全文获取类型
收费全文 | 1111篇 |
免费 | 133篇 |
国内免费 | 172篇 |
出版年
2024年 | 3篇 |
2023年 | 16篇 |
2022年 | 21篇 |
2021年 | 26篇 |
2020年 | 28篇 |
2019年 | 45篇 |
2018年 | 37篇 |
2017年 | 49篇 |
2016年 | 40篇 |
2015年 | 39篇 |
2014年 | 51篇 |
2013年 | 116篇 |
2012年 | 55篇 |
2011年 | 45篇 |
2010年 | 40篇 |
2009年 | 89篇 |
2008年 | 71篇 |
2007年 | 82篇 |
2006年 | 56篇 |
2005年 | 63篇 |
2004年 | 54篇 |
2003年 | 35篇 |
2002年 | 38篇 |
2001年 | 25篇 |
2000年 | 32篇 |
1999年 | 35篇 |
1998年 | 24篇 |
1997年 | 21篇 |
1996年 | 15篇 |
1995年 | 23篇 |
1994年 | 7篇 |
1993年 | 20篇 |
1992年 | 19篇 |
1991年 | 6篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 11篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 9篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有1416条查询结果,搜索用时 15 毫秒
991.
Donald R. Drake 《植被学杂志》1998,9(1):103-112
Abstract. Hawaiian ecosystems are prone to invasion by alien plant species. I compared the seed rain, seed bank, and vegetation of a native Hawaiian forest to examine the potential role that seed ecology plays in allowing alien species to invade native forest. Absolute cover of seed plants in the forest was 126 %, annual seed rain was 5 713 seeds m-2 yr-1, and the mean density of seedlings emerging from the seed bank averaged across four seasons was 1 020/m2. The endemic tree Metrosideros polymorpha was the most abundant species in the vegetation, seed rain and winter seed bank. Overall, native seed plants comprised 95 % of the relative cover in the vegetation and 99 % of the seeds in the seed rain, but alien species comprised 67 % of the seeds in the seed bank. Alien species tended to form persistent seed banks while native species formed transient or pseudo-persistent seed banks. Dominance of the seed bank by alien species with persistent seed banks suggests that aliens are favorably placed to increase in abundance in the vegetation if the forest is disturbed. 相似文献
992.
Does fragmentation of Urtica habitats affect phytophagous and predatory insects differentially? 总被引:3,自引:0,他引:3
Effects of habitat fragmentation on the insect community of stinging nettle (Urtica dioica L.) were studied, using 32 natural nettle patches of different area and degree of isolation in an agricultural landscape.
Habitat fragmentation reduced the species richness of Heteroptera, Auchenorrhyncha, and Coleoptera, and the abundance of populations.
Habitat isolation and area reduction did not affect all insect species equally. Monophagous herbivores had a higher probability
of absence from small patches than all (monophagous and polyphagous) herbivore species, and the percentage of monophagous
herbivores increased with habitat area. Abundance and population variability of species were negatively correlated and could
both be used as a predictor of the percentage of occupied habitats. Species richness of herbivores correlated (positively)
with habitat area, while species richness of predators correlated (negatively) with habitat isolation. In logistic regressions,
the probability of absence of monophagous herbivores from habitat patches could only be explained by habitat area (in 4 out
of 10 species) and predator absence probability only by habitat isolation (in 3 out of 14 species). Presumably because of
the instability of higher-trophic-level populations and dispersal limitation, predators were more affected by habitat isolation
than herbivores, while they did not differ from herbivore populations with respect to abundance or variability. Thus increasing
habitat connectivity in the agricultural landscape should primarily promote predator populations.
Received: 4 February 1998 / Accepted: 4 May 1998 相似文献
993.
We investigated the reproductive system of the threatened taxon Dombeya acutangula ssp. acutangula Cav. (Sterculiaceae), an endemic tree of the Mascarene archipelago (Indian Ocean). A controlled crossing experiment was performed
in two natural populations located in the remnants of the low-elevation dry forest on the island of La Réunion. Active pollination,
probably mainly by insects, was necessary for reproduction in this species. Individuals varied in their degree of self-sterility
from 0 to 100%. Outcrossing between nearby individuals produced lower seed set than did crosses between more distant individuals
within one of the two tested populations. The variation in reproductive success on selfing and in the different types of crosses
could result from inbreeding depression causing embryo death, and we provide evidence that progenies from selfing have lower
seed size and quality. However, for inbreeding depression to account for the dramatic variation in seed set found in our crossing
experiment, the distribution of genetic load and number of lethal factors required appear unrealistic. We favour an alternative
interpretation, that D. acutangula possesses an incompatibility system similar to that found in other Sterculiaceae species such as Theobroma cacao L. Such an incompatibility system allows a certain amount of selfing, and different individuals vary in their degree of self-incompatibility.
The low success of crosses among close neighbours in one population suggests that there was spatial structure for incompatibility
alleles in that population. This could partly explain the decline of the species in fragmented and disturbed habitats, since
relatedness at incompatibility loci may increase in small or isolated population and thus reduce mate availability.
Received: 2 March 1998 / Accepted: 3 August 1998 相似文献
994.
995.
996.
Leaves as islands for microbes 总被引:2,自引:0,他引:2
Summary The equilibrium theory of island biogeography provided a framework for describing the colonization of apple leaf islands by filamentous fungi. Surface sterilization of living leaves in situ by hydrogen peroxide allowed the colonization process to be followed from its inception. Numbers of species of filamentous fungi per leaf fluctuated from 6 to 21 during the first 2 weeks of colonization and equilibrated at about 12 by the third week. Turnover occurred in species composition at equilibrium. The equilibrium number of species was not related to leaf area.The presence of an equilibrium condition with turnover on the leaf surfaces is consistent with two key tenets of the theory of island biogeography. However, the apparent back of a species-area relationship is inconsistent with the island model. 相似文献
997.
Linda J. S. Allen 《Journal of mathematical biology》1987,24(6):617-625
Sufficient conditions are derived for persistence and extinction of a population inhabiting several islands. Discrete reaction-diffusion population models are analyzed which describe growth and diffusion of a population on a group of islands or a patch environment. A critical patch number is defined as the number of islands below which the population goes extinct on that group of islands. It is shown that population persistence on one island leads to population persistence for the entire archipelago. Both single-species and multi-species models are discussed. 相似文献
998.
David H. Kesler 《Hydrobiologia》1981,80(1):63-66
Corynoneura scutellata Winnertz (Chironomidae: Diptera) grazing rates were determined radiometrically at different temperatures. The resulting temperature-grazing function was used to calculate C. scutellata grazing rates in the littoral zone of a Rhode Island lake. These grazing rates were compared to periphyton net accumulation rates, grazing rate: accumulation rate ratios ranged from 3–15% with a maximum of 70%. 相似文献
999.
R E Ferrell R Chakraborty H Gershowitz W S Laughlin W J Schull 《American journal of physical anthropology》1981,55(3):351-358
The Eskimos of St. Lawrence Island have been typed for genetic variation at 44 discrete genetic loci. Three private polymorphisms, at the 2,3-diphosphoglycerate mutase, peptidase B, and purine nucleoside phosphorylase loci, have been observed, which may be useful in future studies of genetic relationships between Eskimos and other circumpolar populations. Genetic distance analysis reveals a close relationship between the St. Lawrence Island Eskimos and other Eskimo populations and that the Eskimo populations form a distinct cluster from Amerindian populations. The St. Lawrence Island Eskimos appear to be more similar to Asiatic Eskimos than to other groups. Caucasian admixture in this population is estimated to be between 2 and 7%. 相似文献
1000.