全文获取类型
收费全文 | 431篇 |
免费 | 27篇 |
国内免费 | 16篇 |
出版年
2023年 | 5篇 |
2022年 | 8篇 |
2021年 | 7篇 |
2020年 | 13篇 |
2019年 | 17篇 |
2018年 | 18篇 |
2017年 | 14篇 |
2016年 | 12篇 |
2015年 | 8篇 |
2014年 | 20篇 |
2013年 | 27篇 |
2012年 | 10篇 |
2011年 | 15篇 |
2010年 | 11篇 |
2009年 | 21篇 |
2008年 | 35篇 |
2007年 | 29篇 |
2006年 | 24篇 |
2005年 | 29篇 |
2004年 | 20篇 |
2003年 | 25篇 |
2002年 | 19篇 |
2001年 | 10篇 |
2000年 | 12篇 |
1999年 | 8篇 |
1998年 | 9篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1995年 | 8篇 |
1994年 | 2篇 |
1993年 | 6篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 6篇 |
1989年 | 1篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有474条查询结果,搜索用时 781 毫秒
91.
Shock loading of toxic substances into natural waterways is apervasive practice which has substantial impacts on their biota.This paper describes the effects of shock (pulse) pollution loadingfrom two major industries on a river and wetland system in southernOntario, Canada. The assessment of shock loading frequencyindicated that sporadic discharges of polluted water occurred onaverage once every other day during the 38 days of monitoring inthe period April, 1986 to November, 1987.To estimate the frequency and intensity of the shock loads, anautomatic pump sampler that was activated by a thresholdconductivity was constructed and deployed. Samples were withdrawnfrom the river when the specific conductivity of the streamexceeded a threshold value of two times background. Backgroundspecific conductivity ranged between 200–250 S cm-1The impact of these shock load discharges on a downstream aquaticplant community in a Class One (Provincially Significant) wetlandwas examined. The area of Thompsons Creek upstream of the CyanamidChemical Company displayed a normal complement of aquatic organismswhile downstream of the companys main discharges for a distance ofnearly 2 km, the stream was barren of nearly all aquatic life.A reef-like structure in which nickel, chromium, and lead reachedrespectively 2900, 2100 and 2210 mg kg-1 dry weight ofsediment was located in the Welland River near the outfall of theAtlas-Mansfield storm sewer adjacent to the Atlas Specialty SteelsCompany. The average flow rate at the outfall of theAtlas-Mansfield storm sewer into the Welland River wasapproximately 28400 m3 day-1 in 1986 (MOE 1987). Asmall area referred to as the Atlas reef because of the high ironconcentrations giving the sediments a cement-like condition, wasdevoid of all higher aquatic plants. 相似文献
92.
93.
GIACOMO SANTINI LORENZO TUCCI LORENZO OTTONETTI FILIPPO FRIZZI 《Ecological Entomology》2007,32(3):319-326
Abstract. 1. The organisation of an ant assemblage inhabiting an olive orchard in central Italy was analysed and patterns of dominance among ant species were described in order to assess (i) the relationship between thermal dependency and degree of behavioural dominance, and (ii) the relationship between dominance and discovery ability.
2. Activity patterns of the most abundant species on trees were examined in a sample of 120 trees during spring and summer. The degree of behavioural dominance and the ability of different species to discover new food sources were assessed using tuna baiting on a subset of 80 trees.
3. Different ant species showed contrasting patterns of activity. Some species (such as Lasius lasioides , Camponotus lateralis , and Camponotus piceus ) were most active during the warmer part of the day, while others restricted their activity to the cooler hours ( Camponotus aethiops and Plagiolepis pygmaea ). Some species (such as Crematogaster scutellaris ) were active irrespective of the time of day.
4. No clear relationship was observed between temperature of maximal activity and degree of behavioural dominance. There was, however, a positive relationship between behavioural dominance and thermal range of activity. A positive relationship between dominance and ability to find resources, with the most behaviourally dominant and aggressive species being most efficient in finding food items, was also observed.
5. The results support the idea that the temperature–dominance relationship is much more complex in Mediterranean-type habitats than in other ecosystems. Of particular interest is the positive dominance–discovery relationship. This finding contrasts with previous investigations, which reported a negative relationship between dominance and discovery ability and suggested that this pattern plays a role in promoting the coexistence of species in ant communities. 相似文献
2. Activity patterns of the most abundant species on trees were examined in a sample of 120 trees during spring and summer. The degree of behavioural dominance and the ability of different species to discover new food sources were assessed using tuna baiting on a subset of 80 trees.
3. Different ant species showed contrasting patterns of activity. Some species (such as Lasius lasioides , Camponotus lateralis , and Camponotus piceus ) were most active during the warmer part of the day, while others restricted their activity to the cooler hours ( Camponotus aethiops and Plagiolepis pygmaea ). Some species (such as Crematogaster scutellaris ) were active irrespective of the time of day.
4. No clear relationship was observed between temperature of maximal activity and degree of behavioural dominance. There was, however, a positive relationship between behavioural dominance and thermal range of activity. A positive relationship between dominance and ability to find resources, with the most behaviourally dominant and aggressive species being most efficient in finding food items, was also observed.
5. The results support the idea that the temperature–dominance relationship is much more complex in Mediterranean-type habitats than in other ecosystems. Of particular interest is the positive dominance–discovery relationship. This finding contrasts with previous investigations, which reported a negative relationship between dominance and discovery ability and suggested that this pattern plays a role in promoting the coexistence of species in ant communities. 相似文献
94.
The present study evaluated the generality of ecosystem engineering processes by examining the influence of sympatric burrowing
shrimps (Callianassidae) and intertidal seagrasses (Zosteraceae) on benthic assemblage composition in two temperate regions,
south-eastern New Zealand and north-western U.S.A. In each region, intertidal macrofauna assemblage composition was determined
at sites of different burrowing shrimp/seagrass density and where both species co-occured, in three different size estuaries/tidal
inlets, on two occasions. Results from both regions showed that the presence of shrimps and seagrasses consistently influenced
the composition of the associated infaunal assemblages at all sites, in both summer and winter. Macrofauna assemblages at
shrimp sites were significantly different to those at seagrass-only and mixed sites, whereas the composition of the latter
sites was similar. The differences observed between sites were best explained by sediment variables. In New Zealand, % fines
and seagrass debris showed the highest correlation to differences in assemblage composition, and in the U.S.A. % fines, %
carbon and sediment turnover (by shrimp) appeared to be the most important environmental parameters measured. Four to six
taxa exhibited the greatest discriminating significance (including corophiid amphipods, spionid polychaetes and oligochaetes)
for dissimilarities in assemblage composition observed at the different sites, with generally lower abundances at shrimp than
at seagrass sites. The present study highlights the functional importance of seagrasses and bioturbating shrimps as ecosystem
engineers in soft-sediment environments, and reveals the generality of their influence on associated macro-invertebrate assemblages.
The findings also allow for further development of a heuristic model for ecosystem engineering by shrimp and seagrass which
indicate that numerical models that aim to explore the relationship between ecosystem engineer populations and habitat modification
should be expanded to capture the interaction of co-occurring engineers and be both spatially and temporally explicit. 相似文献
95.
Jerzy Dzik Daniel Drygant 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1986,19(2):133-141
A cluster of thirteen elements of Panderodus unicosfatus (Branson & Mehl) from the Llandoverian of Po-dolia, Ukraine, is shown to represent an almost complete apparatus; only one element is lacking. The restored original arrangement of the elements is consistent with the model of the conodont apparatus with seven pairs of elements ordered in two parallel rows, with cusps opposing in every pair. The apparatus of Panderodus is closely similar to that of the Cambrian 'protoconcdonts' and their Recent chaetognath successors. Nevertheless, general trends in the evolution of elements of the Panderodontidae towards increased elongation, smoothness of the cusp curvature, and depth of the basal cavity indicate that the grasping apparatus of Panderodus developed independently of, and convergently to, that of the Chaetog-natha. 相似文献
96.
97.
An important effort has been made to develop diversity indices suitable to monitor the loss of biodiversity due to anthropogenic impacts in an accurate and comprehensible way. Here, N90, a diversity index based on the species’ contribution to the similarity between samples in a group, is presented. N90 uses the results of the classic Similarity Percentage analysis and a jack-knife routine to calculate the average and a dispersion value of the number of species contributing up to the ninety percent of the similarity in a group of samples. N90 is applied to two groups of samples subjected to contrasting levels of bottom trawl fishing pressure using time series of experimental bottom trawl surveys of the Balearic Islands. The results are compared to those obtained using more ‘traditional’ diversity indices such as species richness, Shannon–Wienner, Simpson, Pielou, and Margalef diversity indices. The N90 diversity index displayed a clear response to fishing pressure with significantly lower values in impacted communities, while the ‘traditional’ diversity indices showed almost null sensitivity to fishing pressure. In addition, N90 also detects indirect fishing impacts by fluctuating in response to environmental variation in impacted areas, making this index sensitive to the synergies between climate and fishing impact at community level. The application of the N90 diversity index to the case study shows that it may be an alternative to ‘traditional’ diversity indices when trying to monitor fishing impacts and the effects of environmental changes. Its units, number of species, and the corresponding summary list of species facilitate the interpretability of the results, improving the communication to managers and stakeholders. 相似文献
98.
《Palaeoworld》2022,31(3):521-541
Early Eocene climate and vegetation evolution in northern Yakutia (Eastern Siberia) are quantitatively studied for the first time based on paleobotanical records, using the coexistence approach (CA) for paleoclimate, the plant functional type approach (PFT) and the integrated plant record method (IPR). Paleobotanical data of this time-interval were obtained from 18 palynofloras of the coal beds outcropping on the bank of the Bykovskaya Channel in the area of Bulunkan Bay (Lena River Delta). The results indicate the persistence of warm temperate and at times possibly nearly tropical, overall humid climate conditions throughout the early Eocene, and a relatively strong seasonality in precipitation. The vegetation reconstructed is in agreement with the climate conditions determined by the CA. Analyses of the PFT diversity spectra indicate the prevalence of mixed warm temperate evergreen-deciduous forests. Based on the IPR method, plant biomes varied from mixed mesophytic forests during warmer intervals to broadleaved deciduous forest/mixed mesophytic forest ecotone during cooler episodes. The presence of mangroves in our early Eocene records is possibly related to hyperthermal events such as the Paleocene–Eocene Thermal Maximum. 相似文献
99.
100.
The importance of substrate type, shading and scorching for the attractiveness of dead wood to saproxylic beetles 总被引:1,自引:0,他引:1
J. Hjltn T. Johansson O. Alinvi K. Danell J.P. Ball R. Pettersson H. Gibb J. Hilszczaski 《Basic and Applied Ecology》2007,8(4):364-376
Modern forestry management has reduced the amount of dead wood in forest ecosystems and this has become a serious threat to flora and fauna. Efforts are therefore being made to reverse this trend but one problem is that we still lack detailed knowledge regarding the substrate requirements of many saproxylic species. In a field experiment, conducted in three forest types (forest reserve, mature managed forest and clear-cut), we evaluated the value, from a conservation perspective, of different substrate types (logs, snags and tops) of Norway Spruce, Picea abies, and if the quality of spruce logs as saproxylic habitats can be improved by simple log treatments (scorching and shading). We collected 9982 individuals representing 262 saproxylic beetle species in window traps. Both substrate type and, to a lesser extent, log treatment had a significant effect on the abundance and species richness of saproxylic beetles attracted to the different dead wood substrates. However, more importantly, the composition of the beetle assemblages differed significantly between both substrates and log treatments. Snags, logs and tops all attracted significantly different beetle assemblages and scorched logs differed from untreated control logs. Sixteen red listed species were trapped, with the highest number (11 species) being found on scorched logs. We found strong evidence that some species preferred a specific substrate type, mainly logs, in some cases treated logs (scorched or shaded), but not snags, the substrate commonly provided for conservation purposes on e.g. clear-cuts. This stresses the importance of conducting forestry in such a way that a multitude of both forest habitats and dead wood substrates are available continuously in the forest landscape to maintain biodiversity. 相似文献