2,617 research outputs found
Preventing extinction and outbreaks in chaotic populations
Interactions in ecological communities are inherently nonlinear and can lead
to complex population dynamics including irregular fluctuations induced by
chaos. Chaotic population dynamics can exhibit violent oscillations with
extremely small or large population abundances that might cause extinction and
recurrent outbreaks, respectively. We present a simple method that can guide
management efforts to prevent crashes, peaks, or any other undesirable state.
At the same time, the irregularity of the dynamics can be preserved when chaos
is desirable for the population. The control scheme is easy to implement
because it relies on time series information only. The method is illustrated by
two examples: control of crashes in the Ricker map and control of outbreaks in
a stage-structured model of the flour beetle Tribolium. It turns out to be
effective even with few available data and in the presence of noise, as is
typical for ecological settings.Comment: 10 pages, 6 figure
Commodity Markets, Price Limiters and Speculative Price Dynamics
We develop a behavioral commodity market model with consumers, producers and heterogeneous speculators to characterize the nature of commodity price fluctuations and to explore the efectiveness of price stabilization schemes. Within our model, nonlinear interactions between market participants can create either bull or bear markets, or irregular price fluctuations between bulland bear markets. Both the imposition of a bottoming price level (to support producers) or a topping price level (to protect consumers) can reduce market price volatility. However, simple policy rules, such as price limiters, may have unexpected consequences in a complex environment: a minimum price level decreases the average price while a maximum price limit increases the average price. In addition, price limiters influence the price dynamics in an intricate way and may cause volatility clustering.commodity markets; price stabilization; simple limiters; technical and fundamental anaysis; bifurcation analysis; chaos control
Total Value of Phosphorus Recovery
Phosphorus (P) is a critical, geographically concentrated, nonrenewable resource necessary to support global food production. In excess (e.g., due to runoff or wastewater discharges), P is also a primary cause of eutrophication. To reconcile the simultaneous shortage and overabundance of P, lost P flows must be recovered and reused, alongside improvements in P-use efficiency. While this motivation is increasingly being recognized, little P recovery is practiced today, as recovered P generally cannot compete with the relatively low cost of mined P. Therefore, P is often captured to prevent its release into the environment without beneficial recovery and reuse. However, additional incentives for P recovery emerge when accounting for the total value of P recovery. This article provides a comprehensive overview of the range of benefits of recovering P from waste streams, i.e., the total value of recovering P. This approach accounts for P products, as well as other assets that are associated with P and can be recovered in parallel, such as energy, nitrogen, metals and minerals, and water. Additionally, P recovery provides valuable services to society and the environment by protecting and improving environmental quality, enhancing efficiency of waste treatment facilities, and improving food security and social equity. The needs to make P recovery a reality are also discussed, including business models, bottlenecks, and policy and education strategies
Metabolic Futile Cycles and Their Functions: A Systems Analysis of Energy and Control
It has long been hypothesized that futile cycles in cellular metabolism are
involved in the regulation of biochemical pathways. Following the work of
Newsholme and Crabtree, we develop a quantitative theory for this idea based on
open-system thermodynamics and metabolic control analysis. It is shown that the
{\it stoichiometric sensitivity} of an intermediary metabolite concentration
with respect to changes in steady-state flux is governed by the effective
equilibrium constant of the intermediate formation, and the equilibrium can be
regulated by a futile cycle. The direction of the shift in the effective
equilibrium constant depends on the direction of operation of the futile cycle.
High stoichiometric sensitivity corresponds to ultrasensitivity of an
intermediate concentration to net flow through a pathway; low stoichiometric
sensitivity corresponds to super-robustness of concentration with respect to
changes in flux. Both cases potentially play important roles in metabolic
regulation. Futile cycles actively shift the effective equilibrium by expending
energy; the magnitude of changes in effective equilibria and sensitivities is a
function of the amount of energy used by a futile cycle. This proposed
mechanism for control by futile cycles works remarkably similarly to kinetic
proofreading in biosynthesis. The sensitivity of the system is also intimately
related to the rate of concentration fluctuations of intermediate metabolites.
The possibly different roles of the two major mechanisms for cellular
biochemical regulation, namely reversible chemical modifications via futile
cycles and shifting equilibrium by macromolecular binding, are discussed.Comment: 11 pages, 5 figure
Langevin dynamics with dichotomous noise; direct simulation and applications
We consider the motion of a Brownian particle moving in a potential field and
driven by dichotomous noise with exponential correlation. Traditionally, the
analytic as well as the numerical treatments of the problem, in general, rely
on Fokker-Planck description. We present a method for direct numerical
simulation of dichotomous noise to solve the Langevin equation. The method is
applied to calculate nonequilibrium fluctuation induced current in a symmetric
periodic potential using asymmetric dichotomous noise and compared to
Fokker-Planck-Master equation based algorithm for a range of parameter values.
Our second application concerns the study of resonant activation over a
fluctuating barrier.Comment: Accepted in Journal of Statistical Mechanics: Theory and Experimen
Gastrointestinal pathologistsâ perspective on managing risk in the distal esophagus: convergence on a pragmatic approach
Here, we discuss recent updates and a continuing controversy in the diagnosis and management of Barrett’s esophagus, specifically the recommendation that the irregular Zâ line not be biopsied, the diminished status of ultrashortâ segment Barrett’s esophagus, the evidence basis for excluding and including the requirement of goblet cells for the diagnosis of Barrett’s esophagus, and the conclusion that histologically confirmed lowâ grade dysplasia is best managed with endoscopic ablation rather than surveillance. We reference the American Gastroenterological Association and College of Gastroenterology and the British Society of Gastroenterology guidelines throughout, with the thesis that the field is converging on the concept of applying scarce medical resources to the diagnosis, surveillance, and therapy of patients most likely to derive benefit.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146991/1/nyas13680.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146991/2/nyas13680_am.pd
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