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101.
Using the formulation of latent failure times, the problem of inference on survival time in the presence of competing risks is discussed. Even for the simplified problem where dose and other covariates are fixed, it is not well known that the distributions of the latent failure times are not identifiable from the usual competing risks data. Bounds for the joint survival function and marginal survival functions for the latent failure times are given, and an example illustrates the determination of empirical bounds that estimate the true bounds. In general, the bounds are wide and therefore not very useful. Some alternate approaches are briefly mentioned. 相似文献
102.
L. LaFleur T. Bousquet K. Ramage B. Brunck T. Davis W. Luksemburg B. Peterson 《Chemosphere》1990,20(10-12)
Analysis of 2378-TCDD and 2378-TCDF on the ppq-level in foods with high fat content is described. The migration over twelve days of 2378-TCDD, 2378-TCDF and 1278-TCDF from bleached paperboard cartons into whole milk was found to be exponential with time. A survey of selected fatty foods shows the presence of 2378-TCDD and 2378-TCDF in foodstuff. 相似文献
103.
104.
105.
Competitive organization of the soft-bottom macrobenthic communities of southern California lagoons 总被引:5,自引:0,他引:5
C. H. Peterson 《Marine Biology》1977,43(4):343-359
The sandy-bottom macrobenthic community of Mugu Lagoon, a relatively pristine southern California (USA) marine lagoon, demonstrated (1) nearly constant community composition over 37 months of observation, (2) relatively little temporal variability in the population densities of the most abundant species over 37 months, and (3) a pattern of depth stratification in which very little vertical overlap existed among the six most abundant species. The only two species whose vertical distributions overlapped broadly showed horizontal spatial segregation, each abundant in different areas within the sand habitat. These community characteristics imply the importance of biological factors in structuring the sand benthos. The relatively large volume required for living space by these macrofauna suggests that competition for space may be the biological factor most important in determining the observed temporal and spatial abundance patterns. The muddy-sand community and the mud community of Mugu Lagoon also revealed similar patterns of stratification: new abundant species replaced species at the same sedimentary level while not greatly affecting species populations at other non-overlapping levels. In the sand community of Tijuana Slough, two of the abundant species of Mugu Lagoon's sand community were nearly absent as an apparent result of human over-exploitation. Probably in response, densities of species living at the sedimentary levels normally occupied by the missing species were much higher than would be predicted if competition for space were unimportant. In field experiments, removal of the deposit feeder Callianassa californiensis resulted in high recruitment of Sanguinolaria nuttallii, whereas control areas showed no S. nuttallii recruitment. Experiments also suggest that negative intraspecific interactions between Cryptomya californica individuals may explain the observed rapid emigration from areas of artificially high density. Perhaps the relatively great environmental predictability of southern California lagoons has permitted competitive interactions to play a singnificant role in determining the temporal and spatial abundance patterns of the soft-bottom macrobenthos. 相似文献
106.
Biogeography of diseases: a framework for analysis 总被引:1,自引:0,他引:1
Peterson AT 《Die Naturwissenschaften》2008,95(6):483-491
A growing body of literature offers a framework for understanding geographic and ecological distributions of species; a few applications of this framework have treated disease transmission systems and their geography. The general framework focuses on interactions among abiotic requirements, biotic constraints, and dispersal abilities of species as determinants of distributional areas. Disease transmission systems have key differences from other sorts of biological phenomena: Interactions among species are particularly important, interactions may be stable or unstable, abiotic conditions may be relatively less important in shaping disease distributions, and dispersal abilities may be quite variable. The ways in which these differences may influence disease transmission geography are complex; I illustrate their effects by means of worked examples regarding West Nile Virus, plague, filoviruses, and yellow fever. 相似文献
107.
Chapin FS Peterson G Berkes F Callaghan TV Angelstam P Apps M Beier C Bergeron Y Crépin AS Danell K Elmqvist T Folke C Forbes B Fresco N Juday G Niemelä J Shvidenko A Whiteman G 《Ambio》2004,33(6):344-349
The arctic tundra and boreal forest were once considered the last frontiers on earth because of their vast expanses remote from agricultural land-use change and industrial development. These regions are now, however, experiencing environmental and social changes that are as rapid as those occurring anywhere on earth. This paper summarizes the role of northern regions in the global system and provides a blueprint for assessing the factors that govern their sensitivity to social and environmental change. 相似文献
108.
Spread of metals through an invertebrate food chain as influenced by a plant that hyperaccumulates nickel 总被引:6,自引:0,他引:6
Lynsey R. Peterson Victoria Trivett Alan J.M. Baker Carlos Aguiar A. Joseph Pollard 《Chemoecology》2003,13(2):103-108
Summary. Hyperaccumulation of metals in the shoot system of
plants is uncommon, yet taxonomically and geographically widespread. It
may have a variety of functions, including defense against herbivores.
This study investigated the effects of hyperaccumulation on metal
concentrations across trophic levels. We collected plant material, soil,
and invertebrates from Portuguese serpentine outcrops whose vegetation
is dominated by the nickel hyperaccumulator Alyssum pintodasilvae.
Samples were analyzed for nickel, chromium, and cobalt. Grasshoppers,
spiders, and other invertebrates collected from sites where A.
pintodasilvae was common had significantly elevated concentrations of
nickel, compared to nearby sites where this hyperaccumulator was not
found. Chromium and cobalt, occurring in high concentrations in the
serpentine soil but not accumulated by A. pintodasilvae, were not
elevated in the invertebrates. Therefore, it appears likely that a flux
of nickel to herbivore and carnivore trophic levels is specifically
facilitated by the presence of plants that hyperaccumulate this metal.
The results may be relevant to the development of phytoremediation and
phytomining technologies, which use plants to extract metals from the
soil.
Reveived 22 August 2002; accepted 2 April 2003.
R1D="
Correspondence to: A. J. Pollard, e-mail:joe.pollard@furman.edu 相似文献
109.
Sanitary landfills are the major method used today for the disposal and management of municipal solid waste. Decomposition of waste and rainfall generate leachate at the bottom of landfills, causing groundwater contamination. In this study, leachate from a municipal landfill site was treated by electrochemical oxidation in a pilot scale flow reactor, using oxide-coated titanium anode. The experiments were conducted under a constant flow rate of 2000 lh(-1) and the effect of current density on chemical oxygen demand, total organic carbon, color and ammonium removal was investigated. At a current density of 116.0 mA cm(-2) and 180 min of processing, the removal rates achieved were 73% for COD, 57% for TOC, 86% for color and 49% for ammonium. The process proved effective in degrading leachate, despite this effluent's usual refractoriness to treatment. 相似文献
110.
Peterson MM Horst GL Shea PJ Comfort SD Peterson RK 《Environmental pollution (Barking, Essex : 1987)》1996,93(1):57-62
Cost-effective and environmentally acceptable methods are needed to remediate munitions-contaminated soil. Some perennial grass species are tolerant of soil contaminants and may promote remediation because of their high water use and extensive fibrous root systems. The effects of 2,4,6-trinitrotoluene (TNT) and its reduction product, 4-amino-2,6-dinitrotoluene (4ADNT), on germination and early seedling development of tall fescue (Festuca arundinacea Schreb.) were determined. Tall fescue seeds were germinated in nutrient-free agar containing 0-60 mg TNT litre(-1) or 0-15 mg 4ADNT litre(-1). Germination decreased linearly as TNT concentration increased but was not significantly affected by 4ADNT at these concentrations. Concentrations less than 30 mg TNT litre(-1) or 7.5 mg 4ADNT litre(-1) had little effect on seedling growth and development. Higher TNT or 4ADNT concentrations substantially delayed seedling development, caused abnormal radicle tissue development, and reduced secondary root and shoot growth. Seedling respiration rates decreased linearly with increasing TNT concentration. Experiments indicate that tall fescue may be grown in soils that maintain soil solution concentrations of 30 mg TNT litre(-1) or less. 相似文献