全文获取类型
收费全文 | 8191篇 |
免费 | 781篇 |
国内免费 | 1041篇 |
出版年
2024年 | 46篇 |
2023年 | 250篇 |
2022年 | 221篇 |
2021年 | 315篇 |
2020年 | 343篇 |
2019年 | 441篇 |
2018年 | 307篇 |
2017年 | 311篇 |
2016年 | 340篇 |
2015年 | 313篇 |
2014年 | 363篇 |
2013年 | 569篇 |
2012年 | 317篇 |
2011年 | 389篇 |
2010年 | 304篇 |
2009年 | 400篇 |
2008年 | 387篇 |
2007年 | 425篇 |
2006年 | 417篇 |
2005年 | 398篇 |
2004年 | 341篇 |
2003年 | 302篇 |
2002年 | 275篇 |
2001年 | 224篇 |
2000年 | 230篇 |
1999年 | 194篇 |
1998年 | 176篇 |
1997年 | 128篇 |
1996年 | 128篇 |
1995年 | 110篇 |
1994年 | 106篇 |
1993年 | 107篇 |
1992年 | 121篇 |
1991年 | 94篇 |
1990年 | 76篇 |
1989年 | 68篇 |
1988年 | 51篇 |
1987年 | 47篇 |
1986年 | 35篇 |
1985年 | 46篇 |
1984年 | 47篇 |
1983年 | 26篇 |
1982年 | 45篇 |
1981年 | 43篇 |
1980年 | 31篇 |
1979年 | 25篇 |
1978年 | 14篇 |
1975年 | 10篇 |
1974年 | 13篇 |
1973年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Kamlesh R. Chauhan 《Biocontrol Science and Technology》2008,18(7):727-731
Field-collected female goldeneyed lacewing (Chrysopa oculata) strongly avoided ovipositing on substrates exposed over 24 h to three of four lady beetle species tested, resulting in circa 3-fold higher egg deposition on control versus coccinellid-exposed substrates. The effect was stage-specific for Coleomegilla maculata; it was observed for adult and third-instar larvae but not for unfed first-instar larvae. 相似文献
942.
Studies were conducted to determine the natural occurrence of entomopathogenic fungi and their potential for utilisation in biological control of the woolly aphid (Ceratovacuna lanigera Zehntner) in Indian sugarcane plantations. A fungus Gibberella fujikuroi (Sawada) Wr. (Synonym: Fusarium verticillioides (Sacc.) Nirenberg, Synonym: Fusarium moniliforme (Sheld.) emend. Snyd. and Hans.) was found to be a promising biocontrol agent. It was further characterised and its field performance was studied. Reductions of aphid populations up to 60% were observed when the fungus was applied as two sprays at a week interval in the field. 相似文献
943.
Summary . The Cox hazards model ( Cox, 1972 , Journal of the Royal Statistical Society, Series B 34, 187–220) for survival data is routinely used in many applied fields, sometimes, however, with too little emphasis on the fit of the model. A useful alternative to the Cox model is the Aalen additive hazards model ( Aalen, 1980 , in Lecture Notes in Statistics-2 , 1–25) that can easily accommodate time changing covariate effects. It is of interest to decide which of the two models that are most appropriate to apply in a given application. This is a nontrivial problem as these two classes of models are nonnested except only for special cases. In this article we explore the Mizon–Richard encompassing test for this particular problem. It turns out that it corresponds to fitting of the Aalen model to the martingale residuals obtained from the Cox regression analysis. We also consider a variant of this method, which relates to the proportional excess model ( Martinussen and Scheike, 2002 , Biometrika 89, 283–298). Large sample properties of the suggested methods under the two rival models are derived. The finite-sample properties of the proposed procedures are assessed through a simulation study. The methods are further applied to the well-known primary biliary cirrhosis data set. 相似文献
944.
Summary . We develop methods for competing risks analysis when individual event times are correlated within clusters. Clustering arises naturally in clinical genetic studies and other settings. We develop a nonparametric estimator of cumulative incidence, and obtain robust pointwise standard errors that account for within-cluster correlation. We modify the two-sample Gray and Pepe–Mori tests for correlated competing risks data, and propose a simple two-sample test of the difference in cumulative incidence at a landmark time. In simulation studies, our estimators are asymptotically unbiased, and the modified test statistics control the type I error. The power of the respective two-sample tests is differentially sensitive to the degree of correlation; the optimal test depends on the alternative hypothesis of interest and the within-cluster correlation. For purposes of illustration, we apply our methods to a family-based prospective cohort study of hereditary breast/ovarian cancer families. For women with BRCA1 mutations, we estimate the cumulative incidence of breast cancer in the presence of competing mortality from ovarian cancer, accounting for significant within-family correlation. 相似文献
945.
946.
In this paper we address the effects of anthropogenic disturbance and replacement of Brazilian Coastal sandy vegetation (restingas) on dung beetles communities. We sampled dung beetles in the four main vegetative physiognomies of Guriri Island, Espírito Santo State: forest restinga, restinga Clusia, disturbed restinga (from burning events), and pastures. We placed four sets of two pitfall traps (baited with horse and human dung) in four independent areas of each vegetation type, and collected 14,534 individuals of 13 dung beetle species. Neither log10 of individuals nor log10 of species richness were good predictors of restinga disturbance. However, a significant amount of variation in dung beetle abundance and richness could be explained by bait type. Ordination of these sites using hybrid multidimensional scaling revealed a gradient of habitat disturbance from undisturbed restinga samples to pasture. Dung beetle communities along this gradient demonstrated a complete turnover in species composition, from restinga‐specialists to invasive and generalists species respectively. This complete turnover signals the local extirpation of forest‐adapted species in disturbed and converted areas. Only a single dung beetle species in preserved restingas is protected by Brazilian law (Dichotomius schiffleri). Given the extent of the clearing of restinga habitat, the conservation status of dung beetles associated with restinga forest gives cause for concern. 相似文献
947.
Stochastic P systems and the simulation of biochemical processes with dynamic compartments 总被引:3,自引:0,他引:3
We introduce a sequential rewriting strategy for P systems based on Gillespie's stochastic simulation algorithm, and show that the resulting formalism of stochastic P systems makes it possible to simulate biochemical processes in dynamically changing, nested compartments. Stochastic P systems have been implemented using the spatially explicit programming language MGS. Implementation examples include models of the Lotka-Volterra auto-catalytic system, and the life cycle of the Semliki Forest virus. 相似文献
948.
Nicki J. Whitehouse Peter G. Langdon Richard Bustin Sarah Galsworthy 《Biodiversity and Conservation》2008,17(9):2055-2078
We review the uses of fossil insects, particularly Coleoptera (beetles) and Chironomidae (non-biting midges) from ancient
deposits to inform the study of wetland ecosystems and their ecological and restoration processes. In particular, we focus
on two contrasting ecosystems, drawing upon research undertaken by us on British raised mire peats and shallow lake systems,
one an essentially terrestrial ecosystem, the other aquatic, but in which wetland insects play an important and integral part.
The study of raised mire peats suggests that faunal stability is a characteristic of these wetland systems, over what appear
to be extensive periods of time (up to several millennia), whilst studies of shallow lake ecosystems over recent timescales
indicates that faunal instability appears to be more common, usually driven by increasing eutrophication. Drawing upon a series
of fossil Coleoptera records spanning several thousand years from Hatfield Moors, south Yorkshire, we reconstruct in some
detail the mire’s ontogeny and fluctuations in site hydrology and vegetation cover, illustrating the intimate association
between substrate, topography and peat development. A comparison between fossil and modern beetle populations indicates that
the faunal characteristics of this mire and its adjacent neighbour, Thorne Moors, become established during the early phases
of peat development, including its rare endemics, and that the faunal biodiversity on the sites today is dictated by complex
site histories. The over-riding characteristic of these faunas is of stability over several thousand years, which has important
implications for the restoration of degraded sites, especially those where refugial areas are limited. In contrast, analyses
of fossil Chironomidae from shallow lakes allow researchers to track changes in limnological status and while attempts have
been made to reconstruct changes in nutrient levels quantitatively, the chironomids respond indirectly to such changes, typically
mediated through complex ecosystem dynamics such as changes in fish and/or macrophyte communities. These changes are illustrated
via historic chironomid stratigraphies and diversity indices from a range of shallow lakes located across Britain: Slapton
Ley, Frensham Great Pond, Fleet Pond, Kyre Pool and Barnes Loch. These sites have shown varying degrees of eutrophication
over recent timescales which tends to be associated with a decline in chironomid diversity. While complex functional processes
exist within these ecosystems, our evidence suggests that one of the key drivers in the loss of shallow lake chironomid diversity
appears to be the loss of aquatic macrophytes. Overall, while chironomids do show a clear response to altered nutrient regimes,
multi-proxy reconstructions are recommended for a clear interpretation of past change. We conclude that if we are to have
a better understanding of biota at the ecosystem level we need to know more of the complex interactions between different
insect groups as well as with other animal and plant communities. A palaeoecological approach is thus crucial in order to
assess the role of insect groups in ecosystem processes, both in the recent past and over long time scales, and is essential
for wetland managers and conservation organisations involved in long term management and restoration of wetland systems 相似文献
949.
950.
Sarah A. Bailey Kanavillil Nandakumar † Ian C. Duggan ‡ Colin D. A. van Overdijk Thomas H. Johengen David F. Reid Hugh J. MacIsaac 《Diversity & distributions》2005,11(5):453-460
Ships that enter the Great Lakes laden with cargo carry only residual ballast water and sediment in ballast tanks. These ships are designated ‘no ballast on board’ (NOBOB) and constitute > 90% of inbound traffic. We conducted in situ experiments using emergence traps to assess the viability and the introduction potential of invertebrate diapausing stages present in ships’ ballast sediment. All trials commenced while vessels operated on the lower lakes (Erie, Ontario) and were completed 6–11 days later at ports on the upper lakes (Michigan, Lake Superior). Eight trials were conducted on four ships using five different ballast sediments. Hatching was observed on every ship, although not from all sediments on all ships. Overall hatch rates were very low (0.5 individuals per 500 g sediment), typically involving activation of < 0.05% of total eggs present. Five species of rotifers and copepod nauplii were hatched from ballast sediments, although only one or two species typically hatched from any one sediment. Results of this study indicate that hatching of diapausing eggs contained in ballast sediment of NOBOB ships poses a relatively low risk of invasion to the Great Lakes. However, as reproduction may occur in tanks, and non‐indigenous species may be involved in numerous introduction events, the risk posed by this vector is small but potentially important. While dormancy is a characteristic enabling enhanced survival during transportation in ballast tanks, it becomes a hindrance for introduction. 相似文献