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This paper investigates the scope for stabilizing commodity prices by buffer stock interventions. Optimal control is used, rather than simulation, to evaluate the scope for intervention at best. Applications to a world copper market model show that substantial but extensive gains to stabilization are possible. These gains cannot be realised by traditional bandwidth decision rules. An important difficulty is setting the level at which prices should be stabilized. The lack of this information severely reduces the achievable gains. 相似文献
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The Laurentian Great Lakes represent 20 percent of the surface fresh water of the world. They are unique because of their long flushing times (in excess of 175 years for Lake Superior), their relatively short mixing times and their consequent propensity to accumulate persistent chemicals and respond very slowly to decreased chemical loadings. Lake Ontario, as the last lake in the system, receives the drainage from the other four Great Lakes in addition to significant chemical contributions from within its drainage basin particularly from its main tributary, the Niagara River.
This paper evaluates environmental levels of TCOD and related compounds within the Lake Ontario ecosystem by considering temporal trends, the application of mass loading principles for large lake ecosystems and the examination of a holistic ecosystem perspective on exposure routes. 相似文献
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Hallett Chris S. Hobday Alistair J. Tweedley James R. Thompson Peter A. McMahon Kathryn Valesini Fiona J. 《Regional Environmental Change》2018,18(5):1357-1373
Regional Environmental Change - Regions with a Mediterranean climate are generally predicted to become warmer and drier with climate change. Estuaries in these regions are influenced by a broad... 相似文献
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Thermal volatilization is explored as a means of inferring the chemical composition of atmospheric aerosol particles with
diameters smaller than 10 nm (nanoparticles). Such particles contain too little mass for quantitative chemical determination
by traditional analytical methods. Aerosols were subjected to increasing temperature in an oven and particle loss was measured
as a function of temperature with the TSI model 3025 ultrafine condensation particle counter (UCPC), which is capable of counting
particles with diameters as small as 3 nm. Particle nucleation was observed down stream of the oven when it was heated above
about 400°C. To reduce this artifact, the sample air down stream of the oven was cooled to condense the hot gases and/or the
freshly nucleated particles before they reached the UCPC. Controlled experiments were done with pure ammonium sulfate (NH4)2SO4 particles. The experimental design was optimized based on the known concentration of pure (NH4)2SO4 particles vaporized in the oven and the diffusion of this material to the walls of the sampling tube before the particle
counter. 相似文献
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Novel weapons: invasive plant suppresses fungal mutualists in America but not in its native Europe 总被引:7,自引:0,他引:7
Callaway RM Cipollini D Barto K Thelen GC Hallett SG Prati D Stinson K Klironomos J 《Ecology》2008,89(4):1043-1055
Why some invasive plant species transmogrify from weak competitors at home to strong competitors abroad remains one of the most elusive questions in ecology. Some evidence suggests that disproportionately high densities of some invaders are due to the release of biochemicals that are novel, and therefore harmful, to naive organisms in their new range. So far, such evidence has been restricted to the direct phytotoxic effects of plants on other plants. Here we found that one of North America's most aggressive invaders of undisturbed forest understories, Alliaria petiolata (garlic mustard) and a plant that inhibits mycorrhizal fungal mutualists of North American native plants, has far stronger inhibitory effects on mycorrhizas in invaded North American soils than on mycorrhizas in European soils where A. petiolata is native. This antifungal effect appears to be due to specific flavonoid fractions in A. petiolata extracts. Furthermore, we found that suppression of North American mycorrhizal fungi by A. petiolata corresponds with severe inhibition of North American plant species that rely on these fungi, whereas congeneric European plants are weakly affected. These results indicate that phytochemicals, benign to resistant mycorrhizal symbionts in the home range, may be lethal to na?ve native mutualists in the introduced range and indirectly suppress the plants that rely on them. 相似文献
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The information needed for conducting the risk assessment was provided by the participants in a 2-day workshop. The invited
participants, who possessed knowledge of the St. Croix ecosystem, identified through a group-consensus process a list of stressors
and a list of ecosystem values. They then assigned numerical values to each stressor-ecosystem value pair that reflected the
degree to which the given stressor contributes to ecosystem risk as measured by the given ecosystem value. Based on this information,
the analytical portion of the methodology was then used to rank the ecosystem risks (stressors) when examined from several
different perspectives: immediate impact, time-duration, and management activities. Regardless of the perspective taken, riverway
development emerged as the most significant stressor. 相似文献
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Evaluating the Effectiveness of Corridors: a Genetic Approach 总被引:9,自引:0,他引:9
Abstract: The effectiveness of corridors in maintaining dispersal in fragmented landscapes is a question of considerable conservation and ecological importance. We tested the efficacy of corridors as residual landscape structures in maintaining population structure in the red-backed vole ( Clethrionomys gapperi ), a closed-canopy specialist, and the deer mouse ( Peromyscus maniculatus ), a habitat generalist. In coniferous forests managed for timber production in northeastern Washington, we sampled pairs of populations in three landscape classes: (1) contiguous landscapes, in which sites were located completely within a matrix of closed-canopy forest; (2) corridor landscapes, in which sites were connected by a corridor of closed-canopy forest; and (3) isolated landscapes, in which sites were separated from one another by clearcut or young regeneration stands. For each species, we used four microsatellite loci to quantify genetic distance between population pairs. Nei's genetic distance ( D s ) increased from smallest to largest in the order of contiguous, corridor, and isolated landscapes for C. gapperi. For P. maniculatus, genetic distances across landscape configurations were not significantly different. The differences between the two species indicate that they respond differently to the presence of forest corridors. In managed forests, corridors between unlogged habitats appear to maintain higher population connectivity for C. gapperi than landscapes without corridors. 相似文献
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A method for assessing environmental risk: A case study of Green Bay,Lake Michigan,USA 总被引:4,自引:0,他引:4
Hallett J. Harris Robert B. Wenger Victoria A. Harris David S. Devault 《Environmental management》1994,18(2):295-306
The Science Advisory Board of the US Environmental Protection Agency has recommended that risk reduction strategies become
the centerpiece of environmental protection. The goal in developing such strategies is to identify opportunities for greatest
reduction of ecological risks. This is a perspective that is significantly more comprehensive than the traditional focus on
human health risks arising from environmental degradation. The identification of ecological risks upon which environmental
protection efforts should be focused requires an ecological risk assessment methodology that is based on anthropogenic stressors
affecting an ecosystem and a set of impaired use criteria. A methodology based on this concept is developed and discussed
in this article. The methodology requires that risk values be assigned to each ecosystem stressor-impaired use pair that reflect
the degree to which the given stressor contributes to ecosystem risk as measured by the given impaired use criterion. Once
these data are available, mathematical analyses based on concepts from fuzzy set theory are performed to obtain a ranking
of ecosystem stressors. The methodology has been tested by applying it to a case study involving Green Bay of Lake Michigan.
A workshop was held in which 11 persons with extensive knowledge of the Green Bay ecosystem determined risk values through
a group-consensus process. The analytical portion of the methodology was then used to rank the ecosystem risks (stressors)
from several perspectives, including prevention management and remediation management. The overall conclusion of the workshop
participants was that the fuzzy set decision model is a useful and effective methodology for differentiating environmental
risk. 相似文献