304 research outputs found
Discrete Models of Disease and Competition
The aim of this work is to analyze the influence of the fast development of a disease on competition dynamics. To this end we present two discrete time ecoepidemic models. The first one corresponds to the case of one parasite affecting demography and intraspecific competition in a single host, whereas the second one contemplates the more complex case of competition between two different species, one of which is infected by the parasite. We carry out a complete mathematical analysis of the asymptotic behavior of the solutions of the corresponding systems of difference equations and derive interesting ecological information about the influence of a disease in competition dynamics. This includes an assessment of the impact of the disease on the equilibrium population of both species as well as some counterintuitive behaviors in which although we would expect the outbreak of the disease to negatively affect the infected species, the contrary happens
Aggregation methods in dynamical systems and applications in population and community dynamics
Approximate aggregation techniques allow one to transform a complex system involving many coupled variables into a simpler reduced model with a lesser number of global variables in such a way that the dynamics of the former can be approximated by that of the latter. In ecology, as a paradigmatic example, we are faced with modelling complex systems involving many variables corresponding to various interacting organization levels. This review is devoted to approximate aggregation methods that are based on the existence of different time scales, which is the case in many real systems as ecological ones where the different organization levels (individual, population, community and ecosystem) possess a different characteristic time scale. Two main goals of variables aggregation are dealt with in this work. The first one is to reduce the dimension of the mathematical model to be handled analytically and the second one is to understand how different organization levels interact and which properties of a given level emerge at other levels. The review is organized in three sections devoted to aggregation methods associated to different mathematical formalisms: ordinary differential equations, infinite-dimensional evolution equations and difference equations
Global Production Increased by Spatial Heterogeneity in a Population Dynamics Model
Spatial and temporal heterogeneity are often described as important factors having a strong impact on biodiversity. The effect of heterogeneity is in most cases analyzed by the response of biotic interactions such as competition of predation. It may also modify intrinsic population properties such as growth rate. Most of the studies are theoretic since it is often difficult to manipulate spatial heterogeneity in practice. Despite the large number of studies dealing with this topics, it is still difficult to understand how the heterogeneity affects populations dynamics. On the basis of a very simple model, this paper aims to explicitly provide a simple mechanism which can explain why spatial heterogeneity may be a favorable factor for production.We consider a two patch model and a logistic growth is assumed on each patch. A general condition on the migration rates and the local subpopulation growth rates is provided under which the total carrying capacity is higher than the sum of the local carrying capacities, which is not intuitive. As we illustrate, this result is robust under stochastic perturbations
Multitrophic Interactions in the Sea: Assessing the Effect of Infochemical-Mediated Foraging in a 1-d Spatial Model
The release of chemicals following herbivore grazing on primary producers may provide feeding cues to carnivorous predators, thereby promoting multitrophic interactions. In particular, chemicals released following grazing on phytoplankton by microzooplankton herbivores have been shown to elicit a behavioural foraging response in carnivorous copepods, which may use this chemical information as a mechanism to locate and remain within biologically productive patches of the ocean. In this paper, we use a 1D spatial reaction-diffusion model to simulate a tri-trophic planktonic system in the water column, where predation at the top trophic level (copepods) is affected by infochemicals released by the primary producers forming the bottom trophic level. The effect of the infochemical-mediated predation is investigated by comparing the case where copepods forage randomly to the case where copepods adjust their vertical position to follow the distribution of grazing-induced chemicals. Results indicate that utilization of infochemicals for foraging provides fitness benefits to copepods and stabilizes the system at high nutrient load, whilst also forming a possible mechanism for phytoplankton bloom formation. We also investigate how the copepod efficiency to respond to infochemicals affects the results, and show that small increases (2%) in the ability of copepods to sense infochemicals can promote their persistence in the system. Finally we argue that effectively employing infochemicals for foraging can be an evolutionarily stable strategy for copepods
Highlights from the Pierre Auger Observatory
The Pierre Auger Observatory is the world's largest cosmic ray observatory.
Our current exposure reaches nearly 40,000 km str and provides us with an
unprecedented quality data set. The performance and stability of the detectors
and their enhancements are described. Data analyses have led to a number of
major breakthroughs. Among these we discuss the energy spectrum and the
searches for large-scale anisotropies. We present analyses of our X
data and show how it can be interpreted in terms of mass composition. We also
describe some new analyses that extract mass sensitive parameters from the 100%
duty cycle SD data. A coherent interpretation of all these recent results opens
new directions. The consequences regarding the cosmic ray composition and the
properties of UHECR sources are briefly discussed.Comment: 9 pages, 12 figures, talk given at the 33rd International Cosmic Ray
Conference, Rio de Janeiro 201
The Pierre Auger Observatory III: Other Astrophysical Observations
Astrophysical observations of ultra-high-energy cosmic rays with the Pierre
Auger ObservatoryComment: Contributions to the 32nd International Cosmic Ray Conference,
Beijing, China, August 201
Measurement of the Depth of Maximum of Extensive Air Showers above 10^18 eV
We describe the measurement of the depth of maximum, Xmax, of the
longitudinal development of air showers induced by cosmic rays. Almost four
thousand events above 10^18 eV observed by the fluorescence detector of the
Pierre Auger Observatory in coincidence with at least one surface detector
station are selected for the analysis. The average shower maximum was found to
evolve with energy at a rate of (106 +35/-21) g/cm^2/decade below 10^(18.24 +/-
0.05) eV and (24 +/- 3) g/cm^2/decade above this energy. The measured
shower-to-shower fluctuations decrease from about 55 to 26 g/cm^2. The
interpretation of these results in terms of the cosmic ray mass composition is
briefly discussed.Comment: Accepted for publication by PR
A search for point sources of EeV photons
Measurements of air showers made using the hybrid technique developed with
the fluorescence and surface detectors of the Pierre Auger Observatory allow a
sensitive search for point sources of EeV photons anywhere in the exposed sky.
A multivariate analysis reduces the background of hadronic cosmic rays. The
search is sensitive to a declination band from -85{\deg} to +20{\deg}, in an
energy range from 10^17.3 eV to 10^18.5 eV. No photon point source has been
detected. An upper limit on the photon flux has been derived for every
direction. The mean value of the energy flux limit that results from this,
assuming a photon spectral index of -2, is 0.06 eV cm^-2 s^-1, and no celestial
direction exceeds 0.25 eV cm^-2 s^-1. These upper limits constrain scenarios in
which EeV cosmic ray protons are emitted by non-transient sources in the
Galaxy.Comment: 28 pages, 10 figures, accepted for publication in The Astrophysical
Journa
Reconstruction of inclined air showers detected with the Pierre Auger Observatory
We describe the method devised to reconstruct inclined cosmic-ray air showers
with zenith angles greater than detected with the surface array of
the Pierre Auger Observatory. The measured signals at the ground level are
fitted to muon density distributions predicted with atmospheric cascade models
to obtain the relative shower size as an overall normalization parameter. The
method is evaluated using simulated showers to test its performance. The energy
of the cosmic rays is calibrated using a sub-sample of events reconstructed
with both the fluorescence and surface array techniques. The reconstruction
method described here provides the basis of complementary analyses including an
independent measurement of the energy spectrum of ultra-high energy cosmic rays
using very inclined events collected by the Pierre Auger Observatory.Comment: 27 pages, 19 figures, accepted for publication in Journal of
Cosmology and Astroparticle Physics (JCAP
The exposure of the hybrid detector of the Pierre Auger Observatory
The Pierre Auger Observatory is a detector for ultra-high energy cosmic rays.
It consists of a surface array to measure secondary particles at ground level
and a fluorescence detector to measure the development of air showers in the
atmosphere above the array. The "hybrid" detection mode combines the
information from the two subsystems. We describe the determination of the
hybrid exposure for events observed by the fluorescence telescopes in
coincidence with at least one water-Cherenkov detector of the surface array. A
detailed knowledge of the time dependence of the detection operations is
crucial for an accurate evaluation of the exposure. We discuss the relevance of
monitoring data collected during operations, such as the status of the
fluorescence detector, background light and atmospheric conditions, that are
used in both simulation and reconstruction.Comment: Paper accepted by Astroparticle Physic
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