Analytical solution of a neutral model of biodiversity.

International audienceThe unified neutral model of biodiversity proposed by S. Hubbell is solved analytically: The distributions of species abundance in the metacommunity and in a local community are calculated exactly as a function of speciation and migration rates and of the size of the community. In the limit of large population sizes the densities of species of given relative abundance are found to be given by universal functions depending only on two parameters

A niche remedy for the dynamical problems of neutral theory

We demonstrate how niche theory and Hubbell's original formulation of neutral theory can be blended together into a general framework modeling the combined effects of selection, drift, speciation, and dispersal on community dynamics. This framework connects many seemingly unrelated ecological population models, and allows for quantitative predictions to be made about the impact of niche stabilizing and destabilizing forces on population extinction times and abundance distributions. In particular, the existence of niche stabilizing forces in our blended framework can simultaneously resolve two major problems with the dynamics of neutral theory, namely predictions of species lifetimes that are too short and species ages that are too long.Comment: 47 pages, 4 figures, Accepted to Theoretical Ecolog

Ad-blocking Games: Monetizing Online Content Under the Threat of Ad Avoidance

Much of the Internet economy relies on online advertising for monetizing digital content: Users are expected to accept the presence of online advertisements in exchange for content being free. However, online advertisements have become a serious problem for many Internet users: while some are merely annoyed by the incessant display of distracting ads cluttering Web pages, others are highly concerned about the privacy implications - as ad providers typically track users' behavior for ad targeting purposes. Similarly, security problems related to technologies and practices employed for online advertisement have frustrated many users. Consequently, a number of software solutions have emerged that block online ads from being downloaded and displayed on users' screens as they browse the Web. We focus on these advertisement avoidance technologies for online content and their economic ramifications for the monetization of websites. More specifically, our work addresses the interplay between users' attempts to avoid commercial messages and content providers' design of countermeasures. Our investigation is substantiated by the development of a game-theoretic model that serves as a framework usable by content providers to ponder their options to mitigate the consequences of ad avoidance techniques. We complement our analytical approach with simulation results, addressing different assumptions about user heterogeneity. Our findings show that publishers who treat each user individually, and strategically deploy fee-financed or ad-financed monetization strategy, obtain higher revenues, compared to deploying one monetization strategy across all users. In addition, our analysis shows that understanding the distribution of users' aversion to ads and valuation of the content is essential for publishers to make a well-informed decision

Stochastic Spatial Models in Ecology: A Statistical Physics Approach

Ecosystems display a complex spatial organization. Ecologists have long tried to characterize them by looking at how different measures of biodiversity change across spatial scales. Ecological neutral theory has provided simple predictions accounting for general empirical patterns in communities of competing species. However, while neutral theory in well-mixed ecosystems is mathematically well understood, spatial models still present several open problems, limiting the quantitative understanding of spatial biodiversity. In this review, we discuss the state of the art in spatial neutral theory. We emphasize the connection between spatial ecological models and the physics of non-equilibrium phase transitions and how concepts developed in statistical physics translate in population dynamics, and vice versa. We focus on non-trivial scaling laws arising at the critical dimension of spatial neutral models, and their relevance for biological populations inhabiting two-dimensional environments. We conclude by discussing models incorporating non-neutral effects in the form of spatial and temporal disorder, and analyze how their predictions deviate from those of purely neutral theories.MAM is grateful to the Spanish-MINECO for financial support (under Grant FIS2013-43201-P; FEDER funds

Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data

Mars’s seismic activity and noise have been monitored since January 2019 by the seismometer of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander. At night, Mars is extremely quiet; seismic noise is about 500 times lower than Earth’s microseismic noise at periods between 4 s and 30 s. The recorded seismic noise increases during the day due to ground deformations induced by convective atmospheric vortices and ground-transferred wind-generated lander noise. Here we constrain properties of the crust beneath InSight, using signals from atmospheric vortices and from the hammering of InSight’s Heat Flow and Physical Properties (HP3) instrument, as well as the three largest Marsquakes detected as of September 2019. From receiver function analysis, we infer that the uppermost 8–11 km of the crust is highly altered and/ or fractured. We measure the crustal diffusivity and intrinsic attenuation using multiscattering analysis and find that seismic attenuation is about three times larger than on the Moon, which suggests that the crust contains small amounts of volatiles

Analytical solution of a neutral model of biodiversity.

International audienceThe unified neutral model of biodiversity proposed by S. Hubbell is solved analytically: The distributions of species abundance in the metacommunity and in a local community are calculated exactly as a function of speciation and migration rates and of the size of the community. In the limit of large population sizes the densities of species of given relative abundance are found to be given by universal functions depending only on two parameters

Influence of chiral interactions on the winding and localization of a polymer chain on a rigid rod molecule

The problem of winding of a long polymer chain around an attractive rigid rod is studied in relation with the localization transition exhibited by this system. The role of chiral and polar interactions is considered. The results are compared with previous models of adsorption of a polymer and of helix-coil transition.On étudie le problème de l'enroulement d'une longue chaîne de polymère autour d'une barre rigide, en relation avec la transition de localisation présentée par ce système. Le rôle des interactions chirales et polaires est considéré. Ces résultats sont comparés à des modèles précédemment proposés pour décrire l'adsorption d'un polymère et la transition hélice-pelote

Origin of the incommensurate phase of quartz : II. Interpretation of inelastic neutron scattering data

The results of an inelastic neutron scattering investigation of the low-frequency modes of $\beta$ quartz, described in the preceding paper [1], are interpreted using two different approaches : i) a phenomenological model directly derived from a Landau-Ginzburg type expansion of the free energy ; this model is only relevant for the long-wavelength part of the phonon spectrum but it allows an easy connection with thermodynamical data ; ii) a microscopic lattice dynamical model, which is an extension of the Grimm-Dorner model ; it is shown that the main properties of the low-frequency phonon spectrum and, in particular, the softening of a $\Sigma_2$ mode at an incommensurate wave vector close to the zone-center, can be underdtood by analysing the motions of nearly rigid SiO$_4$ tetrahedra.Les résultats de l'investigation par diffusion inélastique des neutrons des modes de basse fréquence du quartz $\beta$, décrits dans l'article précédent [1], sont interprétés à l'aide de deux approches différentes: i) un modèle phénoménologique, directement issu d'un développement du type Landau-Ginzburg de l'énergie libre ; ce modèle n'est valable que pour la partie du spectre relatif aux phonons de grande longueur d'onde, mais il permet d'établir une connexion aisée avec les données thermodynamiques ; ii) un modèle microscopique de dynamique de réseau, qui est une extension du modèle de Grimm-Dorner (modèle à tétraèdres rigides) ; on montre que les principales caractéristiques du spectre des phonons de basse fréquence, et en particulier l'amollissement d'un mode $\Sigma_2$ à un vecteur d'onde incommensurable près du centre de zone, peut être compris par une analyse des mouvements de tétraèdres SiO$_4$ presque rigides