Order statistics for d-dimensional diffusion processes

We present results for the ordered sequence of first passage times of arrival of N random walkers at a boundary in Euclidean spaces of d dimensions

Coagulation reaction in low dimensions: Revisiting subdiffusive A+A reactions in one dimension

We present a theory for the coagulation reaction A+A -> A for particles moving subdiffusively in one dimension. Our theory is tested against numerical simulations of the concentration of $A$ particles as a function of time (``anomalous kinetics'') and of the interparticle distribution function as a function of interparticle distance and time. We find that the theory captures the correct behavior asymptotically and also at early times, and that it does so whether the particles are nearly diffusive or very subdiffusive. We find that, as in the normal diffusion problem, an interparticle gap responsible for the anomalous kinetics develops and grows with time. This corrects an earlier claim to the contrary on our part.Comment: The previous version was corrupted - some figures misplaced, some strange words that did not belong. Otherwise identica

Caza comercial en los bosques de Monte Mitra, Guinea Ecuatorial: alcance e impacto

Understanding the exploitation of bushmeat by commercial hunters is fundamental to resolving hunting sustainability issues in African rainforests. The objective of this study was to examine the impact of hunters operating from the village of Sendje in the Monte Mitra region, Republic of Equatorial Guinea. Offtake patterns of 42 hunters were studied over a period of 16 months. A total of 3,053 animals of 58 species were hunted during 1,914 hunting days. This represented around 11,376 kg of bushmeat or 2,219 animals extracted per annum. Most captures were mammals (43 species, 79%), constituting 90% of the biomass hunted, of these 30% were ungulates and 27% were rodents. Hunters used 17 hunt camps within the 1,010 km2 total study area. Hunting activity fell from the start to the end of the study, with fewer hunting days, biomass and captures being recorded per month. Captures fell from 700 animals in the first month to less than 100 during the last month. Per hunter, returns diminished from 21 in the first month to around 13 animals from the third month. Average body mass of prey also declined throughout the study period. The principal hunting method was cable snaring —over 100 million snare nights were estimated. An average hunter extracted around 50 animals or 271 kg of bushmeat per annum. Hunter and camp differences were significant. Most carcasses were sold for the city market or to villagers, and the proportion of carcasses sold to market was positively correlated with the species body mass. Capture rates and vulnerability were dependent on prey size since medium–sized animals were more vulnerable to be caught than small or large–bodied animals. Harvest sustainability was calculated for 14 mammals and it was seen that the situation was unsustainably for 5 species due to the extent and impact of hunting. The bay duiker (Cephalophus dorsalis) was by far the most heavily exploited species. Conservation of the Monte Mitra region is impossible unless the hunting for profit issue is resolved in Sendje and adjoining villages.Entender la explotación de la carne de selva por parte de cazadores comerciales es fundamental para resolver las cuestiones de sostenibilidad referentes a la caza en los bosques húmedos de África. El objetivo de este estudio fue examinar el impacto de la actividad de los cazadores de la aldea de Sendje, en la región del Monte Mitra, República de Guinea Ecuatorial. Se estudiaron los patrones de caza de 42 cazadores durante un periodo de 16 meses. Se cazaron un total de 3.053 animales de 58 especies en 1.914 jornadas de caza, lo que representa aproximadamente 11.376 kg de carne de selva o 2.219 animales extraídos por año. La mayoría de capturas fueron mamíferos (43 especies, 79%), que constituyeron el 90% de la biomasa cazada, y entre ellos un 30% de ungulados y un 27% de roedores. Los cazadores utilizaron 17 campos de caza dentro de un área de estudio con una extensión total de 1.010 m2. La actividad de caza fue disminuyendo desde el inicio del estudio hasta al final del mismo, con menos días de caza, biomasa y capturas registradas por mes. Las capturas disminuyeron desde 700 animales durante el primer mes a menos de 100 en el último. Por cazador, el rendimiento diminuyó de 21 animales en el primer mes a 13 en el tercero. La media de masa corporal de las presas también disminuyó a lo largo del periodo de estudio. El método de caza más utilizado fue el cepo (se estimó la existencia de alrededor de 100 millones de cepos noche). En promedio, cada cazador extrajo alrededor de 50 animales o 271 kg de carne de selva por año. Las diferencias entre campos de caza y cazadores fueron significativas. La mayoría de piezas fueron vendidas al mercado de la ciudad o a los aldeanos, y la proporción de piezas vendidas al mercado estuvo correlacionada positivamente con la masa corporal de las mismas. Los índices de captura y vulnerabilidad dependieron del tamaño de las presas ya que los animales de tamaño medio resultaron más vulnerables que los pequeños o grandes. Se calculó la sostenibilidad de la caza para 14 mamíferos en los bosques de Monte Mitra, Guinea Ecuatorial, resultando insostenible para cinco especies por su extensión e impacto. Cephalophus dorsalis fue la especie explotada con mayor intensidad. La conservación de la región del monte Mitra es imposible a no ser que el problema de la caza de carne de selva para su comercialización se resuelva en Sendje y pueblos vecinos

Order statistics of the trapping problem

When a large number N of independent diffusing particles are placed upon a site of a d-dimensional Euclidean lattice randomly occupied by a concentration c of traps, what is the m-th moment of the time t_{j,N} elapsed until the first j are trapped? An exact answer is given in terms of the probability Phi_M(t) that no particle of an initial set of M=N, N-1,..., N-j particles is trapped by time t. The Rosenstock approximation is used to evaluate Phi_M(t), and it is found that for a large range of trap concentracions the m-th moment of t_{j,N} goes as x^{-m} and its variance as x^{-2}, x being ln^{2/d} (1-c) ln N. A rigorous asymptotic expression (dominant and two corrective terms) is given for for the one-dimensional lattice.Comment: 11 pages, 7 figures, to be published in Phys. Rev.

Comment on "Mean First Passage Time for Anomalous Diffusion"

We correct a previously erroneous calculation [Phys. Rev. E 62, 6065 (2000)] of the mean first passage time of a subdiffusive process to reach either end of a finite interval in one dimension. The mean first passage time is in fact infinite.Comment: To appear in Phys. Rev.

Computational modelling of epithelial cell monolayers during infection with Listeria monocytogenes

Intracellular bacterial infections alter the normal functionality of human host cells and tissues. Infection can also modify the mechanical properties of host cells, altering the mechanical equilibrium of tissues. In order to advance our understanding of host–pathogen interactions, simplified in vitro models are normally used. However, in vitro studies present certain limitations that can be alleviated by the use of computer-based models. As complementary tools these computational models, in conjunction with in vitro experiments, can enhance our understanding of the mechanisms of action underlying infection processes. In this work, we extend our previous computer-based model to simulate infection of epithelial cells with the intracellular bacterial pathogen Listeria monocytogenes. We found that forces generated by host cells play a regulatory role in the mechanobiological response to infection. After infection, in silico cells alter their mechanical properties in order to achieve a new mechanical equilibrium. The model pointed the key role of cell–cell and cell–extracellular matrix interactions in the mechanical competition of bacterial infection. The obtained results provide a more detailed description of cell and tissue responses to infection, and could help inform future studies focused on controlling bacterial dissemination and the outcome of infection processes. © 2022 The Author(s

A Stiff Extracellular Matrix Favors the Mechanical Cell Competition that Leads to Extrusion of Bacterially-Infected Epithelial Cells

Cell competition refers to the mechanism whereby less fit cells (“losers”) are sensed and eliminated by more fit neighboring cells (“winners”) and arises during many processes including intracellular bacterial infection. Extracellular matrix (ECM) stiffness can regulate important cellular functions, such as motility, by modulating the physical forces that cells transduce and could thus modulate the output of cellular competitions. Herein, we employ a computational model to investigate the previously overlooked role of ECM stiffness in modulating the forceful extrusion of infected “loser” cells by uninfected “winner” cells. We find that increasing ECM stiffness promotes the collective squeezing and subsequent extrusion of infected cells due to differential cell displacements and cellular force generation. Moreover, we discover that an increase in the ratio of uninfected to infected cell stiffness as well as a smaller infection focus size, independently promote squeezing of infected cells, and this phenomenon is more prominent on stiffer compared to softer matrices. Our experimental findings validate the computational predictions by demonstrating increased collective cell extrusion on stiff matrices and glass as opposed to softer matrices, which is associated with decreased bacterial spread in the basal cell monolayer in vitro. Collectively, our results suggest that ECM stiffness plays a major role in modulating the competition between infected and uninfected cells, with stiffer matrices promoting this battle through differential modulation of cell mechanics between the two cell populations

Radiocarbon dating reveals different past managements of adjacent forest soils in the Campine region, Belgium

The soils of adjacent first generation monospecific stands of Scots pine (Pinus sylvestris L.) and pedunculate oak (Quercus robur L.) in the Campine region, Belgium, apparently developed under the same forming factors, were studied for carbon dynamics to disentangle eventual different past land uses. In fact, visual observations suggested that the soil under pine experienced substantial addition of organic matter and ploughing, such to be considered a plaggen, opposite to the soil under oak, which is inexplicably much poorer in C. In order to prove this hypothesis, the soil organic carbon was quantified by horizons and, both bulk soil organic matter (SOM) and the least mobile SOM fractions - the humic acid and the unextractable fractions - were radiocarbon dated. Surprising was the marked difference between the mean SOM age from the two stands. In fact, while under oak this age is a few years or decades, under pine it amounts to more than a millennium, so confirming the hypothesis of a confined C supply occurred mainly in the Middle Age, or later using partly humified matter. The mean residence time (MRT) of SOM in the organic layers matches almost perfectly with that estimated via a mass balance approach and, as expected, was much lower in the oaks than in the pines. The humic acid fraction, generally the most stable fraction of SOM, in terms of both mobility and degradability, reflects the behaviour of the bulk SOM, showing higher radiocarbon ages under pine. The findings of this work indicate that the large human-induced additions of organic material in the area now occupied by the pine stand, probably occurred in the Middle Age and it continues to strongly affect the present soil C pools and their dynamics. Any study dealing with budgets and dynamics of C in soil should avail itself of a careful reconstruction of the land uses and management history, in order to provide reliable conclusions about the real role of the current vegetation on soil carbon. Crown Copyright (c) 2008 Published by Elsevier B.V. All rights reserved

Average shape of fluctuations for subdiffusive walks

We study the average shape of fluctuations for subdiffusive processes, i.e., processes with uncorrelated increments but where the waiting time distribution has a broad power-law tail. This shape is obtained analytically by means of a fractional diffusion approach. We find that, in contrast with processes where the waiting time between increments has finite variance, the fluctuation shape is no longer a semicircle: it tends to adopt a table-like form as the subdiffusive character of the process increases. The theoretical predictions are compared with numerical simulation results.Comment: 4 pages, 6 figures. Accepted for publication Phys. Rev. E (Replaced for the latest version, in press.) Section II rewritte

Critical dimensions for random walks on random-walk chains

The probability distribution of random walks on linear structures generated by random walks in $d$-dimensional space, $P_d(r,t)$, is analytically studied for the case $\xi\equiv r/t^{1/4}\ll1$. It is shown to obey the scaling form $P_d(r,t)=\rho(r) t^{-1/2} \xi^{-2} f_d(\xi)$, where $\rho(r)\sim r^{2-d}$ is the density of the chain. Expanding $f_d(\xi)$ in powers of $\xi$, we find that there exists an infinite hierarchy of critical dimensions, $d_c=2,6,10,\ldots$, each one characterized by a logarithmic correction in $f_d(\xi)$. Namely, for $d=2$, $f_2(\xi)\simeq a_2\xi^2\ln\xi+b_2\xi^2$; for $3\le d\le 5$, $f_d(\xi)\simeq a_d\xi^2+b_d\xi^d$; for $d=6$, $f_6(\xi)\simeq a_6\xi^2+b_6\xi^6\ln\xi$; for $7\le d\le 9$, $f_d(\xi)\simeq a_d\xi^2+b_d\xi^6+c_d\xi^d$; for $d=10$, $f_{10}(\xi)\simeq a_{10}\xi^2+b_{10}\xi^6+c_{10}\xi^{10}\ln\xi$, {\it etc.\/} In particular, for $d=2$, this implies that the temporal dependence of the probability density of being close to the origin $Q_2(r,t)\equiv P_2(r,t)/\rho(r)\simeq t^{-1/2}\ln t$.Comment: LATeX, 10 pages, no figures submitted for publication in PR