Hubbell's neutral theory of biodiversity has generated much debate over the need for niches to explain biodiversity patterns. Discussion of the theory has focused on its neutrality assumption, i.e. the functional equivalence of species in competition and dispersal. Almost no attention has been paid to another critical aspect of the theory, the assumptions on the nature of the speciation process. In the standard version of the neutral theory each individual has a fixed probability to speciate. Hence, the speciation rate of a species is directly proportional to its abundance in the metacommunity. We argue that this assumption is not realistic for most speciation modes because speciation is an emergent property of complex processes at larger spatial and temporal scales and, consequently, speciation rate can either increase or decrease with abundance. Accordingly, the assumption that speciation rate is independent of abundance (each species has a fixed probability to speciate) is a more natural starting point in a neutral theory of biodiversity. Here we present a neutral model based on this assumption and we confront this new model to 20 large data sets of tree communities, expecting the new model to fit the data better than Hubbell's original model. We find, however, that the data sets are much better fitted by Hubbell's original model. This implies that species abundance data can discriminate between different modes of speciation, or, stated otherwise, that the mode of speciation has a large impact on the species abundance distribution. Our model analysis points out new ways to study how biodiversity patterns are shaped by the interplay between evolutionary processes (speciation, extinction) and ecological processes (competition, dispersal). S n (n 0 ; J)0 X
In this paper we present the quasi-Gaussian entropy theory in a comprehensive and consistent way, introducing a new derivation of the theory very suited for applications to molecular systems, and addressing its use in the case of multi-phase systems. A general derivation of the possible confinement of the system within a part of phase space is given, and for water it is shown that for this a hard sphere excluded volume model can be used. To obtain the temperature dependence of the pressure, a new differential equation is derived, and besides the previously introduced Gaussian and Gamma states, in this paper we also describe a new statistical state, the Inverse Gaussian state. We discuss the properties of these different statistical states and for water compare their thermodynamics with experimental data, finding that both the Gamma and Inverse Gaussian states are excellent descriptions. © 1997 American Institute of Physics. ͓S0021-9606͑97͒50305-9͔
The potential of molecular dynamics (MD) simulation for the study and prediction of particle/particle and particle/wall interaction in the wide context of technology has been explored. The present study concerns the nature of adsorbed water and its effect on the interaction between two surfaces. Computer models of two opposing (1,0,-1) crystal surfaces of R-quartz (dimensions 5.49 × 4.91 nm) were constructed and up to 1500 water molecules positioned between the surfaces. The simulations were performed in the NVT ensemble in "math mode" at a temperature of 300 K. The axial profiles of density and mobility (the latter resolved in planar and axial components) in the adsorbed layers were studied. The separation between the crystal surfaces was varied, monitoring the adsorbed layer morphology and the forces acting on the crystals. Most of the simulations shown are with 1500 molecules between the plates, giving around 3.1 adsorbed monolayers, corresponding to a relative saturation (humidity) of 67% according to the BET isotherm. The density profiles show an ordered packing of molecules in the first two adsorbed layers with density peaks considerably higher than in bulk water and a low molecular mobility. The density tails off to zero, and the mobility rises to above that of bulk water at the surface of the adsorbed layer, which was clearly defined but undulating. Determination of the forces acting on the crystals was difficult due to strong fluctuations on a short time scale, so only simulations for long times yielded statistically significant average forces. At a surface separation of 3 nm, spontaneous bridge forming took place, paired with significant attractive forces between the crystals. The nature of the bridge is discussed. The observed bridging and resulting surface/surface force are shown to be roughly consistent with expectations based on macroscopic theory represented by the BET isotherm, the Kelvin equation (using the surface tension of bulk water), and a bridging force calculated from pressure-deficiency and surface tension contributions.
Summary1. The allometry of metabolic rate has long been one of the key relationships in ecology. While its existence is generally agreed on, the exact value of the scaling exponent, and the key mechanisms that determine its value, are still hotly debated. 3.Here we reconstruct the model and derive the exponent in a clearer and much more straightforward way that requires weaker assumptions than the original model. Specifically, self-similarity of the network is not required. Our formulation can even be used if one or several assumptions of West et al . (1997) are considered invalid. 4. Moreover, we provide a formula for the proportionality constant (i.e. the intercept of the allometric scaling relation) that shows explicitly where factors as temperature and stoichiometry affect metabolism.
Application of the quasiGaussian entropy theory to the calculation of thermodynamic properties of water and methane in the liquid and gas phase A new theory is presented for calculating the Helmholtz free energy based on the potential energy distribution function. The usual expressions of free energy, internal energy and entropy involving the partition function are rephrased in terms of the potential energy distribution function, which must be a near Gaussian one, according to the central limit theorem. We obtained expressions for the free energy and entropy with respect to the ideal gas, in terms of the potential energy moments. These can be linked to the average potential energy and its derivatives in temperature. Using thermodynamical relationships we also produce a general differential equation for the free energy as a function of temperature at fixed volume. In this paper we investigate possible exact and approximated solutions. The method was tested on a theoretical model for a solid ͑classical harmonic solid͒ and some experimental liquids. The harmonic solid has an energy distribution, which can be derived exactly from the theory. Experimental free energies of water and methanol could be reproduced very well over a temperature range of more than 300 K. For water, where the appropriate experimental data were available, also the energy and heat capacity could be reproduced very well.
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