全文获取类型
收费全文 | 2112篇 |
免费 | 94篇 |
国内免费 | 179篇 |
专业分类
2385篇 |
出版年
2023年 | 32篇 |
2022年 | 40篇 |
2021年 | 38篇 |
2020年 | 37篇 |
2019年 | 48篇 |
2018年 | 74篇 |
2017年 | 45篇 |
2016年 | 47篇 |
2015年 | 43篇 |
2014年 | 92篇 |
2013年 | 138篇 |
2012年 | 99篇 |
2011年 | 81篇 |
2010年 | 67篇 |
2009年 | 103篇 |
2008年 | 117篇 |
2007年 | 97篇 |
2006年 | 90篇 |
2005年 | 95篇 |
2004年 | 67篇 |
2003年 | 66篇 |
2002年 | 72篇 |
2001年 | 35篇 |
2000年 | 37篇 |
1999年 | 35篇 |
1998年 | 29篇 |
1997年 | 27篇 |
1996年 | 32篇 |
1995年 | 30篇 |
1994年 | 36篇 |
1993年 | 28篇 |
1992年 | 30篇 |
1991年 | 26篇 |
1990年 | 16篇 |
1989年 | 24篇 |
1988年 | 25篇 |
1987年 | 21篇 |
1986年 | 24篇 |
1985年 | 44篇 |
1984年 | 38篇 |
1983年 | 23篇 |
1982年 | 42篇 |
1981年 | 33篇 |
1980年 | 25篇 |
1979年 | 22篇 |
1978年 | 19篇 |
1977年 | 11篇 |
1976年 | 14篇 |
1975年 | 9篇 |
1974年 | 8篇 |
排序方式: 共有2385条查询结果,搜索用时 15 毫秒
1.
2.
3.
A new nematode genus, Nothacrobeles, is proposed in the subfamily Acrobelinae. Four new species are described and one new combination made. The five species exhibit a progression from short-to-long, bifurcate, elaborately fringed labial probolae, the longest probolae resembling those of species of Acrobeles. 相似文献
4.
5.
6.
Ishizue Adachi 《Population Ecology》1989,31(2):343-352
The study was carried out in two (A and B) citrus groves to clarify the spatial distribution patterns of eggs and larvae, and to analyse the mortality process of eggs. From the analysis by using the mean density and the mean crowding, it was clarified that the distributions of eggs were contagious and that larvae were more contagiously distributed than eggs. The τ and z indices showed that the operation of egg mortality was inversely density-dependent in both groves, and that the degree of inverse density-dependence was greater in A than in B grove. The spatial correlations between the emergence holes and the eggs or larvae in each tree, which were analysed by using ω index, showed that the distributions were more overlapping between the emergence holes and the larvae than the eggs. As the result of dividing trees into several groups according to the number of emergence holes, it was clarified that the survival rates of eggs were positively correlated with the number of emergence holes. In conclusion, inversely density-dependent mortality process was considered to be caused by lower mortality rates of eggs in the trees with more emergence holes. Especially in A grove, because the trees with more emergence holes were larger in diameter and more egg oviposition, the inversely density-dependent mortality process was considered to be detected more conspicuously than in B grove. 相似文献
7.
The effects of invasive macrophytes, water level fluctuations and predation on freshwater unionids Pyganodon grandis and Utterbackia imbecillis were studied in three small impoundments in Northeastern Texas in 2003–2005. Mussel density was sampled with quadrats. Mortality,
associated with the water level fluctuations and predation, was estimated by collecting dead shells on the shore at about
two month intervals. In two ponds, horizontal distribution of unionids was limited by dense beds of invasive and noxious macrophytes
(mainly Eurasian watermilfoil Myriophyllum spicatum and American lotus Nelumbo lutea): mussel densities were significantly lower in these macrophyte beds (P < 0.001). In the third pond with the lowest density of macrophytes (stonewort Chara sp.), unionids were distributed more evenly, and the average unionid biomass was the highest among all ponds studied. Vertical
distribution of unionids in all ponds was likely limited by low oxygen at depth >2 m. The total amount of shells found on
the shore per year varied from 0.1% to 28% of the total population in the pond and was negatively correlated with water level
(r = −0.72 to −0.81, P < 0.005). Mammalian predators consumed up to 19% of the total unionid population and predation was facilitated by water level
fluctuations.
Handling editor: K. Martens 相似文献
8.
- Orchids are distributed around the world, however, the factors shaping their specific distribution and habitat preferences are largely unknown. Moreover, many orchids are at risk of becoming threatened as landscapes change, sometimes declining without apparent reason. One important factor affecting plant distribution is nutrient levels in the environment. Nitrates can inhibit not only orchid growth and persistence, but also seed germination.
- We used in vitro axenic cultures to exactly determine the germination sensitivity of seven orchid species to nitrates and correlated this with soil properties of the natural sites and with the species’ habitat preferences.
- We found high variation in response to nitrate between species. Orchids from oligotrophic habitats were highly sensitive, while orchids from more eutrophic habitats were almost insensitive. Sensitivity to nitrate was also associated with soil parameters that indicated a higher nitrification rate.
- Our results indicate that nitrate can affect orchid distribution via direct inhibition of seed germination. Nitrate levels in soils are increasing rapidly due to intensification of agricultural processes and concurrent soil pollution, and we propose this increase could cause a decline in some orchid species.
9.
In soil micromorphology fissures are considered in vertical sections. To get information about the properties of the soil the joint distribution of spatial direction and width of these fissures is of interest. The fissures are mathematically generalized to flat bodies which are defined as stationary weighted surface processes with the weight “thickness”. In a typical point of the surface process suitable, joint parametric distributions of direction and thickness are assumed. The parameters have to be estimated from measurements on vertical sections which are taken from the soil. On these sections only a visible thickness and a visible angle can be observed. The joint distribution of these variables can be expressed by the joint distribution of spatial direction and thickness with the same parameters and in this indirect way the parameters can be estimated. The paper describes how to randomize the vertical section and how to measure the visible variables on the sections. The Chi-Square method is proposed for the parameter estimation. Further it is discussed how to derive good starting values for the numerical procedure. All this is demonstrated in a simulation study using the Bingham-Mardia distribution for the direction and the lognormal distribution for the thickness including a way to correlate the mean thickness and the direction. Finally an application in soil micromorphology is demonstrated for one soil horizon. 相似文献
10.
Résumé Le problème de la lutte biologique en forêt, par utilisation d'entomophages indigènes, est discuté dans le cas d'un hyménoptère,
le diprion du pin,Diprion pini L.
Des études de dynamique des populations montrent que le schéma de progression spatiale des populations de cet insecte dans
le sud du Bassin Parisien semble se renouveler à chaque pullulation. De plus les parasites représentent un véritable “facteur
clé”. Leur incidence varie notamment de génération en génération: en faible nombre au début d'une gradation, ils ne peuvent
alors en empêcher le développement; mais ensuite leur multiplication assure une régression brutale du ravageur.
Dans ces conditions, des interventions assez peu onéreuses et faciles à réaliser, comme de simples transports de matériel
parasité pourraient être efficaces; elles supposent toutefois le développement d'un foyer initial d'infestation. En revanche
l'élevage d'espèces régulatrices commeAchrysocharella ruforum Krausse etPleolophus basizonus Grav. permettrait d'assurer la préservation permanente des massifs forestiers.
Summary The spatial pattern of insect populations progression in southern “Bassin Parisien”, showed by population dynamics studies, seems to happen again for every outbreak. Parasites are moreover a true “key-factor”. Their effect is quite different from one generation to another one; scarce at the beginning of an outbreak, they cannot stop its evolution; but then, their increase allows a brutal collape of the pest. In such a case, cheap and easily achieved operations as simple carryings of parasited insects may be efficient, but they need the development of a first infestation focus. In return breeding of regulating species asAchrysocharella ruforum Krausse andPleolophus basizonus Grav. would be able to sure forest stands permanently.相似文献