全文获取类型
收费全文 | 2834篇 |
免费 | 351篇 |
国内免费 | 259篇 |
出版年
2024年 | 11篇 |
2023年 | 66篇 |
2022年 | 47篇 |
2021年 | 95篇 |
2020年 | 131篇 |
2019年 | 163篇 |
2018年 | 134篇 |
2017年 | 114篇 |
2016年 | 133篇 |
2015年 | 136篇 |
2014年 | 136篇 |
2013年 | 166篇 |
2012年 | 134篇 |
2011年 | 142篇 |
2010年 | 129篇 |
2009年 | 131篇 |
2008年 | 128篇 |
2007年 | 172篇 |
2006年 | 165篇 |
2005年 | 140篇 |
2004年 | 123篇 |
2003年 | 99篇 |
2002年 | 96篇 |
2001年 | 82篇 |
2000年 | 67篇 |
1999年 | 70篇 |
1998年 | 68篇 |
1997年 | 52篇 |
1996年 | 44篇 |
1995年 | 39篇 |
1994年 | 26篇 |
1993年 | 35篇 |
1992年 | 29篇 |
1991年 | 20篇 |
1990年 | 24篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 20篇 |
1986年 | 4篇 |
1985年 | 14篇 |
1984年 | 10篇 |
1983年 | 3篇 |
1982年 | 9篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1974年 | 2篇 |
排序方式: 共有3444条查询结果,搜索用时 15 毫秒
1.
Understanding the state of forest resource utilization in China and correctly evaluating the role and function of forest resources in national economic development are essential for realizing balanced development of forests, the environment, and the economy. This is especially true given the present situation of increasingly scarce forest resources. Using data from Chinese forest industry statistical yearbooks, forestry development reports, and other documents, this paper examines the current state of forest resource utilization in China from the angle of combining quantities and values based on input–output tables. We show that demand for and input use of forest resources varies greatly across industrial sectors; the paper products and furniture manufacturing industries have the greatest direct consumption coefficient for timber use. When considering direct and indirect demand, it is clear that forest resources restrict different industrial sectors in diverse ways. These results provide an important set of reference values regarding the utilization of forest resources and coordinated industrial development in China. 相似文献
2.
Saldanha Bay,South Africa: recovery of Gracilaria verrucosa (Gracilariales,Rhodophyta) 总被引:2,自引:2,他引:0
Since World War II the greater Saldanha Bay lagoon system, South Africa, has been an important Gracilaria producer. Two agar factories, built in the 1960's, used Gracilaria from Saldanha Bay as their raw material. In the early 1970's the industry was destroyed as a result of dredging and marine construction operations to establish a harbor in the bay for loading ore. These environmental changes destroyed stocks and prevented the previously significant beachings of the seaweed from occurring. After a few years of no or very low commercial production, the resource slowly started to recover. The size of Gracilaria drifts increased over the following eight years to approximately one-third of the original output. This trend seems to continue. Although the stocks and resultant drifts are unlikely to recover fully to their original quantity, current production is already sufficient to ensure re-establishment of a seaweed industry in Saldanha Bay. This could have considerable socio-economic impact on the area. 相似文献
3.
4.
《Current biology : CB》2020,30(17):3444-3449.e4
5.
The flowering of Clutia pulchella was studied in a coastal scarp forest community in southern KwaZulu-Natal, South Africa. Sex dependent differences in the
frequency and magnitude of floral output were recorded, with male individuals producing 4.32 times more flowers per leaf axis
than females. Increased node production, as a manifestation of significantly increased branching, allowed for prolific flower
production in males. Energy investment per flower was marginally greater in females (0.045 J/mg), despite copious pollen production
and glandular secretion in males (0.035 J/mg). Differential flower production, reproductive organ dry mass and an unbalanced
branching ratio revealed that “per plant” expenditure towards flowering was more pronounced in males (♀ flowering × 5.04).
The elevated cost of fruit production (♀ flowering × 5.5), inclusive of imperfect fruit set, resulted in overall reproductive
expense being slightly higher for females. Such sex-dependent reproductive investment was shown to have minimal influence
on the population sex ratio, with sex frequencies and nearest neighbor distances yielding a non-significant male bias of 1.16.
No evidence of spatial asymmetry was found.
Received December 1, 1999 Accepted September 27, 2000 相似文献
6.
Leif Egil Loe Atle Mysterud Vebj?rn Veiberg Rolf Langvatn 《Proceedings. Biological sciences / The Royal Society》2009,276(1667):2581-2587
In species with polygynous mating systems, females are regarded as food-limited, while males are limited by access to mates. When local density increases, forage availability declines, while mate access for males may increase due to an increasingly female-biased sex ratio. Density dependence in emigration rates may consequently differ between sexes. Here, we investigate emigration using mark-recovery data from 468 young red deer Cervus elaphus marked in Snillfjord, Norway over a 20-year period when the population size has increased sixfold. We demonstrate a strong negative density-dependent emigration rate in males, while female emigration rates were lower and independent of density. Emigrating males leaving the natal range settled in areas with lower density than expected by chance. Dispersing males moved 42 per cent longer at high density in 1997 (37 km) than at low density in 1977 (26 km), possibly caused by increasing saturation of deer in areas surrounding the marking sites. Our study highlights that pattern of density dependence in dispersal rates may differ markedly between sexes in highly polygynous species. Contrasting patterns reported in small-scale studies are suggestive that spatial scale of density variation may affect the pattern of temporal density dependence in emigration rates and distances. 相似文献
7.
8.
JAMES F. HARWOOD KEHUI CHEN HANS‐GEORG MÜLLER JANE‐LING WANG ROGER I. VARGAS JAMES R. CAREY 《Physiological Entomology》2013,38(1):81-88
The reproductive ability of female tephritids can be limited and prevented by denying access to host plants and restricting the dietary precursors of vitellogenesis. The mechanisms underlying the delayed egg production in each case are initiated by different physiological processes that are anticipated to have dissimilar effects on lifespan and reproductive ability later in life. The egg‐laying abilities of laboratory‐reared females of the Mediterranean fruit fly (Ceratitis capitata Wiedmann) and melon fly (Bactrocera cucurbitae Coquillett) from Hawaii are delayed or suppressed by limiting access to host fruits and dietary protein. In each case, this is expected to prevent the loss of lifespan associated with reproduction until protein or hosts are introduced. Two trends are observed in each species: first, access to protein at eclosion leads to a greater probability of survival and a higher reproductive ability than if it is delayed and, second, delayed host access reduces lifetime reproductive ability without improving life expectancy. When host access and protein availability are delayed, the rate of reproductive senescence is reduced in the medfly, whereas the rate of reproductive senescence is generally increased in the melon fly. Overall, delaying reproduction lowers the fitness of females by constraining their fecundity for the remainder of the lifespan without extending the lifespan. © 2013 The Royal Entomological Society 相似文献
9.
The potential for and constraints on the evolution of compensatory ability in Asclepias syriaca 总被引:2,自引:0,他引:2
To investigate the potential for and constraints on the evolution of compensatory ability, we performed a greenhouse experiment
using Asclepias syriaca in which foliar damage and soil nutrient concentration were manipulated. Under low nutrient conditions, significant genetic
variation was detected for allocation patterns and for compensatory ability. Furthermore, resource allocation to storage was
positively, genetically correlated both with compensatory ability and biomass when damaged, the last two being positively,
genetically correlated with each other. Thus, in the low nutrient environment, compensatory ability via resource allocation
to storage provided greater biomass when damaged. A negative genetic correlation between compensatory ability and plant biomass
when undamaged suggests that this mechanism entailed an allocation cost, which would constrain the evolution of greater compensatory
ability when nutrients are limited. Under high nutrient conditions, neither compensatory ability nor allocation patterns predicted
biomass when damaged, even though genetic variation in compensatory ability existed. Instead, plant biomass when undamaged
predicted biomass when damaged. The differences in outcomes between the two nutrient treatments highlight the importance of
considering the possible range of environmental conditions that a genotype may experience. Furthermore, traits that conferred
compensatory ability did not necessarily contribute to biomass when damaged, demonstrating that it is critical to examine
both compensatory ability and biomass when damaged to determine whether selection by herbivores can favor the evolution of
increased compensation.
Received: 2 April 1999 / Accepted: 21 September 1999 相似文献
10.
Theoretical models of species coexistence between desert mammals have generally been based on a combination of food and microhabitat selection by granivorous rodents. Although these models are applicable in various deserts of the world, they cannot explain resource use by mammals in Neotropical deserts. The present study examines diet composition in a mammal assemblage in the Monte desert, Argentina. The results show that two main strategies are used by these mammals: medium‐sized species (hystricognath rodents: Dolichotis patagonum, Lagostomus maximus, Microcavia australis and Galea musteloides; and an exotic lagomorph: Lepus europaeus) are herbivores, whereas small‐sized species (a marsupial: Thylamys pusillus; and sigmodontine rodents: Graomys griseoflavus, Akodon molinae, Calomys musculinus, Eligmodontia typus) are omnivorous. Small mammals also show a tendency towards granivory (C. musculinus), insectivory (A. molinae and T. pusillus) and folivory (G. griseoflavus). 相似文献