128 research outputs found
Persistence time of SIS infections in heterogeneous populations and networks
For a susceptible-infectious-susceptible (SIS) infection model in a
heterogeneous population, we present simple formulae giving the leading-order
asymptotic (large population) behaviour of the mean persistence time, from an
endemic state to extinction of infection. Our model may be interpreted as
describing an infection spreading through either (i) a population with
heterogeneity in individuals' susceptibility and/or infectiousness; or (ii) a
heterogeneous directed network. Using our asymptotic formulae, we show that
such heterogeneity can only reduce (to leading order) the mean persistence time
compared to a corresponding homogeneous population, and that the greater the
degree of heterogeneity, the more quickly infection will die out
Approximating time to extinction for endemic infection models
Approximating the time to extinction of infection is an important problem in
infection modelling. A variety of different approaches have been proposed in
the literature. We study the performance of a number of such methods, and
characterize their performance in terms of simplicity, accuracy, and
generality. To this end, we consider first the classic stochastic
susceptible-infected-susceptible (SIS) model, and then a multi-dimensional
generalization of this which allows for Erlang distributed infectious periods.
We find that (i) for a below-threshold infection initiated by a small number of
infected individuals, approximation via a linear branching process works well;
(ii) for an above-threshold infection initiated at endemic equilibrium, methods
from Hamiltonian statistical mechanics yield correct asymptotic behaviour as
population size becomes large; (iii) the widely-used Ornstein-Uhlenbeck
diffusion approximation gives a very poor approximation, but may retain some
value for qualitative comparisons in certain cases; (iv) a more detailed
diffusion approximation can give good numerical approximation in certain
circumstances, but does not provide correct large population asymptotic
behaviour, and cannot be relied upon without some form of external validation
(eg simulation studies)
La pesca industrial responsable en Chimbote y su impacto en el producto bruto interno del sector pesquero, en el periodo 2016 – 2017
El problema que aborda la presente investigación fue la falta de conocimiento
sobre el impacto económico de la Pesca Industrial Responsable en Chimbote
en el Producto Bruto Interno Peruano en el periodo 2016-2017. El objetivo
general consistió en determinar el impacto económico que tuvo la Pesca
industrial responsable en Chimbote en el Producto Bruto Interno Peruano en
el periodo 2016-2017. La hipótesis planteada fue que la pesca industrial
responsable en Chimbote tuvo impacto económico positivo en el Producto
Bruto Interno peruano, periodo 2016-2017. El tipo de estudio fue no
experimental, de diseño descriptivo correlacional. Se aplicó encuesta a una
muestra de tamaño de 30 personas. Existió un impacto negativo bajo de la
Pesca Industrial Responsable en el Producto Bruto Interno peruano en el
periodo 2016-2017. (Índice de correlación de Spearman rs = -0.212458).
Existió un impacto negativo bajo de la Pesca Industrial Responsable en el
Producto Bruto Interno Real peruano en el periodo 2016-2017. (rs = -
0.133259). Existió un impacto negativo bajo de la Pesca Industrial
Responsable en el Producto Bruto Interno Nominal peruano en el periodo
2016-2017. (rs = -0.360400). Existió un impacto negativo muy bajo de la Pesca
Industrial Responsable en el Producto Bruto Interno Per cápita peruano en el
periodo 2016-2017. (rs = -0.0832036)
Among-site variability in the stochastic dynamics of East African coral reefs
Coral reefs are dynamic systems whose composition is highly influenced by
unpredictable biotic and abiotic factors. Understanding the spatial scale at
which long-term predictions of reef composition can be made will be crucial for
guiding conservation efforts. Using a 22-year time series of benthic
composition data from 20 reefs on the Kenyan and Tanzanian coast, we studied
the long-term behaviour of Bayesian vector autoregressive state-space models
for reef dynamics, incorporating among-site variability. We estimate that if
there were no among-site variability, the total long-term variability would be
approximately one third of its current value. Thus among-site variability
contributes more to long-term variability in reef composition than does
temporal variability. Individual sites are more predictable than previously
thought, and predictions based on current snapshots are informative about
long-term properties. Our approach allowed us to identify a subset of possible
climate refugia sites with high conservation value, where the long-term
probability of coral cover <= 0.1 was very low. Analytical results show that
this probability is most strongly influenced by among-site variability and by
interactions among benthic components within sites. These findings suggest that
conservation initiatives might be successful at the site scale as well as the
regional scale.Comment: 97 pages, 49 figure
Effects of human land use and temperature on community dynamics in European forests
Climate change and human land use are thought to play a dominant role in the dynamics of European central-latitude forests in the Holocene. A wide range of mathematical and statistical models have been used to study the effects of these variables on forest dynamics, including physiologically-based simulations and phenomenological community models. However, for statistical analysis of pollen count data, compositional data analysis is particularly well suited, because pollen counts give only relative information. We studied the effects of changes in human land use and temperature on European central-latitude forest dynamics at 7 sites over most of the last , using a stochastic model for compositional dynamics of pollen count data. Our approach has a natural ecological interpretation in terms of relative proportional population growth rates, and does not require information on pollen production, dispersal, or deposition. We showed that the relative proportional population growth rates of Fagus and Picea were positively affected by intensified human land use, and that those of Tilia and Ulmus were negatively affected. Also, the relative proportional population growth rate of Fagus was negatively affected by increases in temperature above about . Overall, the effects of temperature on the rate of change of forest composition were more important than those of human land use. Although there were aspects of dynamics, such as short-term oscillations, that our model did not capture, our approach is broadly applicable and founded on ecological principles, and gave results consistent with current thinking
Semi-stochastic models for Salmonella infection within finishing pig units in the UK
A multi-group semi-stochastic model is formulated to describe Salmonella dynamics on a pig herd within the UK and assess whether farm structure has any effect on the dynamics. The models include both direct transmission and indirect (via free-living infectious units in the environment and airborne infection). The basic reproduction number [Formula: see text] is also investigated. The models estimate approximately 24.6% and 25.4% of pigs at slaughter weight will be infected with Salmonella within a slatted-floored and solid-floored unit respectively, which corresponds to values found in previous abattoir and farm studies, suggesting that the model has reasonable validity. Analysis of the models identified the shedding rate to be of particular importance in the control of Salmonella spread, a finding also evident in an increase in the [Formula: see text] value
Sex-specific effects of mitochondrial haplotype on metabolic rate in Drosophila melanogaster support predictions of the Mother's Curse hypothesis
Evolutionary theory proposes that maternal inheritance of mitochondria will facilitate the accumulation of mitochondrial DNA (mtDNA) mutations that are harmful to males but benign or beneficial to females. Furthermore, mtDNA haplotypes sampled from across a given species distribution are expected to differ in the number and identity of these ‘male-harming’ mutations they accumulate. Consequently, it is predicted that the genetic variation which delineates distinct mtDNA haplotypes of a given species should confer larger phenotypic effects on males than females (reflecting mtDNA mutations that are male-harming, but female-benign), or sexually antagonistic effects (reflecting mutations that are male-harming, but female-benefitting). These predictions have received support from recent work examining mitochondrial haplotypic effects on adult life-history traits in Drosophila melanogaster. Here, we explore whether similar signatures of male-bias or sexual antagonism extend to a key physiological trait—metabolic rate. We measured the effects of mitochondrial haplotypes on the amount of carbon dioxide produced by individual flies, controlling for mass and activity, across 13 strains of D. melanogaster that differed only in their mtDNA haplotype. The effects of mtDNA haplotype on metabolic rate were larger in males than females. Furthermore, we observed a negative intersexual correlation across the haplotypes for metabolic rate. Finally, we uncovered a male-specific negative correlation, across haplotypes, between metabolic rate and longevity. These results are consistent with the hypothesis that maternal mitochondrial inheritance has led to the accumulation of a sex-specific genetic load within the mitochondrial genome, which affects metabolic rate and that may have consequences for the evolution of sex differences in life history
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