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71.
72.
Shrub encroachment in North American grasslands: shifts in growth form dominance rapidly alters control of ecosystem carbon inputs 总被引:4,自引:0,他引:4
ALAN K. KNAPP JOHN M. BRIGGS† SCOTT L. COLLINS‡ STEVEN R. ARCHER§ M. SYNDONIA BRET-HARTE¶ BRENT E. EWERS DEBRA P. PETERS DONALD R. YOUNG†† GAIUS R. SHAVER‡‡ ELISE PENDALL MEAGAN B. CLEARY 《Global Change Biology》2008,14(3):615-623
Shrub encroachment into grass-dominated biomes is occurring globally due to a variety of anthropogenic activities, but the consequences for carbon (C) inputs, storage and cycling remain unclear. We studied eight North American graminoid-dominated ecosystems invaded by shrubs, from arctic tundra to Atlantic coastal dunes, to quantify patterns and controls of C inputs via aboveground net primary production (ANPP). Across a fourfold range in mean annual precipitation (MAP), a key regulator of ecosystem C input at the continental scale, shrub invasion decreased ANPP in xeric sites, but dramatically increased ANPP (>1000 g m−2 ) at high MAP, where shrub patches maintained extraordinarily high leaf area. Concurrently, the relationship between MAP and ANPP shifted from being nonlinear in grasslands to linear in shrublands. Thus, relatively abrupt (<50 years) shifts in growth form dominance, without changes in resource quantity, can fundamentally alter continental-scale pattern of C inputs and their control by MAP in ways that exceed the direct effects of climate change alone. 相似文献
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ALAN W. OWEN DAVID A.T. HARPER RACHEL A. HEATH 《Lethaia: An International Journal of Palaeontology and Stratigraphy》2008,41(2):173-184
The shelly fauna of the lowest part of the Sælabonn Formation in the Hadeland district of the Oslo Region provides a rare insight into the shallow-water biota of the earliest Silurian and hence the start of biotic recovery after the end Ordovician extinction event. It is dominated by the brachiopods Dalmanella cf. pectinoides Bergström, Coolinia cf. columbana (Reed), Leptaena cf. haverfordensis Bancroft and Zygospiraella scotica (Salter) together with the trilobite Acernaspis elliptifrons (Esmark). The first three species in this unique association belong to eurytopic Ordovician survivor genera which were also present in the underlying unit in Hadeland and continued to thrive during the Silurian. Significantly, Zygospiraella and Acernaspis have no unequivocal Ordovician record but diversified rapidly and became common during the early Silurian (Rhuddanian) in many parts of the world. Moreover, Acernaspis and Zygospiraella were highly eurytopic, pioneer taxa that were among the first elements of the Rhuddanian shelly fauna to appear in many parts of the world especially around the margins of the remnant Iapetus Ocean, following the drowning of areas previously emergent during the late Ordovician sea-level lowstand. The Hadeland fauna provides evidence of a previously undocumented route (among several) by which life ultimately returned to the global diversity plateau established in the mid-Ordovician. 相似文献
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SARAH E. STEWART ALAN W. OWEN 《Lethaia: An International Journal of Palaeontology and Stratigraphy》2008,41(2):139-146
A deep‐water Konservat Lagerstätte from the lower Caradoc (Sandbian) at Girvan is dominated by the trilobite Diacanthaspis trippi, the carpoids Anatifopsis n. sp.? and a new genus of ctenocystoid together with the polyplacophoran Solenocaris solenoides and the brachiopod Onniella williamsi. Most of these are multi‐element organisms, with many specimens preserved in an articulated state in finely laminated rocks, indicating minimal disturbance and suggesting that the fauna is largely an in situ association. It contains few of the species known from other deep‐water sites of similar age at Girvan which contain diverse assemblages of trilobites and brachiopods absent from the Lagerstätte. The taphonomy of the site indicates preservation by rapid burial followed by early diagenesis under dysaerobic conditions. It provides a ‘taphonomic window’ on otherwise unknown faunas from distal shelf facies on the Ordovician Laurentian margin, and, moreover, is an important reminder of the hidden biodiversity that resided in thin‐shelled, multi‐element organisms. 相似文献
77.
A falconiform Miocene fossil bird from North America, represented by a nearly complete tarsometatarsus with phalanges in place, is that of a hawk that converged on the living Secretarybird Sagittarius serpentarius. It is described as a new genus and species, Apatosagittarius terrenus. The palaeoenvironment indicates a Miocene setting similar to the present day habitat of the living Secretarybird, characterized by extensive grasslands and savannas. 相似文献
78.
ALAN MOLUMBY 《Ecological Entomology》1995,20(2):159-168
Abstract.
- 1 Life tables and rates of parasitism were tabulated from mud nests built by Trypoxylon politum (Hymenoptera: Sphecidae) at nine different nesting sites from Missouri and Mississippi.
- 2 Most developmental mortality occurred either during the first two instars of development, or during the inactive prepupal phase. The majority (76%) of deaths were caused by insect parasitoids and cleptoparasites. Levels of parasitism and survivorship varied among nesting sites, and among locations within the two sites surveyed at a fine spatial scale.
- 3 Total developmental mortality, K, was positively associated with the number of hosts (immature T.politum) per site. Within one of two sites sampled at a fine spatial scale, K was negatively associated with the local density of hosts. Levels of total parasitism were positively associated with host population size, and negatively associated with local host density within one of the two sites sampled at a fine spatial scale.
- 4 Levels of parasitism by Melittobia (Hymenoptera: Eulophidae) were positively associated with the number of hosts per site, but negatively associated with the local density of hosts within sites. Melittobia parasitism was also negatively associated with the local density of old nesting material within sites.
- 5 Parasitism by Melittobia was a function of both the numbers of nests per quadrat and the mean nest size per quadrat at one of the two sites surveyed at a fine scale. At the other site, parasitism by Melittobia was a function of mean nest size per quadrat.
- 6 The life cycle and nesting behaviour of T.politum, in relation to the regulation of its numbers, is discussed.
79.
Abstract.
- 1 We describe a new technique for sampling arthropods from grassland and other vegetation, using a modified petrol-driven suction apparatus called a‘Blow & Vac’.
- 2 The new apparatus was tested by comparing its efficiency in extracting arthropods from vegetation with that of a conventional Dietrick, or‘D-Vac’, suction sampler.
- 3 Unimpeded air velocity inside the suction tube was approximately 4 times that of the D-Vac.
- 4 In field trials on semi-natural grasslands, the Blow & Vac caught more individuals of most spider and certain beetle species compared to the D-Vac and approximately equal numbers of Auchenorrhyncha.
- 5 We conclude that the‘Blow & Vac’has considerable potential for sampling arthropod populations. Its low cost and weight make it a suitable alternative to the conventional D-Vac apparatus.
80.