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991.
We studied physiognomy‐specific (i.e., gaps vs. understory) responses of birds to low harvest (18.7 m3/ha), reduced‐impact logging by comparing 3500 mist net captures in control and cut blocks of an Amazonian terra firme forest in Brazil at 20–42 mo postharvest. Species richness did not differ significantly between control (92 species) and cut (85) forest based on rarefaction to 1200 captures. Fifty‐six percent of all species were shared between control and cut forest, compared to the 64 percent shared between control blocks. Higher captures of nectarivores and frugivores in cut forest likely occurred as a consequence of postharvest resource blooms. Higher captures of some insectivores in cut as compared to control forest were unexpected, attributable to increased wandering or shifts from association with midstory to understory as a consequence of habitat alteration. Logging influenced capture rates for 21 species, either consistently, or via positive interaction with physiognomy or time (13 species higher in cut forest and 8 species higher in control forest). Cut understory sites had lower diversity (H′) and scaled dominance than understory and gap sites in control forest. Temporal changes in captures may have resulted from successional dynamics in cut forest: two guilds and three species increased in abundance. Increases in abundances of guilds and particular species were more prevalent in control than in cut forest, suggesting that logging displaced birds to control forest. In general, the effects of logging were relatively minor; low harvest rates and reduced‐impact methods may help to retain aspects of avian biodiversity in Amazon forest understories. 相似文献
992.
993.
JOHN J. GILBERT 《Freshwater Biology》2007,52(8):1417-1426
1. One at a time during the reproductive period of amictic females, oocytes fill with yolk and undergo a mitotic maturation division (oogenesis), are oviposited as single cells, and then develop parthenogenetically into females. Sexual reproduction in Brachionus and several other genera is initiated when amictic females are crowded and oviposit some eggs induced to differentiate into mictic females. Mictic females produce haploid eggs that can develop parthenogentically into males or be fertilised and develop into diapausing embryos called resting eggs. 2. This study examines the time when oocytes in amictic females respond to maternal population density. Is the fate of all oocytes in the germarium irreversibly determined during the early postnatal life of the mother, or is each oocyte labile until just before oviposition? In the former case, the probability of an amictic female producing a mictic daughter at any time throughout her reproductive period would reflect the population density she experienced while young and not that at the time she oviposited an egg. 3. Amictic females of two clones of a Florida strain of B. calyciflorus were cultured singly from birth at a low or high density (in a large or small volume) until about halfway through their reproductive period and then switched (experimental treatment), or not (control treatment), to the other density condition. The results indicate that the female fate of an oocyte is determined by maternal population density during oogenesis. Eggs oviposited soon after transfer from low to high density had the same, or a higher, probability of becoming mictic females compared with those produced by control females kept at the high density; eggs oviposited after transfer from the high to the low density had the same low probability of becoming mictic females as those produced by control females kept at the low density. 4. Control females kept at the high density were less likely to produce mictic daughters as they aged. This decline is not because of a decreased propensity of older females to respond to crowding, as older females responded maximally when transferred from a low to a high population density. 5. As oocytes in amictic females respond to maternal population density only during oogenesis, there is a negligible lag between the population‐density signal in the environment and the commitment to sexual reproduction. This minimises the obligatory two‐generation lag between this signal and production of resting eggs, and thus reduces the possibility that crowding will lead to food limitation before production of these eggs. 相似文献
994.
J. Dumas † J. G. Bassenave M. Jarry ‡ L. Barrière S. Glise 《Journal of fish biology》2007,70(6):1734-1758
Egg-to-fry development and survival of brown trout Salmo trutta were compared in two rivers of the Pyrenean piedmont, the Nive d'Arnéguy, with few human activities, and the Nive des Aldudes, with many anthropogenic activities including 11 fish farms and two sewage treatment plants (STPs). Survival was estimated between spawning (early December) and emergence time (early March) by means of capsules, 7 cm3 in volume, filled with green eggs then inserted into the gravel of artificial redds at spawning sites. In the Nive des Aldudes, three sets of conditions were studied: a fish farm near the springs, a fish farm and an STP in a village, and a series of fish farms and an STP in a village. In each situation, two artificial redds were created upstream and two others downstream from the fish farms. In the Nive d'Arnéguy, four sites were equipped: each of the two downstream sites with two artificial redds, and the two upstream sites with one redd. Substratum characteristics (proportion of fine particles) and the quality of surface and interstitial water (oxygen content, ammonia and nitrite nitrogen) were periodically measured. There was no redd substratum difference between sites upstream and downstream of fish farms. Survival to fry emergence was higher in the Nive d'Arnéguy (63·5%) than in the Nive des Aldudes (47·7%). In this latter river, the nitrogen released from the STPs was 0·5% that from fish farms. Fish farming impaired survival close to fish farm effluents (31·6%), as compared to survival upstream (63·6%), and induced a development delay during the yolk-sac fry stage. These differences were mainly linked to a drop in the dissolved oxygen content in interstitial water induced by the nitrogen flow in surface and interstitial waters. 相似文献
995.
Half-sib groups of juvenile brown trout Salmo trutta (0+ years) having fathers with different characteristics were compared in several respects. Two kinds of experiments were performed, aquarium observations in the laboratory and stocking in a semi-natural stream. It was found in the aquarium experiments that the offspring of fathers with larger adipose fins were more active and were probably better at defending territories. Offspring of males chosen by females had higher growth rate. The feeding rate was higher in juveniles having a less aggressive father or a father that was chosen by the mother. The stream experiments showed that offspring of dominant males had higher growth rates, and that offspring of less aggressive fathers had higher survival. Thus, characteristics of the males (fathers) influences the quality of the offspring, and there seem to be benefits for the females both to choose among potential mates and to spawn with the most dominant male. As these male characteristics are never regarded during the process of artificial breeding in hatcheries, the results indicate a basis for divergence between hatchery and wild populations. 相似文献
996.
L.B. Dobbss L.O. Medici L.E.P. Peres L.E. Pino-Nunes V.M. Rumjanek A.R. Façanha & L.P. Canellas 《The Annals of applied biology》2007,151(2):199-211
The presence of ramified and abundant lateral roots is fundamental to plant growth on highly weathered soils. In this work, the effects of humic acid (HA), fulvic acid and hexanic–methanolic (HM) extract, after alkaline extraction from topsoil of seven different oxisols, on the development of roots of Arabidopsis thaliana was evaluated. Furthermore, we used another emergent plant model, that is, micro‐tom (MT) tomatoes with size similar to that of Arabidopsis to test the effects of HA. It was observed that both humic fractions and the HM extract were able to change the root development, improving the number of lateral roots and their development in comparison to control plants. The promotion of root growth by the three organic matter fractions was higher than that observed by 10?6 mol L?1 indole acetic acid. The treatment of MT tomato mutant, less sensitive to auxin, with HA did not promote the emergence of lateral roots, being an additional indication of auxin‐like activities of HA. However, some organic matter fractions exhibited, besides promotion of lateral roots number, increase in the length of principal root, which is not a typical auxin effect, indicating that these substances could contain other physiologically active substances. 相似文献
997.
A literature review of data on nitrate uptake by phytoplankton suggests that nitrate levels above 20 μmol N·L?1 generally stimulated uptake rates in cultured unicellular algae and natural phytoplankton communities. This phenomenon indicates that phytoplankton cells acclimate to elevated nitrate levels by increasing their uptake capacity in a range of concentrations previously considered to be saturating. Cyanobacteria and flagellates were found to present a considerable capacity for acclimation, with low (0.1–2 μmol N·L?1) half‐saturation values (Ks) at low (5–20 μmol N·L?1) substrate levels and high (1–80 μmol N·L?1) Ks values at high (30–100 μmol N·L?1) substrate levels. However, some diatom genera (Rhizosolenia, Skeletonema, Thalassiosira) also appeared to possess a low affinity nitrate uptake system (Ks between 18 and 120 μmol N·L?1), which can help resolve the paradox of their presence in enriched seas. It follows that present models of nitrate uptake can severely underestimate the effects of high nitrate concentrations on phytoplankton dynamics and development. A more adequate approach would be to consider the possibility of multiphasic uptake involving several phase transitions as nitrate concentrations increased. Because it is a nonlinear phenomenon featuring strong thresholds, this effect appears to override that of other variables, such as irradiance, temperature, and cell size. Within the present context of eutrophication and for a range of concentrations that is becoming more and more ecologically relevant, equations are tentatively presented as a first approach to estimate Ks from ambient nitrate concentrations. 相似文献
998.
The influence of salt and nitrogen on herbivore abundance: direct and indirect effects 总被引:4,自引:0,他引:4
We report on the influence of experimentally increased interstitial salinity and plant nitrogen on the abundance of the delphacid
planthopper, Prokelisia marginata (Van Duzee) (Homoptera: Delphacidae), which feeds on salt marsh cordgrass, Spartina alterniflora. We also report the effects of these treatments on parasitism of P. marginata eggs by the fairyfly parasitoid, Anagrus sophiae (Hymenoptera: Mymaridae). Soil salinity was significantly elevated following the addition of salt pellets broadcast over
the ground and plant foliar nitrogen was significantly increased after the addition of fertilizer. The addition of fertilizer
increased P. marginata densities on Spartina but addition of salt did not. Neither treatment significantly affected levels of egg parasitism by A. sophiae. In this system direct effects of plants on their herbivores via changes in plant chemistry appear more important than indirect
effects of plants on herbivores via their natural enemies.
Received: 1 August 1997 / Accepted: 29 September 1997 相似文献
999.
Animal, fungal, and bacterial consumers can have dramatic effects on the structure of plant communities, often by consuming
dominant competitors and indirectly increasing the abundance of inferior competitors. We investigated the role of a consumer
plant, the parasite Cuscuta salina, on plant zonation in a western salt marsh. Cuscuta had a strong host species preference in experiments, disproportionally infecting Salicornia virginica, the dominant competitor in most of the marsh. In plots with Cuscuta, which infected 18% of our study area over a 3-year period, Salicornia cover decreased and the cover of Arthrocnemum increased substantially in comparison to plots without Cuscuta. Deep in the Salicornia zone, the cover of Arthrocnemum in Cuscuta-infected plots increased by 558% in 1 year relative to uninfected plots. At the ecotone, the cover of Arthrocnemum in Cuscuta-infected plots increased by only 41% during the same time interval. These data suggest that the relative benefit of a consumer
to a less-preferred, subordinate competitor may be strongest where competition is the most asymmetrical as predicted by recent
theoretical models. By weakening the competitive dominant, which in the absence of the parasite can create virtual monocultures,
Cuscuta enhanced community diversity and altered the ecotone between Salicornia and Arthrocnemum. Cuscuta patches were highly dynamic at the ecotone between Salicornia and Arthrocnemum, and thus the changes we measured in our sample plots were likely to be representative of large portions of the marsh. Our
findings emphasize the importance of trophic interactions in salt marsh structure and zonation.
Received: 23 April 1997 / Accepted: 10 October 1997 相似文献
1000.
Interspecific variation in the escape responses of aphids: effect on risk of predation from foliar-foraging and ground-foraging predators 总被引:1,自引:0,他引:1
A series of laboratory experiments was conducted to determine the effect of interspecific differences on prey defensive behavior
on the susceptibility of two aphid species (Acyrthosiphon pisum and A. kondoi) to a ground-foraging predator, Harpalus pennsylvanicus, and a foliar-foraging predator, Coccinella septempunctata. These organisms are representative of a biologically and economically important predator/prey system in alfalfa. The primary
defensive behavior of both aphid species toward C. septempunctata was to “drop” from the plant. Both aphid species were significantly more likely to drop from the plant in the presence of
C. septempunctata. However, when C. septempunctata was present, a significantly lower proportion of A. kondoi individuals dropped (0.42 ± 0.07) compared to A. pisum (0.73 ± 0.08). As a result of their lower propensity to drop from the plant A. kondoi individuals are significantly more likely to be consumed by C. septempunctata. Conversely, the higher propensity of A. pisum individuals to drop increased their susceptibility to ground-foraging predators. When A. pisum was the prey species, ground-foraging predators made a significant contribution to overall aphid suppression and there was
a significant synergistic interaction between ground and foliar-foraging predators. When A. kondoi was the prey there was no interaction between the predator species. As either a cause or consequence of its higher propensity
to drop, A. pisum seems to be more adapted for survival and dispersal off the plant. In comparison to A. kondoi individuals, A. pisum individuals relocate plants more quickly (63 ± 41 s vs. 164 ± 39 s), disperse farther (18 ± 1.7 cm vs. 13 ± 0.66 cm), and
survive longer (37 ± 2.0 h vs. 25 ± 2.0) off the plant. This study demonstrates the importance of prey defensive behavior
in determining the susceptibility of a prey species to a multiple-predator complex.
Received: 24 February 1997 / Accepted:17 December 1997 相似文献