全文获取类型
收费全文 | 2154篇 |
免费 | 215篇 |
国内免费 | 227篇 |
专业分类
2596篇 |
出版年
2024年 | 10篇 |
2023年 | 25篇 |
2022年 | 29篇 |
2021年 | 38篇 |
2020年 | 52篇 |
2019年 | 79篇 |
2018年 | 71篇 |
2017年 | 90篇 |
2016年 | 67篇 |
2015年 | 64篇 |
2014年 | 78篇 |
2013年 | 172篇 |
2012年 | 96篇 |
2011年 | 84篇 |
2010年 | 70篇 |
2009年 | 123篇 |
2008年 | 112篇 |
2007年 | 136篇 |
2006年 | 119篇 |
2005年 | 117篇 |
2004年 | 87篇 |
2003年 | 80篇 |
2002年 | 82篇 |
2001年 | 76篇 |
2000年 | 68篇 |
1999年 | 61篇 |
1998年 | 51篇 |
1997年 | 44篇 |
1996年 | 28篇 |
1995年 | 35篇 |
1994年 | 21篇 |
1993年 | 34篇 |
1992年 | 33篇 |
1991年 | 32篇 |
1990年 | 24篇 |
1989年 | 24篇 |
1988年 | 28篇 |
1987年 | 19篇 |
1986年 | 26篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 10篇 |
1982年 | 18篇 |
1981年 | 24篇 |
1980年 | 9篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有2596条查询结果,搜索用时 15 毫秒
41.
Impacts of dust on plant health,survivorship and plant communities in semi‐arid environments 下载免费PDF全文
Mamoru Matsuki Mark R. Gardener Andrew Smith Robert K. Howard Aaron Gove 《Austral ecology》2016,41(4):417-427
There is a general perception that dust accumulation on plant surfaces causes negative impacts to plants. Consequently, it is common for environmental regulatory agencies to apply vegetation monitoring requirements to oil, gas and mining developments. We use two independent, medium‐term monitoring studies in semi‐arid Australia to examine this relationship at two scales: plant health and survivorship of a threatened subspecies (Tetratheca paynterae paynterae: Elaeocarpaceae) at Windarling Range between 2003 and 2014; and changes in plant health and floristic composition on Barrow Island between 2009 and 2014. Accumulation of dust decreased rapidly with distance from source. At Windarling Range, even at the site with the highest dust load, there was no significant impact on Tetratheca paynterae paynterae compared with the less dusty sites for 10 years. Similarly, there was no significant effect between distance from the source of dust and floristic composition on Barrow Island for 5 years. The probability of plants transitioning to a lower health condition between one year and the next did not appear to be related to dust load. This is further supported by comparing the same site before and after paving the road (removal of dust source), which showed no clear trends. Trends in plant health are likely to be driven more by the variability of cumulative rainfall in the preceding 5 months than dust load. The observed temporal variation in the mean dust load may also be related to variation in rainfall. In conclusion, in these case studies from semi‐arid Australia, we find no evidence to support the perception that, under the observed climatic condition and dust deposition rates up to 20 or 77 g m?2 per month at Windarling Range and Barrow Island, respectively, dust accumulation on plants causes negative impacts. 相似文献
42.
Significant variations in four biological measures of water quality with stream order and river basin were demonstrated for streams of the Black and Osage river basins of Missouri. Water quality criteria specific for each order and basin were then developed.Benthic macroinvertebrates from springs and stream orders 3–8 in the two river basins were sampled quarterly for one year with riffle nets and artificial substrate samplers. A total of 548 samples were taken at 137 stations. The average annual macroinvertebrate density, index of diversity, number of taxa, and number of mayfly and stonefly taxa were determined for each station. These measures showed significant differences (p < 0.05) across stream order within and between the two river basins. Total taxa, total mayfly and stonefly taxa, and diversity were highest in orders 4 and 5 with decreased values in lower and higher stream orders. Maximum organism densities occurred in intermediate order streams. These differences were attributed to the succession of physical changes from headwaters to mouth within each river and to the unique geomorphology of each catchment basin.Water quality criteria based on three of the four measures described above (with 95% confidence limits) were established for each stream order in each river basin. Criteria for the Osage River basin were then used to identify three streams in the basin affected by environmental disturbances (stream impoundment, channelization and sewage discharge). The use of order- and basin-specific criteria assures that the biological differences between streams caused by environmental disturbance can be distinguished from the natural biological differences between streams of different orders and drainages. 相似文献
43.
Feral cat Felis catus predation on seabirds has been well documented; however, details regarding shifts in feral cat diet in relation to seabird availability, seabird predation rate and impact on seabird population dynamics are scarce. Here, we present data documenting a seasonal shift in feral cat diet at Juan de Nova Island, Mozambique Channel. We also quantify sooty tern Sterna fuscata predation by feral cats and examine the impact on sooty terns over both the short term (by removing individual cats from sub-colonies) and over the longer term by highlighting their influence on population growth rate ( λ ) using a deterministic matrix model. Cat diet shifted dramatically from insects, rats and mice outside the tern breeding season to primarily terns when terns were breeding. The predation rate of sooty terns at Juan de Nova was estimated at 5.94 terns cat−1 day−1 , with a proportion of these (22%) being killed without being consumed ('surplus kills'). When only one cat was removed from each sub-colony, tern predation declined tenfold in the short term. From our matrix model, the annual growth rate for sooty terns was 1.01 in the absence of cat predation. It remained above one until a predation impact equivalent to approximately three times the estimated cat density (12.04 per km2 ) was incorporated. Our results demonstrate that cats preferentially predate and have an impact on breeding sooty terns at Juan de Nova, and that an increase in cat density could lead to negative effects on population growth, despite the large breeding tern population. 相似文献
44.
Gene E. Likens Donald C. Buso Brian K. Dresser Emily S. Bernhardt Robert O. Hall Jr Kate H. Macneale Scott W. Bailey 《Restoration Ecology》2004,12(3):419-428
Ground and pelletized Wollastonite (Wo; CaSiO3) was added to a 50‐m reach of an anthropogenically acidified stream within the Hubbard Brook Experimental Forest, New Hampshire, to evaluate its buffering and restoration potential. The Wo was highly effective in raising the pH, acid‐neutralizing capacity (ANC), dissolved inorganic carbon (DIC), and Ca2+ concentrations of the stream water, but during the short duration of the experiment had no discernable effect on the stream biota. After initial, spike‐like fluctuations in pH and concentrations of ANC, DIC, and Ca2+, the relatively slow dissolution rates of the Wo dampened extreme concentrations and contributed to relatively long‐lasting (4 months) amelioration of streamwater acidity. Changes in concentrations of Ca2+, dissolved Si, ANC, and DIC were inversely related to streamflow. After several high, stream‐discharge events, concentrations quickly and consistently returned to pre‐event conditions. 相似文献
45.
Competition is believed to be a central force limiting local diversity and controlling the structure of plant communities.
However, it has been proposed that the stressed understory environment limits total understory plant density to such low levels
that competitive exclusion cannot be an important factor limiting the local diversity of understory plants. To evaluate the
importance of inter-seedling competition, we performed a seedling competition experiment with five shade-tolerant species
in a tropical moist forest in Panama. Three-month-old seedlings were transplanted into the forest singly or with their roots
intertwined with a single conspecific or heterospecific seedling in all pairwise species combinations. If competition is important,
performance (survival, stem height, and number of leaves after one and six years) would be expected to be lowest with a conspecific
neighbor and greatest without a neighbor. The experiment was replicated in five 0.24-m2 plots at each of 20 sites in tall secondary forest. To test whether seedling performance differed among treatments we fitted
linear mixed models (LMM) and generalized linear mixed models (GLMM), treating species identity and microsite (site and plot)
as random effects. The five shade-tolerant study species all experienced good establishment with relatively high survival
and growth rates. The neighbor treatment consistently affected seedling performance, but the effect was always very small,
both in absolute terms and relative to the much stronger species and microsite effects. Seedlings with a conspecific neighbor
consistently performed worse than seedlings with a heterospecific neighbor, but having no neighbor generally did not cause
superior performance relative to the other treatments. We conclude that direct competitive interactions are relatively unimportant
among understory plants in humid tropical forests. 相似文献
46.
中国大陆青冈分布区东缘大金山岛种群种子形态变异 总被引:3,自引:0,他引:3
在大陆青冈分布区东缘的大金山岛种群中按不同坡向取样,旨在揭示相对均一条件下青冈种子的形态变异及变异的来源,以从不同侧面研究环境对形态变异的作用。大金山岛种群青冈种子的长度、直径和重量分别为1.557±0.128cm、1.175±0.112cm、1.321±0.310g,处于青冈种子正常分布范围内。单颗种子的长度、直径符合正态分布,而重量则明显偏离正态分布。不同坡向青冈种子间差异很小,表明微地形的影响较小;种群变异的来源株内和株间约各占一半,说明种子表型具有一定的可塑性,主要由于种子处于植株不同部位的缘故。 相似文献
47.
48.
In Central Kalimantan, Indonesia, the water of lakes and rivers showed high dissolved organic carbon (DOC) concentration,
5-50mgC/l. There was a clear relationship between DOC concentration and pH. DOC also contributed to low water transparency
in the studied lakes. Water colors measured by spectrophotometer showed a strong relationship with DOC concentration, indicating
high applicability of water color measurement for estimation of DOC. The development of a simple but quick estimation of DOC
will contribute to understanding the seasonal dynamics of DOC, which might regulate both abiotic and biotic conditions in
aquatic ecosystems in this area. 相似文献
49.
J. LOBON-CERVIA† 《Functional ecology》2007,21(1):117-124
50.
为探究滨豇豆(Vignamarina)的生态生物学特性及其对热带珊瑚岛的适应性,对西沙群岛野生滨豇豆的叶片形态解剖结构、生理特征和营养成分进行了分析。结果表明,滨豇豆具有叶片厚、比叶面积小、栅栏组织发达、气孔密度大、气孔面积指数大等形态特征,对减少蒸腾、保持水分起到重要作用。滨豇豆叶片的SOD活性和脯氨酸(Pro)含量高,丙二醛(MDA)含量低,表明抗氧化能力较强;滨豇豆叶片养分含量适中但其生境土壤养分含量很低,说明其对养分有着良好的吸收利用能力,利于适应贫瘠的环境。因此,滨豇豆具有能够较好地适应珊瑚岛礁高温、干旱、贫瘠生境的形态解剖结构和生理特征,加之其具有良好的固氮和养分利用能力,可作为热带珊瑚岛植被恢复的工具种。 相似文献