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

Le développement de l'embryon zygotique chez Vitis vinifera L.

The development of the zygotic embryo of Vitis vinifera L.The embryonic development of Vitis vinifera from the zygote to the mature embryo has been studied. The planes of early cell divisions are irregular, thus leading to formation of a large globular proembryo comprising about 1500 undifferentiated cells above a bulky suspensor. Organogenesis of the embryo sta rts with initiation of the cotyledons and constitution of the radicular meristem. The intercotyledonary zone, future shoot apex, is formed by about 300 cells. In this zone, cells remain undifferentiated throughout embryogenesis; its organization begins to be completed only after germination. The radicular meristem, on the other hand, is functioning from the embryonic stage. The meristem of the embryonic radicle and the meristem of the adventitious root are identical in their ('open' type) organization

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

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

Transition rugueuse et localisation pour une singularité linéaire dans un espace à deux ou trois dimensions

The continuous Gaussian approximation is used to describe the thermodynamical properties of an interfacial line in the two-dimensional Ising model ; the exact results concerning the roughening transition of the Abraham model are recovered for TR ⪡ Tc. Furthermore the correlation function for the displacements of the interfacial points is calculated and the influence of a uniform applied field is also studied. The same model is extended to the case of a linear singularity in the three-dimensional space : it is shown that one can get a roughening transition and the free energy exhibits an essential singularity - exp - (A/TR-T).Le modèle gaussien continu est utilisé pour décrire les propriétés thermodynamiques de la ligne d'interface dans le modèle d'Ising à deux dimensions ; on retrouve les résultats exacts concernant la transition rugueuse du modèle d'Abraham pour TR ⪡ Tc On calcule en outre la fonction de corrélation de position des points de l'interface et on étudie l'influence d'un champ extérieur uniforme. Le même modèle est étendu au cas d'une singularité linéaire dans l'espace d = 3 : on montre qu'on peut avoir une transition rugueuse avec une énergie libre possédant une singularité en exp- (A/TR-T)

Birefringence study of the tricritical point of KDP

The tricritical behaviour of KDP has been investigated using birefringence measurements under high pressure (up to 0.5 GPa) and d.c. electric field (up to 100 kV/m). The behaviour of the order parameter and the 3-dimensional phase diagram have been deduced from these data. The experimental results are consistently interpreted in the frame of a phenomenological Landau theory both in the critical and the tricritical regions. The numerical values of the Landau coefficients are compared to those obtained by other authors and other techniques.Le comportement tricritique du KDP a été étudié en utilisant les mesures de biréfringence sous haute pression (jusqu'a 0,5 GPa) et sous champ électrique continu (jusqu'a 100 kV/m). Le comportement du paramètre d'ordre et du diagramme de phase tricritique a été déduit de ces mesures. Les résultats expérimentaux sont interprétés de façon cohérente dans le cadre de la théorie phénoménologique de Landau pour l'ensemble des deux régions critique et tricritique. Les valeurs numériques des coefficients de Landau sont comparées à celles obtenues par d'autres auteurs et d'autres techniques

CALCULATION OF THE ELASTIC AND ELECTROSTATIC ENERGIES OF AN INCLUSION IN A FERROELECTRIC OR A PIEZOELECTRIC CRYSTAL

Previous optical observations have shown that the first order structural phase transitions of some ferroelectric (e.g. KD2PO4) or piezoelectric (e.g. quartz) crystals present some similarity with martensitic transformations : two phases with different lattice parameters coexist over a small temperature range and the low temperature phase can be twinned ; (this twinning corresponds to alternate shears and electric polarization in the case of KD2PO4). In contrast with martensites both elastic and electric properties are important in determining the morphology of the heterophase structure. In particular the habit plane separating the two phases is no longer a plane without long range strains when electric energies are not negligible. In this case one has to determine the configuration for which the sum of the elastic and electric energies is minimum : this can be done for an inclusion of the new phase in an infinite matrix of the parent phase by extending Khachaturyan's theory to take into account the electric and piezeoelectric properties. We present an outline of this theory in the case of crystals of any symmetry. The effect of a uniform applied stress or of an electric field is also discussed

Critical behaviour of compressible Ising models at marginal dimensionalities

Renormalization group methods are applied to study the critical behaviour of a compressible n-component Ising model with short range interactions at d = 4 and a one component Ising model with dipolar interactions at d = 3. The recursion equations are exactly solved in the case of an elastic system of spherical symmetry (d = 4) or cylindrical symmetry (d = 3); new types of logarithmic corrections, corresponding to a Fisher renormalization at marginal dimensions, are found. It is shown that the system exhibits a first-order transition for constant pressure external conditions or when anisotropy is taken into account. The relevance of the calculations to the critical behaviour of uniaxial ferroelectrics is discussed.Les méthodes du groupe de renormalisation sont appliquées à l'étude du comportement critique du modèle d'Ising compressible à n composantes avec interaction à courte distance à d = 4 et du modèle d'Ising à une composante avec interactions dipolaires à d = 3. Les équations de récurrence sont résolues exactement dans le cas d'un système élastique de symétrie sphérique (d = 4) ou de symétrie cylindrique (d = 3); de nouveaux types de corrections logarithmiques sont obtenus, correspondant à une renormalisation de Fisher pour la dimension marginale. On montre que le système présente une transition du premier ordre dans des conditions de pression extérieure constante ou quand l'anisotropie est prise en compte. On discute l'intérêt des présents calculs pour l'étude du comportement critique des ferro-électriques uniaxiaux