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991.
Ronald E. Wyzga 《人类与生态风险评估》1999,5(3):493-499
The new annual PM2.5 standard will be the most difficult particulate matter (PM) standard to satisfy. At issue is whether the extant health evidence supports the rationale for this standard being the controlling standard. Indeed the key issue is whether this standard will result in the most cost-effective way to protect public health. This paper examines the health literature and concludes that the evidence for the annual PM2.5 standard is weak. The bulk of the health evidence is related to daily exposures to PM10 and larger particle sizes, and there is no rational way to decide upon the correct level for this standard. It is unclear whether the most restrictive PM2.5 standard will be protective of public health. Clearly research is needed to determine the correct PM metric, averaging time, and level for a standard. To date such research has been limited. 相似文献
992.
Christine M. Bridges 《Aquatic Ecology》1999,33(2):205-211
The presence of environmental contaminants may alter predator-prey interactions among aquatic species by altering activity
levels of predators or prey, or by altering predator avoidance behavior. The outcome of a predatory encounter may be dependent
upon whether both species are exposed to a contaminant simultaneously, or whether exposure occurs only in one of the species.
In a laboratory experiment, I used the insecticide carbaryl to examine predation of southern leopard frog tadpoles (Rana sphenocephala)
by adult red-spotted newts (Notophthalmus viridescens) under four conditions: both tadpoles and newts exposed, neither tadpoles
nor newts exposed, and either newts or tadpoles only exposed. After one hour, exposed newts consumed half as many tadpoles
as non-exposed newts. Carbaryl potentially affected newt activity enough to reduce time spent searching for prey, or may have
altered the speed and coordination necessary to capture tadpoles. After six hours, non-exposed and exposed newts consumed
similar numbers of tadpoles, most likely indicating recovery from exposure. After 24 h, predation rates were lowest when both
newts and tadpoles were simultaneously either exposed or not exposed, and were greatest when newts and tadpoles were not exposed
simultaneously. This study suggests that when tadpoles and newts are exposed to a sublethal level of a contaminant simultaneously,
that predation rates do not differ from those observed under natural conditions, but exposure of either predator or prey at
different times can disrupt predator-prey dynamics.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
993.
Abstract: After exposure to full sunlight under natural conditions, attached leaves of the common meadow weed Potentilla reptans show non-uniform ("patchy") chlorophyll fluorescence quenching in the early stages of fluorescence transients. These areas of bright fluorescence can be readily reproduced in detached leaves that are allowed to wilt on the laboratory bench in weak light. The extent and duration of the patchiness increases with increasing water stress (higher relative saturation deficits). Images captured during saturating flashes show that the patches also display slow development of non-photochemical quenching, consistent with the possibility that photosynthetic metabolism is impaired in these areas. Wilted Potentilla leaves readily regain full turgor when petioles are placed in water, and uniform chlorophyll fluorescence is recovered with in 30mm. Epidermal impressions reveal closed stomata over areas of both low and high fluorescence in wilted leaves. Because highly fluorescent patches also persist when wilted tissues are exposed to high CO2 (i.e., patchiness is unlikely to be due to local differences in CO2 supply) the data suggest direct effects of water stress on metabolism in wilted leaves. Leaf transverse sections show that although major veins may isolate areas of the lamina, minor veins do not. Relationships to leaf anatomy are discussed. 相似文献
994.
Indirect interactions between populations of different species can be important in structuring natural communities. Indirect
effects are either mediated by changes in population densities (trophic or density-mediated effects) or by changes in the
behavior of species that are not trophically connected (behavioral or trait-mediated effects). We reviewed the literature
on aphids and their parasitoids to explore the various possible indirect interactions that can occur in such communities.
The review was motivated by our study of a particular aphid–parasitoid community in a natural (i.e., nonagricultural) habitat,
and by the wealth of information that exists about aphid–parasitoid systems in agricultural settings. We focused our review
on aphid–parasitoid interactions, but considered how these were influenced by the other aphid natural enemies and also by
aphid mutualists and host plants. We conclude that indirect effects are likely to have a major effect in structuring aphid–parasitoid
communities, and that the latter are a valuable model system for testing ideas about community interactions.
Received: December 20, 1998 / Accepted: January 12, 1999 相似文献
995.
Jean Witz 《Analytical biochemistry》1999,270(2):201-206
I examined the penetration of the hydrodynamic flow into a polymer matrix immobilized by grafting to a surface, such as used in optical biosensors designed to measure binding reactions in real time. I show that the flow penetrates with an appreciable velocity into a region located at the tip of such a polymer brush, corresponding to about 10 to 15% of the total mass of the grafted polymer. Furthermore, under the conditions recommended for kinetic measurements, the concentrations of both polymer and immobilized ligands are low in these regions of the matrix, where crowding effects are negligible. Under such conditions, the hydrodynamic flow penetrating into the dextran matrix flow will bring the analytes close to their targets, thus considerably reducing transport problems. 相似文献
996.
水稻品种和稻虱缨小蜂对褐飞虱协同作用的模拟分析 总被引:3,自引:0,他引:3
利用我们先前的试验结果和文献资料,组建了水稻品种-褐飞虱-稻虱缨小蜂3者相互关系的模拟模型.模拟分析表明水稻品种能直接和间接地通过物理或化学特性影响稻虱缨小蜂的寄生作用,其中以影响褐飞虱卵垂直分布型、稻虱缨小蜂的迁入时间、未成熟期历期和怀印量等的水稻品种特性起主要作用.同时,模拟分析还表明中等抗性的水稻品种,配以增强稻虱缨小蜂寄生作用的品种特性,就能有效地控制褐飞虱的危害.文中还就协调水稻品种抗性与天敌协同控制褐飞虱的途径进行了讨论. 相似文献
997.
沪191麻疹疫苗免疫持久性和影响因素的评价 总被引:11,自引:0,他引:11
1991~1998年,我们对荆州区川店镇503名6~15月龄儿童进行了现行沪191麻疹疫苗血清流行病学效果观察,结果表明,初次免疫后1个月麻疹IgG抗体阳转率为9165%,GMT为1∶26674,达保护滴度者比例为465%。随着时间的推移,第4年上述指标迅速下降到4686%、1∶1274和185%,第6年时低至2943%、1∶489和136%。02ml、03ml和05ml麻疹疫苗组的近期和远期效果是类似的,初免后1个月时IgG滴度越高,其免疫持久性越好;初免月龄是影响麻苗免疫效果的主要原因,6月龄初免组的免疫效果明显低于≥8月龄组。结果提示麻苗8月龄初免是可行的。 相似文献
998.
晚粳,早籼光敏核不育水稻幼穗分化后光周期效应的比较 总被引:1,自引:0,他引:1
报道了日长对不同晚粳(Oryzasativasub.japonica)及早籼(O.sativasub.indica)水稻幼穗发育的影响。发现在晚粳中,二次枝梗期之后所给予的长日处理除延迟幼穗分化进程外,还会影响颍花的发育,表现为颍花密度及结实率下降。而早籼稻中则无明显的类似效应。因此日长对幼穗发育的影响与日长对幼穗分化诱导的影响有一定的相关性。同时,长日对晚粳幼穗发育的影响与引起光敏不育表现的时期相同,表明光敏不育水稻的雄性不育是该突变体雄蕊发育对长日抑制幼穗发育影响的一种特殊反应。 相似文献
999.
Zooplankton in the main channel of the Nakdong River and in three tributaries was sampled from June 1994 to September 1995. Planktonic rotifers (Brachionus spp., Keratella spp., and Polyarthra spp.), cyclopoid nauplii and small cladocerans (Bosmina longirostris) were numerically dominant. There was considerable longitudinal variation of zooplankton biomass in the main channel as well as spatial heterogeneity among the major tributaries. In the middle region of the main channel, between river kilometer (RK) 170 and 150 above the estuary dam, total zooplankton abundance sharply increased from less than 100 ind. L—1 to more than 1,000 ind. L—1. In a downstream direction toward the estuary dam, phytoplankton biomass increased while total zooplankton biomass decreased. However, as shown by the increasing transport of zooplankton biomass, zooplankton was diluted in the reach of the estuary dam. Advective effects from major tributaries appear to be the contributory factor for the higher zooplankton biomass in the middle region. Overall, rather the external factors (flushing, retention) than internal factors (e.g., phytoplankton) appear to be responsible for changes in zooplankton abundance toward the river mouth. 相似文献
1000.
Sadeq R. Chowdhury C.A. McGilchrist 《Biometrical journal. Biometrische Zeitschrift》2001,43(3):271-281
A method for analysing matched case control data in presence of overdispersion or correlation between observations within matched sets is presented. A generalised linear mixed model approach is adapted to account for overdispersion by allowing random exposure effects. Expressions for the elements of the estimation equation are derived and a strategy for achieving efficiency in computing is discussed. Results from simulation studies and two applications are presented. 相似文献