全文获取类型
收费全文 | 2430篇 |
免费 | 38篇 |
专业分类
2468篇 |
出版年
2015年 | 15篇 |
2014年 | 12篇 |
2013年 | 81篇 |
2012年 | 54篇 |
2011年 | 79篇 |
2010年 | 130篇 |
2009年 | 94篇 |
2008年 | 75篇 |
2007年 | 118篇 |
2006年 | 76篇 |
2005年 | 85篇 |
2004年 | 56篇 |
2003年 | 41篇 |
2002年 | 22篇 |
2001年 | 32篇 |
2000年 | 38篇 |
1999年 | 34篇 |
1998年 | 54篇 |
1997年 | 85篇 |
1996年 | 87篇 |
1995年 | 55篇 |
1994年 | 41篇 |
1993年 | 49篇 |
1992年 | 51篇 |
1991年 | 41篇 |
1990年 | 45篇 |
1989年 | 54篇 |
1988年 | 41篇 |
1987年 | 32篇 |
1986年 | 36篇 |
1985年 | 39篇 |
1984年 | 49篇 |
1983年 | 28篇 |
1982年 | 25篇 |
1981年 | 30篇 |
1980年 | 47篇 |
1979年 | 39篇 |
1978年 | 34篇 |
1977年 | 25篇 |
1976年 | 21篇 |
1975年 | 36篇 |
1974年 | 30篇 |
1973年 | 32篇 |
1972年 | 50篇 |
1971年 | 58篇 |
1969年 | 12篇 |
1968年 | 18篇 |
1966年 | 15篇 |
1959年 | 12篇 |
1957年 | 12篇 |
排序方式: 共有2468条查询结果,搜索用时 0 毫秒
21.
PETER D. DWYER 《Austral ecology》1978,3(2):213-232
A trapping study of four species of Rattus and three of Melomys in rainforest and grassland habitats revealed differences in altitudinal distribution and in both gross and subtle habitat selection between species. The species are ranked on a scale of increasing opportunism on the basis of various demographic characteristics (e.g. reproductive rate, life span and population density). Species judged to be more opportunistic occur in habitats subject to more intense disturbance from pigs and humans. It is argued that changes in the pattern of species coexistence have followed overt human impact in the New Guinea highlands. 相似文献
22.
23.
Akaike's information criterion and the histogram 总被引:1,自引:0,他引:1
24.
25.
SUMMARY. 1. The predation-balanced polymorphism of Ceriodaphnia cornuta (a small-eyed horned form and a large-eyed unhorned form) in Lake Gatun, Panama, is re-examined approximately 12 years after the original study (Zaret, 1972a). We consider morphological variability, distribution and fecundity of the population inhabiting Laboratory Bay.
2. The two morphological characters emphasized by Zaret, eye size and horn length, are studied in detail. The C. cornuta population exhibits wide, yet continuous variation; coefficients of variation are 28% or eye diameter and range from 114% to 136% for horn and spine lengths. Phenotypic variation in the population is characterized by: (1) negative correlations between spine lengths and eye size; (2) positive correlations among spines from different body regions (anterior horn, lateral spines, and tail spine); and (3) weak correlations between body length and spine lengths.
3. While the horned and unhorned forms continue to coexist in Laboratory Bay. the frequencies of horned forms have increased in the open water areas since the time of the original study. The increase in horned form frequencies may be due to changes in fish predation intensity. Did vertical distribution patterns of the forms remain very similar to each other.
4. In contrast to previous laboratory findings, egg ratio calculations from field samples revealed similar clutch sizes for the horned and unhorned forms. The egg development time of the horned form, however, was found to be approximately 30% longer than that of the unhorned form, perhaps because of increased moulting difficulties produced by the presence of spines. 相似文献
2. The two morphological characters emphasized by Zaret, eye size and horn length, are studied in detail. The C. cornuta population exhibits wide, yet continuous variation; coefficients of variation are 28% or eye diameter and range from 114% to 136% for horn and spine lengths. Phenotypic variation in the population is characterized by: (1) negative correlations between spine lengths and eye size; (2) positive correlations among spines from different body regions (anterior horn, lateral spines, and tail spine); and (3) weak correlations between body length and spine lengths.
3. While the horned and unhorned forms continue to coexist in Laboratory Bay. the frequencies of horned forms have increased in the open water areas since the time of the original study. The increase in horned form frequencies may be due to changes in fish predation intensity. Did vertical distribution patterns of the forms remain very similar to each other.
4. In contrast to previous laboratory findings, egg ratio calculations from field samples revealed similar clutch sizes for the horned and unhorned forms. The egg development time of the horned form, however, was found to be approximately 30% longer than that of the unhorned form, perhaps because of increased moulting difficulties produced by the presence of spines. 相似文献
26.
PETER G. MAY 《Ecological Entomology》1988,13(2):171-184
ABSTRACT.
- 1 I studied flower selection and foraging energetics of Agraulis vanillae L. (Nymphalidae) and Phoebis sennae (Pieridae), two butterfly species common to north central Florida. I identified the major nectar resources exploited by several populations of these butterflies and, for each plant species, measured available nectar volumes and concentrations, corolla lengths, and density. I quantified foraging behaviour of each butterfly species at each nectar source (flower visitation rate and percentage of foraging time in flight), and used these data to estimate the net rate of energy intake of each butterfly species at each nectar source.
- 2 Estimated mean energy contents of individual flowers of the eleven exploited plant species spanned three orders of magnitude, ranging between 0.015 and 9.27 joules. Mean energy content of individual flowers was strongly correlated with mean foraging profit of both butterfly species.
- 3 Mean nectar volume strongly influenced energy content and varied widely within and among species, ranging from 0.0076 to 1.853 μ1. Nectar concentration varied between 17.1% and 40.4% sucrose-equivalents. Nectar volume was the best single predictor of foraging profitability (correlation coefficients of 0.994 and 0.984 for Phoebis and Agraulis respectively). Corolla length also strongly affected foraging profitability for both butterfly species; flower species with longer corollas were generally more profitable.
- 4 Flower density and nectar concentration showed weak or nonsignificant associations with foraging profitability.
- 5 The usefulness and limitations of these floral characteristics as bases for foraging selectivity, and the selective pressures foraging butterflies might place on the visited plants are discussed.
27.
Are galling insects better protected against parasitoids than exposed feeders?: a test using tenthredinid sawflies 总被引:1,自引:0,他引:1
ABSTRACT.
- 1 Data mostly from the published literature were used to assess the effect of galling on the number of parasitoid species per host species in the phylogeny of nematine sawflies from free external feeders (colonial and solitary) to leaf gallers and shoot gallers.
- 2 The strongest effects of galling were the total elimination of the species-rich cocoon-attacking guild of parasitoids, and eonymphal parasitoids, from the parasitoid community on shoot gallers, all of which are in the genus Euura.
- 3 All tachinid larval parasitoids were also eliminated by the galling habit.
- 4 The cumulative effects of these exclusions resulted in a decline in mean number of parasitoid species per host species from almost sixteen species on external colonial feeders to 4.0 species on shoot gallers.
- 5 General patterns in per cent parasitism by non-tachinid and tachinid larval parasitoids, eonymphal and cocoon parasitoids, on exposed feeders to shoot gallers, showed declines in non-tachinid attack and elimination of tachinid, eonymphal and cocoon parasitoids. But leaf gallers tended to be attacked more than exposed feeders by non-tachinid larval parasitoids.
- 6 The galling habit had a long-term impact by reducing the number of parasitoid species attacking nematine sawfly gallers and per cent mortality inflicted, so that natural enemies may have been important as a selective factor in the evolution of galling nematine sawflies.
28.
On nonparametric discrimination using density differences 总被引:2,自引:0,他引:2
29.
The described fossil testudinids from the Miocene of Africa are reviewed. Geochelone stromeri sp. nov . is described from Lower Pliocene (Langebaanweg) and Miocene (Namib) specimens. Kinixys erosa , an extant species, is reported from Songhor Hill. Chersina sp. is reported from Arrisdrift. Impregnochelys pachytectis gen. et sp. nov . is described from Rusinga Island, Kenya, and is unique in having the anterior shell opening orientated ventrally and in having struts on the internal surface of the neurals, but shares with Kinixys a unique epiplastron shape, a high number of axillary scutes and unique orientation of the head of the femur. 相似文献
30.