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Constant Fraction (constant + fraction)

Distribution by Scientific Domains

Life Sciences	50%
Business, Economics, Finance and Accounting	33%

Selected Abstracts

EVOLUTION O ANTIBIOTIC RESISTANCE BY HUMAN AND BACTERIAL NECHE CONSTRUCTION

EVOLUTION, Issue 3 2005
Maciej F. Boni
Abstract Antibiotic treatment by humans generates strong viability selection for antibiotic-resistant bacterial strains. The frequency of host antibiotic use often determines the strength of this selection, and changing patterns of antibiotic use can generate many types of behaviors in the population dynamics of resistant and sensitive bacterial populations. In this paper, we present a simple model of hosts dimorphic for their tendency to use/avoid antibiotics and bacterial pathogens dimorphic in their resistance/sensitivity to antibiotic treatment. When a constant fraction of hosts uses antibiotics, the two bacterial strain populations can coexist unless host use-frequency is above a critical value; this critical value is derived as the ratio of the fitness cost of resistance to the fitness cost of undergoing treatment. When strain frequencies can affect host behavior, the dynamics may be analyzed in the light of niche construction. We consider three models underlying changing host behavior: conformism, the avoidance of long infections, and adherence to the advice of public health officials. In the latter two, we find that the pathogen can have quite a strong effect on host behavior. In particular, if antibiotic use is discouraged when resistance levels are high, we observe a classic niche-construction phenomenon of maintaining strain polymorphism even in parameter regions where it would not be expected. [source]

Estimating annual N2O emissions from agricultural soils in temperate climates

GLOBAL CHANGE BIOLOGY, Issue 10 2005
Caroline Roelandt
Abstract The Kyoto protocol requires countries to provide national inventories for a list of greenhouse gases including N2O. A standard methodology proposed by the Intergovernmental Panel on Climate Change (IPCC) estimates direct N2O emissions from soils as a constant fraction (1.25%) of the nitrogen input. This approach is insensitive to environmental variability. A more dynamic approach is needed to establish reliable N2O emission inventories and to propose efficient mitigation strategies. The objective of this paper is to develop a model that allows the spatial and temporal variation in environmental conditions to be taken into account in national inventories of direct N2O emissions. Observed annual N2O emission rates are used to establish statistical relationships between N2O emissions, seasonal climate and nitrogen-fertilization rate. Two empirical models, MCROPS and MGRASS, were developed for croplands and grasslands. Validated with an independent data set, MCROPS shows that spring temperature and summer precipitation explain 35% of the variance in annual N2O emissions from croplands. In MGRASS, nitrogen-fertilization rate and winter temperature explain 48% of the variance in annual N2O emissions from grasslands. Using long-term climate observations (1900,2000), the sensitivity of the models with climate variability is estimated by comparing the year-to-year prediction of the model to the precision obtained during the validation process. MCROPS is able to capture interannual variability of N2O emissions from croplands. However, grassland emissions show very small interannual variations, which are too small to be detectable by MGRASS. MCROPS and MGRASS improve the statistical reliability of direct N2O emissions compared with the IPCC default methodology. Furthermore, the models can be used to estimate the effects of interannual variation in climate, climate change on direct N2O emissions from soils at the regional scale. [source]

Spatial and temporal dynamics of methane emissions from agricultural sources in China

GLOBAL CHANGE BIOLOGY, Issue 1 2001
Peter H. Verburg
Summary Agricultural activities contribute significantly to the global methane budget. Agricultural sources of methane are influenced by land-use change, including changes in agricultural area, livestock keeping and agricultural management practices. A spatially explicit inventory of methane emissions from agriculture is made for China taking the interconnections between the different agricultural sources into account. The influence of land-use change on methane emissions is studied by linking a dynamic land-use change model with emission calculations. The land-use change model calculates changes in rice area and livestock numbers for a base-line scenario. Emissions are calculated for 1991 based on land-use statistics and for 2010 based on simulated changes in land-use patterns. Emissions from enteric fermentation and manure management are based on emission factors, while emissions from rice paddies involve the calculation of total organic carbon added to rice paddy soils and assume that a constant fraction is emitted as methane. Spatial patterns of emissions are presented for the different sources. For the land-use scenario considered it is expected that total methane emissions from agricultural sources in China increase by 11% while the relative contribution of rice fields to the emission decreases. Emissions from manure management are expected to become more important. These results indicate that agencies should anticipate changes in source strengths as a consequence of land-use changes when proposing mitigation strategies and future national greenhouse gas budgets. [source]

An elementary model for the dust cycle in galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001
M.G. Edmunds
We give an elementary model for the evolution of dust in galaxies, based on abundance arguments. The model takes account of grain core production in both supernovae and giant stars, and includes mantle growth in the interstellar medium. Destruction of grain cores does not appear to be a dominant effect. We show that a self-consistent picture can be made in which the interstellar dust mass is an approximately constant fraction of the heavy element mass in the interstellar medium. This result is demonstrated to be essentially independent of outflow or inflow of interstellar material. [source]

Ein einheitliches Rentensystem für Ost- und Westdeutschland: Simulationsrechnungen zum Reformvorschlag des Sachverständigenrates

PERSPEKTIVEN DER WIRTSCHAFTSPOLITIK, Issue 1 2010
Axel Börsch-Supan
In this paper, we quantify the effects of this proposal. We show that the direction and size of the effects largely depend on the development of future wages. In the most realistic case we assume that the average wages in East Germany remain a constant fraction of the average wage in the West over time. In this case, the Council's proposal only weakly affects the size of pensions in East and West Germany. Thus, the effect on the contribution rate to the pension system is also weak. On the contrary, if average wages in East and West fully converge in the future, this reform would lead to redistribution from pensioners in the East to pensioners in the West. In the most unrealistic case, where average wages in East and West Germany continue to diverge in the future, pensioners in both East and West Germany would be worse off with the reform. However, contribution rates to the pension system would be relatively lower, leading to redistribution from the old to the young. [source]

High-Water Marks: High Risk Appetites?

THE JOURNAL OF FINANCE, Issue 1 2009
Convex Compensation, Long Horizons, Portfolio Choice
ABSTRACT We study the portfolio choice of hedge fund managers who are compensated by high-water mark contracts. We find that even risk-neutral managers do not place unbounded weights on risky assets, despite option-like contracts. Instead, they place a constant fraction of funds in a mean-variance efficient portfolio and the rest in the riskless asset, acting as would constant relative risk aversion (CRRA) investors. This result is a direct consequence of the in(de)finite horizon of the contract. We show that the risk-seeking incentives of option-like contracts rely on combining finite horizons and convex compensation schemes rather than on convexity alone. [source]