559 research outputs found
Spatial and Ecological Scaling of Stability in Spatial Community Networks
There are many scales at which to quantify stability in spatial and
ecological networks. Local-scale analyses focus on specific nodes of the
spatial network, while regional-scale analyses consider the whole network.
Similarly, species- and community-level analyses either account for single
species or for the whole community. Furthermore, stability itself can be
defined in multiple ways, including resistance (the inverse of the relative
displacement caused by a perturbation), initial resilience (the rate of return
after a perturbation), and invariability (the inverse of the relative amplitude
of the population fluctuations). Here, we analyze the scale-dependence of these
stability properties. More specifically, we ask how spatial scale (local vs
regional) and ecological scale (species vs community) influence these stability
properties. We find that regional initial resilience is the weighted arithmetic
mean of the local initial resiliences. The regional resistance is the harmonic
mean of local resistances, which makes regional resistance particularly
vulnerable to nodes with low stability, unlike regional initial resilience.
Analogous results hold for the relationship between community- and
species-level initial resilience and resistance. Both resistance and initial
resilience are ``scale-free'' properties: regional and community values are
simply the biomass-weighted means of the local and species values,
respectively. Thus, one can easily estimate both stability metrics of whole
networks from partial sampling. In contrast, invariability generally is greater
at the regional and community-level than at the local and species-level,
respectively. Hence, estimating the invariability of spatial or ecological
networks from measurements at the local or species level is more complicated,
requiring an unbiased estimate of the network (i.e. region or community) size
Dispersal-induced resilience to stochastic environmental fluctuations in populations with Allee effect
Many species are unsustainable at small population densities (Allee Effect).
This implies that for population densities below a threshold, named Allee
threshold, the population decreases instead of growing. In a closed local
population, this makes that environmental fluctuations always leads to
extinction. Here, we show how, in spatially extended habitats, dispersal can
lead to a sustainable population in a region, provided the amplitude of
environmental fluctuations is below an extinction threshold. We have identified
two types of sustainable populations: high-density and low-density populations
(through a mean-field approximation, valid in the limit of large dispersal
length). Our results show that close to global extinction patches where
population density is high, low or extinct coexist (even for homogeneous
habitats). The extinction threshold increases proportionally to the squared
root of the dispersal rate, decreases with the Allee threshold, and it is
maximum for characteristic dispersal distances much larger than the spatial
scale of synchrony of environmental fluctuations. The low-density population
solution can be particularly interesting for future applications, as to
understand non-recovery events after harvesting. This theoretical framework
allows novel approaches to address the impact of other factors, as habitat
fragmentation, on the population resilience to environmental fluctuations.Comment: 24 pages, 3 figures, 1 tabl
Domain wall dynamics in expanding spaces
We study the effects on the dynamics of kinks due to expansions and
contractions of the space. We show that the propagation velocity of the kink
can be adiabatically tuned through slow expansions/contractions, while its
width is given as a function of the velocity. We also analyze the case of fast
expansions/contractions, where we are no longer on the adiabatic regime. In
this case the kink moves more slowly after an expansion-contraction cycle as a
consequence of loss of energy through radiation. All these effects are
numerically studied in the nonlinear Klein-Gordon equations (both for the
sine-Gordon and for the phi^4 potential), and they are also studied within the
framework of the collective coordinate evolution equations for the width and
the center of mass of the kink. These collective coordinate evolution equations
are obtained with a procedure that allows us to consider even the case of large
expansions/contractions.Comment: LaTeX, 18 pages, 2 figures, improved version to appear in Phys Rev
Open Problems on Information and Feedback Controlled Systems
Feedback or closed-loop control allows dynamical systems to increase their
performance up to a limit imposed by the second law of thermodynamics. It is
expected that within this limit, the system performance increases as the
controller uses more information about the system. However, despite the
relevant progresses made recently, a general and complete formal development to
justify this statement using information theory is still lacking. We present
here the state-of-the-art and the main open problems that include aspects of
the redundancy of correlated operations of feedback control and the continuous
operation of feedback control. Complete answers to these questions are required
to firmly establish the thermodynamics of feedback controlled systems. Other
relevant open questions concern the implications of the theoretical results for
the limitations in the performance of feedback controlled flashing ratchets,
and for the operation and performance of nanotechnology devices and biological
systems.Comment: LaTeX, 10 pages, 2 figures. Improved version to appear in Entrop
Information in feedback ratchets
Feedback control uses the state information of the system to actuate on it.
The information used implies an effective entropy reduction of the controlled
system, potentially increasing its performance. How to compute this entropy
reduction has been formally shown for a general system, and has been explicitly
computed for spatially discrete systems. Here, we address a relevant example of
how to compute the entropy reduction by information in a spatially continuous
feedback-controlled system. Specifically, we consider a feedback flashing
ratchet, which constitutes a paradigmatic example for the role of information
and feedback in the dynamics and thermodynamics of transport induced by the
rectification of Brownian motion. A Brownian particle moves in a periodic
potential that is switched on and off by a controller, with the latter
performing the switching depending on the system state. We show how the entropy
reduction can be computed from the entropy of a sequence of control actions,
and also discuss the required sampling effort for its accurate computation.
Moreover, the output power developed by the particle against an external force
is investigated, which -- for some values of the system parameters -- is shown
to become larger than the input power due to the switching of the potential:
the apparent efficiency of the ratchet thus becomes higher than one, if the
entropy reduction contribution is not considered. This result highlights the
relevance of including the entropy reduction by information in the
thermodynamic balance of feedback controlled devices, specifically when writing
the second principle: the inclusion of the entropy reduction by information
leads to a well-behaved efficiency over all the range of parameters
investigated.Comment: 17 pages, 13 figure
On the athermal character of structural phase transitions
The significance of thermal fluctuations on nucleation in structural
first-order phase transitions has been examined. The prototype case of
martensitic transitions has been experimentally investigated by means of
acoustic emission techniques. We propose a model based on the mean
first-passage time to account for the experimental observations. Our study
provides a unified framework to establish the conditions for isothermal and
athermal transitions to be observed.Comment: 5 pages, 4 figures, accepted in Phys. Rev. Let
Star-forming outflowing QSOs at z~2: their status, future and environments
Trabajo presentado al Demographics and environment of AGN from multi-wavelength surveys, celebrado en Creta del 21 al 24 de septiembre de 2015.Peer Reviewe
Observations and Theoretical Implications of the Large Separation Lensed Quasar SDSS J1004+4112
We study the recently discovered gravitational lens SDSS J1004+4112, the
first quasar lensed by a cluster of galaxies. It consists of four images with a
maximum separation of 14.62''. The system has been confirmed as a lensed quasar
at z=1.734 on the basis of deep imaging and spectroscopic follow-up
observations. We present color-magnitude relations for galaxies near the lens
plus spectroscopy of three central cluster members, which unambiguously confirm
that a cluster at z=0.68 is responsible for the large image separation. We find
a wide range of lens models consistent with the data, but they suggest four
general conclusions: (1) the brightest cluster galaxy and the center of the
cluster potential well appear to be offset by several kpc; (2) the cluster mass
distribution must be elongated in the North--South direction, which is
consistent with the observed distribution of cluster galaxies; (3) the
inference of a large tidal shear (~0.2) suggests significant substructure in
the cluster; and (4) enormous uncertainty in the predicted time delays between
the images means that measuring the delays would greatly improve constraints on
the models. We also compute the probability of such large separation lensing in
the SDSS quasar sample, on the basis of the CDM model. The lack of large
separation lenses in previous surveys and the discovery of one in SDSS together
imply a mass fluctuation normalization \sigma_8=1.0^{+0.4}_{-0.2} (95% CL), if
cluster dark matter halos have an inner slope -1.5. Shallower profiles would
require higher values of \sigma_8. Although the statistical conclusion might be
somewhat dependent on the degree of the complexity of the lens potential, the
discovery is consistent with the predictions of the abundance of cluster-scale
halos in the CDM scenario. (Abridged)Comment: 21 pages, 24 figures, 5 tables, accepted for publication in Ap
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