1,602 research outputs found
Tradeoffs in Community Properties Through Time in a Desert Rodent Community
Resource limitation represents an important constraint on ecological communities, which restricts the total abundance, biomass, and community energy flux a given community can support. However, the exact relationship among these three measures of biological activity remains unclear. Here we use a simple framework that links abundance and biomass with an energetic constraint. Under constant energetic availability, it is expected that changes in abundance and biomass can result from shifts in the distribution of individual masses. We test these predictions using long-term data from a desert rodent community. Total energy use for the community has not changed directionally for 25 years, but species composition has. As a result, the average body size has decreased by almost 50% and average abundance has doubled. These results lend support to the idea of resource limitation on desert rodent communities and demonstrate that systems are able to maintain community energy flux in the face of environmental change, through changes in composition and structure
Dynamical density-matrix renormalization-group method
I present a density-matrix renormalization-group (DMRG) method for
calculating dynamical properties and excited states in low-dimensional lattice
quantum many-body systems. The method is based on an exact variational
principle for dynamical correlation functions and the excited states
contributing to them. This dynamical DMRG is an alternate formulation of the
correction vector DMRG but is both simpler and more accurate. The finite-size
scaling of spectral functions is discussed and a method for analyzing the
scaling of dense spectra is described. The key idea of the method is a
size-dependent broadening of the spectrum.The dynamical DMRG and the
finite-size scaling analysis are demonstrated on the optical conductivity of
the one-dimensional Peierls-Hubbard model. Comparisons with analytical results
show that the spectral functions of infinite systems can be reproduced almost
exactly with these techniques. The optical conductivity of the Mott-Peierls
insulator is investigated and it is shown that its spectrum is qualitatively
different from the simple spectra observed in Peierls (band) insulators and
one-dimensional Mott-Hubbard insulators.Comment: 16 pages (REVTEX 4.0), 10 figures (in 13 EPS files
Intra-guild compensation regulatesspecies richness in desert rodents
Evidence from numerous studies suggests that species richness is an emergent property of local communities. The maintenance of species richness, despite changes in species composition and environmental conditions, requires compensatory colonization and extinction events with species coming from a regional pool. Using long-term data from a rodent community in the Chihuahuan Desert, we use randomization methods to test the null hypothesis that changes in species richness occur randomly. We find that the dynamics of species richness differ significantly from a random process, and that these nonrandom dynamics occur largely within the most speciose guild. Finally, we propose a general framework for assessing the importance of species compensation in maintaining biodiversity within local communities. Our results highlight the importance of niche complementarity and compensation in maintaining relatively constant species richness over time
Zero-sum, the niche,and metacommunities: long-term dynamics of community assembly
Recent models of community assembly, structure, and dynamics have incorporated, to varying degrees, three mechanistic processes: resource limitation and interspecific competition, niche requirements of species, and exchanges between a local community and a regional species pool. Synthesizing 30 years of data from an intensively studied desert rodent community, we show that all of these processes, separately and in combination, have influenced the structural organization of this community and affected its dynamical response to both natural environmental changes and experimental perturbations. In addition, our analyses suggest that zero-sum constraints, niche differences, and metacommunity processes are inextricably linked in the ways that they affect the structure and dynamics of this system. Explicit consideration of the interaction of these processes should yield a deeper understanding of the assembly and dynamics of other ecological communities. This synthesis highlights the role that long-term data, especially when coupled with experimental manipulations, can play in assessing the fundamental processes that govern the structure and function of ecological communities
Template-stripped gold surfaces with 0.4 nm rms roughness suitable for force measurements. Application to the Casimir force in the 20-100 nm range
Using a template-stripping method, macroscopic gold surfaces with
root-mean-square (rms) roughness less than 0.4 nm have been prepared, making
them useful for studies of surface interactions in the nanometer range. The
utility of such substrates is demonstrated by measurements of the Casimir force
at surface separations between 20 and 100 nm, resulting in good agreement with
theory. The significance and quantification of this agreement is addressed, as
well as some methodological aspects regarding the measurement of the Casimir
force with high accuracy.Comment: 7 figure
Low-energy properties and magnetization plateaus in a 2-leg mixed spin ladder
Using the density matrix renormalization group technique we investigate the
low-energy properties and the magnetization plateau behavior in a 2-leg mixed
spin ladder consisting of a spin-1/2 chain coupled with a spin-1 chain. The
calculated results show that the system is in the same universality class as
the spin-3/2 chain when the interchain coupling is strongly ferromagnetic, but
the similarity between the two systems is less clear under other coupling
conditions. We have identified two types of magnetization plateau phases. The
calculation of the magnetization distribution on the spin-1/2 and the spin-1
chains on the ladder shows that one plateau phase is related to the partially
magnetized valence-bond-solid state, and the other plateau state contains
strongly coupled S=1 and s=1/2 spins on the rung.Comment: 6 pages with 8 eps figure
Towards the architecture of an instructional multimedia database
The applicability of multimedia databases in education may be extended if they can serve multiple target groups, leading to affordable costs per unit for the user. In this contribution, an approach is described to build generic multimedia databases to serve that purpose. This approach is elaborated within the ODB Project ('Instructional Design of an Optical DataBase'); the term optical refers to the use of optical storage media to hold the audiovisual components. The project aims at developing a database in which a hypermedia encyclopedia is combined with instructional multimedia applications for different target groups at different educational levels. The architecture of the Optical Database will allow for switching between application types while working (for instance from tutorial instruction via the encyclopedia to a simulation and back). For instruction, the content of the database is thereby organized around so-called standard instruction routes: one route per target group. In the project, the teacher is regarded as the manager of instruction.\ud
\ud
From that perspective, the database is primarily organized as a teaching facility. Central to the research is the condition that the architecture of the Optical Database has to enable teachers to select and tailor instruction routes to their needs in a way that is perceived as logical and easy to use
Antiferromagnetically coupled alternating spin chains
The effect of antiferromagnetic interchain coupling in alternating spin
(1,1/2) chains is studied by mean of a spin wave theory and density matrix
renormalization group (DMRG). In particular, two limiting cases are
investigated, the two-leg ladder and its two dimensional (2D) generalization.
Results of the ground state properties like energy, spin gap, magnetizations,
and correlation functions are reported for the whole range of the interchain
coupling . For the 2D case the spin wave results predict a smooth
dimensional crossover from 1D to 2D keeping the ground state always ordered.
For the ladder system, the DMRG results show that any drives the
system to a gapped ground state. Furthermore the behaviour of the correlation
functions closely resemble the uniform spin-1/2 ladder. For lower
than 0.3, however, the gap behaves quadratically as . Finally, it is argued that the behaviour of the spin gap for an
arbitrary number of mixed coupled spin chains is analogous to that of the
uniform spin-1/2 chains.Comment: 5 pages, 7 ps-figure
Primordial magnetic fields and the HI signal from the epoch of reionization
The implication of primordial magnetic-field-induced structure formation for
the HI signal from the epoch of reionization is studied. Using semi-analytic
models, we compute both the density and ionization inhomogeneities in this
scenario. We show that: (a) The global HI signal can only be seen in emission,
unlike in the standard CDM models, (b) the density perturbations
induced by primordial fields, leave distinctive signatures of the magnetic
field Jeans' length on the HI two-point correlation function, (c) the length
scale of ionization inhomogeneities is \la 1 \rm Mpc. We find that the peak
expected signal (two-point correlation function) is in
the range of scales for magnetic field strength in the
range . We also discuss the
detectability of the HI signal. The angular resolution of the on-going and
planned radio interferometers allows one to probe only the largest magnetic
field strengths that we consider. They have the sensitivity to detect the
magnetic field-induced features. We show that thefuture SKA has both the
angular resolution and the sensitivity to detect the magnetic field-induced
signal in the entire range of magnetic field values we consider, in an
integration time of one week.Comment: 19 pages, 5 figures, to appear in JCA
Recommended from our members
Radio identifications in the NEP Deep Field
We have imaged the AKARI Deep Field with the GMRT radio telescope at 610 MHz, detecting 1224 radio components, which are optically identified with 455 optical galaxies having a mean r' magnitude brighter of 22.5 (to a completeness limit of 25.4 mag), and an average redshift ∼ 0.8
- …