7,681 research outputs found
Demographic hallmarks of an overbrowsed population state in American ginseng
Effects of high deer herbivory in North America on populations of favored plant browse species have been well-documented, however since less palatable plants now dominate the understory, we asked whether these species could be vulnerable as well, and if so, what symptoms might signal that this was occurring? Using American ginseng (Panax quinquefoliusL.) as our representative less palatable understory plant, we compared two subpopulations within a single natural population that were differentially exposed to browse; one isolated from deer by growing atop a large, flat-topped boulder, and a browse-exposed subpopulation in the surrounding low-lying area. We tested the hypothesis that deer effects would be manifested in all parts of the life history; through reduced growth, survival and reproduction. In turn, we hypothesized that browse would reduce population growth rates, and that differences in stage structure of the population would be produced. Taking advantage of a 20 year record of formal demographic censusing, we showed that browse effects were manifested primarily in reduced size-specific growth, while size-specific fertility and survival were relatively unaffected by exposure to browse. Demographically, these differences in growth were sufficient to drive population size reductions of 4.5%/y in the off rock subpopulation while the on rock plants slowly increased in number. High browse off the rock resulted in high proportions of plants in a stunted juvenile state in the off rock population relative to the on rock plants. A high proportion of juveniles is therefore a clear symptom of an understory subjected to chronic overbrowsing, providing land managers a rapid way to assess whether deer could be impacting understory biodiversity. The sharp demographic contrasts we observed between browsed and unbrowsed subpopulations also implies that promotion of refugia within managed lands will likely become increasingly important management tools for biodiversity preservation as long as unchecked deer populations persis
Demographic hallmarks of an overbrowsed population state in American ginseng
Effects of high deer herbivory in North America on populations of favored plant browse species have been well-documented, however since less palatable plants now dominate the understory, we asked whether these species could be vulnerable as well, and if so, what symptoms might signal that this was occurring? Using American ginseng (Panax quinque- foliusL.) as our representative less palatable understory plant, we compared two sub- populations within a single natural population that were differentially exposed to browse; one isolated from deer by growing atop a large, flat-topped boulder, and a browse-exposed subpopulation in the surrounding low-lying area. We tested the hypothesis that deer ef- fects would be manifested in all parts of the life history; through reduced growth, survival and reproduction. In turn, we hypothesized that browse would reduce population growth rates, and that differences in stage structure of the population would be produced. Taking advantage of a 20 year record of formal demographic censusing, we showed that browse effects were manifested primarily in reduced size-specific growth, while size-specific fertility and survival were relatively unaffected by exposure to browse. Demographically, these dif- ferences in growth were sufficient to drive population size reductions of 4.5%/y in the off rock subpopulation while the on rock plants slowly increased in number. High browse off the rock resulted in high proportions of plants in a stunted juvenile state in the off rock population relative to the on rock plants. A high proportion of juveniles is therefore a clear symptom of an understory subjected to chronic overbrowsing, providing land managers a rapid way to assess whether deer could be impacting understory biodiversity. The sharp demographic contrasts we observed between browsed and unbrowsed subpopulations also implies that promotion of refugia within managed lands will likely become increasingly important management tools for biodiversity preservation as long as unchecked deer populations persist
Solar system constraints on the Dvali-Gabadadze-Porrati braneworld theory of gravity
A number of proposals have been put forward to account for the observed
accelerating expansion of the Universe through modifications of gravity. One
specific scenario, Dvali-Gabadadze-Porrati (DGP) gravity, gives rise to a
potentially observable anomaly in the solar system: all planets would exhibit a
common anomalous precession, dw/dt, in excess of the prediction of General
Relativity. We have used the Planetary Ephemeris Program (PEP) along with
planetary radar and radio tracking data to set a constraint of |dw/dt| < 0.02
arcseconds per century on the presence of any such common precession. This
sensitivity falls short of that needed to detect the estimated universal
precession of |dw/dt| = 5e-4 arcseconds per century expected in the DGP
scenario. We discuss the fact that ranging data between objects that orbit in a
common plane cannot constrain the DGP scenario. It is only through the relative
inclinations of the planetary orbital planes that solar system ranging data
have sensitivity to the DGP-like effect of universal precession. In addition,
we illustrate the importance of performing a numerical evaluation of the
sensitivity of the data set and model to any perturbative precession.Comment: 9 pages, 2 figures, accepted for publication in Phys. Rev.
Length scale dependence of dynamical heterogeneity in a colloidal fractal gel
We use time-resolved dynamic light scattering to investigate the slow
dynamics of a colloidal gel. The final decay of the average intensity
autocorrelation function is well described by , with and
decreasing from 1.5 to 1 with increasing . We show that the dynamics is not
due to a continuous ballistic process, as proposed in previous works, but
rather to rare, intermittent rearrangements. We quantify the dynamical
fluctuations resulting from intermittency by means of the variance
of the instantaneous autocorrelation function, the analogous of
the dynamical susceptibility studied in glass formers. The amplitude
of is found to grow linearly with . We propose a simple --yet
general-- model of intermittent dynamics that accounts for the dependence
of both the average correlation functions and .Comment: Revised and improved, to appear in Europhys. Let
Origin of entropy convergence in hydrophobic hydration and protein folding
An information theory model is used to construct a molecular explanation why
hydrophobic solvation entropies measured in calorimetry of protein unfolding
converge at a common temperature. The entropy convergence follows from the weak
temperature dependence of occupancy fluctuations for molecular-scale volumes in
water. The macroscopic expression of the contrasting entropic behavior between
water and common organic solvents is the relative temperature insensitivity of
the water isothermal compressibility. The information theory model provides a
quantitative description of small molecule hydration and predicts a negative
entropy at convergence. Interpretations of entropic contributions to protein
folding should account for this result.Comment: Phys. Rev. Letts. (in press 1996), 3 pages, 3 figure
A statistical mechanics framework for static granular matter
The physical properties of granular materials have been extensively studied
in recent years. So far, however, there exists no theoretical framework which
can explain the observations in a unified manner beyond the phenomenological
jamming diagram [1]. This work focuses on the case of static granular matter,
where we have constructed a statistical ensemble [2] which mirrors equilibrium
statistical mechanics. This ensemble, which is based on the conservation
properties of the stress tensor, is distinct from the original Edwards ensemble
and applies to packings of deformable grains. We combine it with a field
theoretical analysis of the packings, where the field is the Airy stress
function derived from the force and torque balance conditions. In this
framework, Point J characterized by a diverging stiffness of the pressure
fluctuations. Separately, we present a phenomenological mean-field theory of
the jamming transition, which incorporates the mean contact number as a
variable. We link both approaches in the context of the marginal rigidity
picture proposed by [3, 4].Comment: 21 pages, 15 figure
Isomorphic classical molecular dynamics model for an excess electron in a supercritical fluid
Ring polymer molecular dynamics (RPMD) is used to directly simulate the
dynamics of an excess electron in a supercritical fluid over a broad range of
densities. The accuracy of the RPMD model is tested against numerically exact
path integral statistics through the use of analytical continuation techniques.
At low fluid densities, the RPMD model substantially underestimates the
contribution of delocalized states to the dynamics of the excess electron.
However, with increasing solvent density, the RPMD model improves, nearly
satisfying analytical continuation constraints at densities approaching those
of typical liquids. In the high density regime, quantum dispersion
substantially decreases the self-diffusion of the solvated electron.
In this regime where the dynamics of the electron is strongly coupled to the
dynamics of the atoms in the fluid, trajectories that can reveal diffusive
motion of the electron are long in comparison to .Comment: 24 pages, 4 figure
Network synchronization of groups
In this paper we study synchronized motions in complex networks in which
there are distinct groups of nodes where the dynamical systems on each node
within a group are the same but are different for nodes in different groups.
Both continuous time and discrete time systems are considered. We initially
focus on the case where two groups are present and the network has bipartite
topology (i.e., links exist between nodes in different groups but not between
nodes in the same group). We also show that group synchronous motions are
compatible with more general network topologies, where there are also
connections within the groups
Derivation of Boltzmann Principle
We present a derivation of Boltzmann principle
based on classical mechanical models of thermodynamics. The argument is based
on the heat theorem and can be traced back to the second half of the nineteenth
century with the works of Helmholtz and Boltzmann. Despite its simplicity, this
argument has remained almost unknown. We present it in a modern, self-contained
and accessible form. The approach constitutes an important link between
classical mechanics and statistical mechanics
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Measles vaccination and antibody response in autism spectrum disorder
OBJECTIVE: To test the hypothesis that measles vaccination was involved in the pathogenesis of autism spectrum disorders (ASD) as evidenced by signs of a persistent measles infection or abnormally persistent immune response shown by circulating measles virus or raised antibody titres in children with ASD who had been vaccinated against measles, mumps and rubella (MMR) compared with controls. DESIGN: Case-control study, community based. METHODS: A community sample of vaccinated children aged 10-12 years in the UK with ASD (n = 98) and two control groups of similar age, one with special educational needs but no ASD (n = 52) and one typically developing group (n = 90), were tested for measles virus and antibody response to measles in the serum. RESULTS: No difference was found between cases and controls for measles antibody response. There was no dose-response relationship between autism symptoms and antibody concentrations. Measles virus nucleic acid was amplified by reverse transcriptase-PCR in peripheral blood mononuclear cells from one patient with autism and two typically developing children. There was no evidence of a differential response to measles virus or the measles component of the MMR in children with ASD, with or without regression, and controls who had either one or two doses of MMR. Only one child from the control group had clinical symptoms of possible enterocolitis. CONCLUSION: No association between measles vaccination and ASD was shown
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