arXiv subjects

D. Valenti

Publications and source records attributed to D. Valenti.

29 records · Page 2Linked to original sources

arXiv2005

Cyclic Fluctuations, Climatic Changes and Role of Noise in Planktonic Foraminifera in the Mediterranean Sea

The study of Planktonic Foraminifera abundances permits to obtain climatic curves on the basis of percentage ratio between tropical and temperate/polar forms. Climatic changes were controlled by several phenomena as: (i) Milankovitch's cycles, produced by variations of astronomical parameters such as precession, obliquity and eccentricity; (ii) continental geodynamic evolution and orogenic belt; (iii) variations of atmospheric and oceanic currents; (iv) volcanic eruptions; (v) meteor impacts. But while astronomical parameters have a quasi-regular periodicity, the other phenomena can be considered as "noise signal" in natural systems. The interplay between cyclical astronomical variations, the "noise signal" and the intrinsic nonlinearity of the ecologic system produces strong glacial or interglacial period according to the stochastic resonance phenomenon.

q-bio.PE↗

arXiv2005

Role of the Colored Noise in Spatio-Temporal Behavior of Two Competing Species

We study the spatial distributions of two randomly interacting species, in the presence of an external multiplicative colored noise. The dynamics of the ecosystem is described by a coupled map lattice model. We find a nonmonotonic behavior in the formation of large scale spatial correlations as a function of the multiplicative colored noise intensity. This behavior is shifted towards higher values of the noise intensity for increasing correlation time of the noise.

cond-mat.stat-mech↗

arXiv2004

Transient behavior of a population dynamical model

The transient behavior of an ecosystem with N random interacting species in the presence of a multiplicative noise is analyzed. The multiplicative noise mimics the interaction with the environment. We investigate different asymptotic dynamical regimes and the role of the external noise on the probability distribution of the local field.

cond-mat.stat-mech↗

arXiv2004

Noise in ecosystems: a short review

Noise, through its interaction with the nonlinearity of the living systems, can give rise to counter-intuitive phenomena such as stochastic resonance, noise-delayed extinction, temporal oscillations, and spatial patterns. In this paper we briefly review the noise-induced effects in three different ecosystems: (i) two competing species; (ii) three interacting species, one predator and two preys, and (iii) N-interacting species. The transient dynamics of these ecosystems are analyzed through generalized Lotka-Volterra equations in the presence of multiplicative noise, which models the interaction between the species and the environment. The interaction parameter between the species is random in cases (i) and (iii), and a periodical function, which accounts for the environmental temperature, in case (ii). We find noise-induced phenomena such as quasi-deterministic oscillations, stochastic resonance, noise-delayed extinction, and noise-induced pattern formation with nonmonotonic behaviors of patterns areas and of the density correlation as a function of the multiplicative noise intensity. The asymptotic behavior of the time average of the \emph{$i^{th}$} population when the ecosystem is composed of a great number of interacting species is obtained and the effect of the noise on the asymptotic probability distributions of the populations is discussed.

q-bio.PE↗

arXiv2004

Nonmonotonic Behavior of Spatiotemporal Pattern Formation in a Noisy Lotka-Volterra System

The noise-induced pattern formation in a population dynamical model of three interacting species in the coexistence regime is investigated. A coupled map lattice of Lotka-Volterra equations in the presence of multiplicative noise is used to analyze the spatiotemporal evolution. The spatial correlation of the species concentration as a function of time and of the noise intensity is investigated. A nonmonotonic behavior of the area of the patterns as a function of both noise intensity and evolution time is found.

cond-mat.stat-mech↗

arXiv2004

Pattern formation and spatial correlation induced by the noise in two competing species

We analyze the spatio-temporal patterns of two competing species in the presence of two white noise sources: an additive noise acting on the interaction parameter and a multiplicative noise which affects directly the dynamics of the species densities. We use a coupled map lattice (CML) with uniform initial conditions. We find a nonmonotonic behavior both of the pattern formation and the density correlation as a function of the multiplicative noise intensity.

cond-mat.stat-mech↗

arXiv2003

Enhancement of the escape time in metastable states with colored noise

We present a study of the escape time from a metastable state in the presence of colored noise, generated by Ornstein-Uhlenbeck process. We analyze the role of the correlated noise and of unstable initial conditions of an overdamped Brownian particle on the enhancement of the average escape time as a function of the noise intensity. We observe the noise enhanced stability (NES) effect for all the initial unstable states and for all values of the correlation time $τ_c$ investigated. We can distinguish two dynamical regimes characterized by: (a) a weak correlated noise and (b) a strong correlated noise, depending on the value of $τ_c$ with respect to the relaxation time. With increasing $τ_c$ we find : (i) a shift of the maximum of the average escape time towards higher values of noise intensity and an enhancement of the value of this maximum; (ii) a broadening of the NES region, which becomes very large in the strong colored noise regime; (iii) in this regime (b), the absence of the peculiar initial condition $x_c$ which separates the set of the initial unstable states lying into the divergency region from those which give only a nonmonotonic behavior of the average escape time.

cond-mat.stat-mech↗

arXiv2003

Role of the initial conditions on the enhancement of the escape time in static and fluctuating potentials

We present a study of the noise driven escape of an overdamped Brownian particle moving in a cubic potential profile with a metastable state. We analyze the role of the initial conditions of the particle on the enhancement of the average escape time as a function of the noise intensity for fixed and fluctuating potentials. We observe the noise enhanced stability effect for all the initial unstable states investigated. For a fixed potential we find a peculiar initial condition $x_c$ which separates the set of the initial unstable states in two regions: those which give rise to divergences from those which show nonmonotonic behavior of the average escape time. For fluctuating potential at this particular initial condition and for low noise intensity we find large fluctuations of the average escape time.

cond-mat.stat-mech↗

arXiv2003

Noise Induced Phenomena in Lotka-Volterra Systems

We study the time evolution of two ecosystems in the presence of external noise and climatic periodical forcing by a generalized Lotka-Volterra (LV) model. In the first ecosystem, composed by two competing species, we find noise induced phenomena such as: (i) quasi deterministic oscillations, (ii) stochastic resonance, (iii) noise delayed extinction and (iv) spatial patterns. In the second ecosystem, composed by three interacting species (one predator and two preys), using a discrete model of the LV equations we find that the time evolution of the spatial patterns is strongly dependent on the initial conditions of the three species.

cond-mat.stat-mech↗

arXiv2003

Stochastic resonance and noise delayed extinction in a model of two competing species

We study the role of the noise in the dynamics of two competing species. We consider generalized Lotka-Volterra equations in the presence of a multiplicative noise, which models the interaction between the species and the environment. The interaction parameter between the species is a random process which obeys a stochastic differential equation with a generalized bistable potential in the presence of a periodic driving term, which accounts for the environment temperature variation. We find noise-induced periodic oscillations of the species concentrations and stochastic resonance phenomenon. We find also a nonmonotonic behavior of the mean extinction time of one of the two competing species as a function of the additive noise intensity.

cond-mat.stat-mech↗

arXiv2003

Stochastic resonance for two competing species in the presence of colored noise

We study the role of multiplicative colored noise for different values of the correlation time $τ_c$ in the dynamics of two competing species, described by generalized Lotka-Volterra equations. The multiplicative colored noise models the interaction between the species and the environment. The interaction parameter between the species is a random process which obeys a stochastic differential equation with a generalized bistable potential in the presence of a periodic driving term, which accounts for the environment temperature variation. The bistable potential is useful to describe the coexistence and exclusion dynamical regimes of the ecosystem. Noise-induced periodic oscillations of the species concentrations and stochastic resonance phenomenon appear due to the presence of the multiplicative noise. We find that for low values of the correlation time $τ_c$ the response of the system coincides with that obtained with multiplicative white noise. For higher values of $τ_c$ the coherent response of the system and the maximum of the signal-to-noise ratio, signature of the stochastic resonance phenomenon, are shifted towards higher values of the noise intensity.

cond-mat.stat-mech↗