399 research outputs found
Spatial patterns in mesic savannas: the local facilitation limit and the role of demographic stochasticity
We propose a model equation for the dynamics of tree density in mesic
savannas. It considers long-range competition among trees and the effect of
fire acting as a local facilitation mechanism. Despite short-range facilitation
is taken to the local-range limit, the standard full spectrum of spatial
structures obtained in general vegetation models is recovered. Long-range
competition is thus the key ingredient for the development of patterns. The
long time coexistence between trees and grass, and how fires affect the
survival of trees as well as the maintenance of the patterns is studied. The
influence of demographic noise is analyzed. The stochastic system, under the
parameter constraints typical of mesic savannas, shows irregular patterns
characteristics of realistic situations. The coexistence of trees and grass
still remains at reasonable noise intensities.Comment: 12 pages, 7 figure
Online games: a novel approach to explore how partial information influences human random searches
Many natural processes rely on optimizing the success ratio of a search
process. We use an experimental setup consisting of a simple online game in
which players have to find a target hidden on a board, to investigate the how
the rounds are influenced by the detection of cues. We focus on the search
duration and the statistics of the trajectories traced on the board. The
experimental data are explained by a family of random-walk-based models and
probabilistic analytical approximations. If no initial information is given to
the players, the search is optimized for cues that cover an intermediate
spatial scale. In addition, initial information about the extension of the cues
results, in general, in faster searches. Finally, strategies used by informed
players turn into non-stationary processes in which the length of each
displacement evolves to show a well-defined characteristic scale that is not
found in non-informed searches.Comment: 17 pages, 10 figure
Vegetation pattern formation in semiarid systems without facilitative mechanisms
Regular vegetation patterns in semiarid ecosystems are believed to arise from
the interplay between long-range competition and facilitation processes acting
at smaller distances. We show that, under rather general conditions, long-range
competition alone may be enough to shape these patterns. To this end we propose
a simple, general model for the dynamics of vegetation, which includes only
long-range competition between plants. Competition is introduced through a
nonlocal term, where the kernel function quantifies the intensity of the
interaction. We recover the full spectrum of spatial structures typical of
vegetation models that also account for facilitation in addition to
competition.Comment: 21 pages, 3 figure
Minimal mechanisms for vegetation patterns in semiarid regions
The minimal ecological requirements for formation of regular vegetation
patterns in semiarid systems have been recently questioned. Against the general
belief that a combination of facilitative and competitive interactions is
necessary, recent theoretical studies suggest that, under broad conditions,
nonlocal competition among plants alone may induce patterns. In this paper, we
review results along this line, presenting a series of models that yield
spatial patterns when finite-range competition is the only driving force. A
preliminary derivation of this type of model from a more detailed one that
considers water-biomass dynamics is also presented. Keywords: Vegetation
patterns, nonlocal interactionsComment: 8 pages, 4 figure
Test Center Location Problem: A bi-objective Model and Algorithms
The optimal placement of healthcare facilities, including the placement of
diagnostic test centers, plays a pivotal role in ensuring efficient and
equitable access to healthcare services. However, the emergence of unique
complexities in the context of a pandemic, exemplified by the COVID-19 crisis,
has necessitated the development of customized solutions. This paper introduces
a bi-objective integer linear programming model designed to achieve two key
objectives: minimizing average travel time for individuals visiting testing
centers and maximizing an equitable workload distribution among testing
centers. To address this problem, we propose a customized local search
algorithm based on the Voronoi diagram. Additionally, we employ an
-constraint approach, which leverages the Gurobi solver. We
rigorously examine the effectiveness of the model and the algorithms through
numerical experiments and demonstrate their capability to identify
Pareto-optimal solutions. We show that while the Gurobi performs efficiently in
small-size instances, our proposed algorithm outperforms it in large-size
instances of the problem
Spatially nonuniform phases in the one-dimensional SU(n) Hubbard model for commensurate fillings
The one-dimensional repulsive SU Hubbard model is investigated
analytically by bosonization approach and numerically using the density-matrix
renormalization-group (DMRG) method for , and 5 for commensurate
fillings where and are relatively prime. It is shown that the
behavior of the system is drastically different depending on whether ,
, or , the umklapp processes are irrelevant, the model is
equivalent to an -component Luttinger liquid with central charge . When
, the charge and spin modes are decoupled, the umklapp processes open a
charge gap for finite , whereas the spin modes remain gapless and the
central charge . The translational symmetry is not broken in the ground
state for any . On the other hand, when , the charge and spin modes are
coupled, the umklapp processes open gaps in all excitation branches, and a
spatially nonuniform ground state develops. Bond-ordered dimerized, trimerized
or tetramerized phases are found depending on the filling.Comment: 10 pages, 11 figure
The influence of isolation on the dynamics of populations and communities
Isolation is defined as the separation in time or space of individuals, populations, or of species within a community. Though isolation can be the result of many ecological processes, its role in affecting the structure and dynamics of populations and communities is not often acknowledged directly. For example, spatial heterogeneity is a frequently recognized as a significant ecological factor, but the effects of spatial heterogeneity are manifested through the isolation that heterogeneity imposes on the focal populations or communities. Isolation is an important, but hidden, component of many other ecological theories and frameworks as well. In this dissertation, I explore the role of isolation per se as an organizing theme in ecology by studying the effects of isolation in time and in space on both populations and communities.
Chapter 1 explores how isolation in time among individuals in a population may affect the population's dynamics and risk of extinction. Through a combination of modeling and meta-analysis, Chapter 1 demonstrates that reproductive asynchrony, a form of temporal isolation, can have profound negative effects at the population level in species that feature annual lifecycles. Chapter 2 reviews and synthesizes the literature on habitat connectivity, the inverse of spatial isolation, and lays out a novel framework for organizing and understanding the different metrics used to measure the connectivity. Chapter 3 examines the role of spatial isolation among species in an assemblage of Costa Rican bark beetles in mediating species interactions. The chapter uses a combination of modeling and field-collected observational data to test the hypothesis that isolation among species in this bark beetle assemblage results in a community that behaves neutrally.
The studies presented in this dissertation represent a broad sweep of the ways in which the concept of isolation may be applied to better understand the dynamics of populations and communities. Individually, each chapter is an original contribution to the ecology literature. Taken together, these papers demonstrate the power of isolation as an organizing theme in ecology and will hopefully stimulate increased research effort and theoretical development around the concept of isolation
Optimal search in interacting populations: Gaussian jumps versus Lévy flights
We investigated the relationships between search efficiency, movement strategy, and nonlocal communication in the biological context of animal foraging. We considered situations where the members of a population of foragers perform either Gaussian jumps or Lévy flights, and show that the search time is minimized when communication among individuals occurs at intermediate ranges, independently of the type of movement. Additionally, while Brownian strategies are more strongly influenced by the communication mechanism, Lévy flights still result in shorter overall search durations. © 2014 American Physical Society.R.M.-G. is supported by the JAEPredoc program of CSIC. R.M.-G. and C.L. acknowledge support from MINECO (Spain) and FEDER (EU) through Grants No. FIS2012-30634 (Intense-COSYP) and No. CTM2012-39025-C02-01 (ESCOLA). J.M.C. is supported by US National Science Foundation Grant No. ABI-1062411.Peer Reviewe
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