藓孢子体种群偏少且孢子体败育率低是其重要的生物学特征. 相似文献
909.
910.
《Saudi Journal of Biological Sciences》2022,29(5):3466-3472
The conservation of biological diversity is gaining an increasing amount of global attention. In particular, in Saudi Arabia, conservation actions have become a topic of focus, with many successful initiatives being implemented. Despite these efforts, several wild animal species in Saudi Arabia still require special attention to ensure their long-term conservation and survival. If not effectively controlled, certain challenges could threaten the conservation status of local wildlife. Various conservation strategies are employed in Saudi Arabia to control these challenges, minimize their influence, and promote conservation action. Such strategies have proven to be effective; however, further efforts are still required, particularly outside protected areas. The conservation of species in critical situations primarily depends on the creation of protected areas and human intervention. Wildlife conservation is a collaborative effort; every individual has a role to play in protecting wild animals in each unique ecosystem to ensure their survival and the sustainability of their habitats for future generations. 相似文献