Optimal phenotypic plasticity in a stochastic environment minimizes the cost/benefit ratio

This paper addresses the question of optimal phenotypic plasticity as a response to environmental fluctuations while optimizing the cost/benefit ratio, where the cost is energetic expense of plasticity, and benefit is fitness. The dispersion matrix \Sigma of the genes' response (H = ln|\Sigma|) is used: (i) in a numerical model as a metric of the phenotypic variance reduction in the course of fitness optimization, then (ii) in an analytical model, in order to optimize parameters under the constraint of limited energy availability. Results lead to speculate that such optimized organisms should maximize their exergy and thus the direct/indirect work they exert on the habitat. It is shown that the optimal cost/benefit ratio belongs to an interval in which differences between individuals should not substantially modify their fitness. Consequently, even in the case of an ideal population, close to the optimal plasticity, a certain level of genetic diversity should be long conserved, and a part, still to be determined, of intra-populations genetic diversity probably stem from environment fluctuations. Species confronted to monotonous factors should be less plastic than vicariant species experiencing heterogeneous environments. Analogies with the MaxEnt algorithm of E.T. Jaynes (1957) are discussed, leading to the conjecture that this method may be applied even in case of multivariate but non multinormal distributions of the responses

Refounding of Activity Concept ? Towards a Federative Paradigm for Modeling and Simulation

Journal : Simulation, Transactions of the Society for Modeling and Simulation InternationalInternational audienceCurrently, the widely used notion of activity is increasingly present in computer science. However, because this notion is used in specific contexts, it becomes vague. Here, the notion of activity is scrutinized in various contexts and, accord-ingly, put in perspective. It is discussed through four scientific disciplines: computer science, biology, economics, and epis-temology. The definition of activity usually used in simulation is extended to new qualitative and quantitative definitions. In computer science, biology and economics disciplines, the new simulation activity definition is first applied critically. Then, activity is discussed generally. In epistemology, activity is discussed, in a prospective way, as a possible framework in models of human beliefs and knowledge

Endocrine Disrupting Chemicals Interfere With Leydig Cell Hormone Pathways During Testicular Descent in Idiopathic Cryptorchidism

Cryptorchidism, a frequent genital malformation in male newborn, remains in most cases idiopathic. On the basis of experimental, epidemiological, and clinical data, it has been included in the testicular dysgenesis syndrome and believed to be influenced, together with genetic and anatomic factors, by maternal exposure to endocrine disrupting chemicals (EDCs). Here, we analyze how EDCs may interfere with the control of testicular descent, which is regulated by two Leydig cell hormones, testosterone, and insulin like peptide 3 (INSL3)

Dynamique des boisements spontanés dans la Chaîne des Puys : une approche par la modélisation

National audienceThe Chaîne des Puys, a volcanic mountain of the French Massif Central, is characterised by widespread natural stands, especially Scots pine stands, that established on former agricultural and pastoral lands these past last decades. Scots pine stands were studied on the basis of a floristic and dendrometric survey and a classification of these stands was proposed. In order to understand the initial conditions that lead to the establishment of stands characterised by a narrow range of ages, a model of the colonisation process was achieved. An individual-based model was set up taking into account growth, competition, mortality and recruitment processes. Simulations were achieved using data from a measured stand and their analysis provided some conclusions from natural stands establishment. Parameters such as the number of parent trees and their spatial distribution as well as the susceptibility of the resident vegetation to colonisation could explain the fast establishment of natural Scots pine stands after field abandonment.La Chaîne des Puys, massif volcanique du Massif Central, se caractérise par une forte présence de boisements spontanés, notamment de pin sylvestre, qui se sont mis en place à la suite de la déprise agricole et pastorale de ces dernières décennies. Nous avons étudié ces boisements sur un plan floristique et sylvicole pour en proposer une typologie. Afin de comprendre les conditions qui ont pu permettre la constitution de ces boisements, caractérisés en particulier par une distribution des âges resserrée, nous avons entrepris la modélisation de la colonisation forestière. Le modèle conçu est un modèle individu-centré et prend en compte les processus de croissance, de compétition, de mortalité et de recrutement. L'analyse de simulations à partir d'un cas réel permet de dégager des enseignements sur la dynamique d'afforestation. Le nombre et la disposition des semenciers ainsi que la résistance du milieu au cours du temps sont des facteurs qui semblent essentiels pour expliquer la mise en place rapide des boisements spontanés de pin dans la Chaîne des Puys

Activatability for simulation tractability of NP problems: Application to Ecology

Dynamics of biological-ecological systems is strongly depending on spatial dimensions. Most of powerful simulators in ecology take into account for system spatiality thus embedding stochastic processes. Due to the difficulty of researching particular trajectories, biologists and computer scientists aim at predicting the most probable trajectories of systems under study. Doing that, they considerably reduce computation times. However, because of the largeness of space, the execution time remains usually polynomial in time. In order to reduce execution times we propose an activatability-based search cycle through the process space. This cycle eliminates the redundant processes on a statistical basis (Generalized Linear Model), and converges to the minimal number of processes required to match simulation objectives.Comment: Complex Systems: Activity-Based Modeling and Simulation, Cargese Interdisciplinary Seminar, Corsica, France., France (2009

Organisms modeling: The question of radial basis function networks

There exists usually a gap between bio-inspired computational techniques and what biologists can do with these techniques in their current researches. Although biology is the root of system-theory and artifical neural networks, computer scientists are tempted to build their own systems independently of biological issues. This publication is a first-step re-evalution of an usual machine learning technique (radial basis funtion(RBF) networks) in the context of systems and biological reactive organisms

Modèle individu-centré de l'invasion par Pinus sylvestris après abandon pastoral dans le Massif Central français

International audienceIn the Chaîne des Puys, a mid-elevation volcanic mountain of the French Massif Central, Scots pine proves to be an 'invasive' species colonizing abandoned lawns or heathlands, and forms in a few years monospecific natural forests. Most of the abandonment occurred 30 to 40 years ago and this process has now stopped. Thus, we lack data on the very first phase of tree colonization. We anticipate that a simulation tool could bring an appreciable help in (i) rebuilding the entire colonization process including the initial phase of pine settlement and (ii) answering questions about the origin of the narrow and unimodal distributions of age of pine stands we observed. In addition, such a simulator could help managers to forecast extension of Scots pine and to predict growth and evolution of present secondary forests.A spatially explicit individual-based model is presented. The model takes into account both space and time and includes growth of trees, seed production and seed dispersal, death and competition between individuals. The influence of the initial parameters are analyzed and elements of validation given. The model was then used to predict tree settlement and stand establishment using the initial conditions from a natural stand studied in the field whose characteristics before abandonment were known (number and age-distribution of trees, location of mother trees, time of abandonment). Three simulations were achieved by using the same initial conditions but following different scenarios for the recruitment process. The scenario of a fluctuating resistance of the resident vegetation (that controls the susceptibility of the environment to tree establishment) seemed as one of the most probable to explain the actual stand characteristics. We thus concluded that dynamic models could be improved by taking into account the resistance of the vegetation to colonization as a fluctuating parameter instead of a static and permanent attribute.Dans la Chaîne des Puys, massif volcanique de moyenne montagne du Massif Central, le pin sylvestre se comporte comme une espèce invasive colonisant pelouses et landes abandonnées et formant en quelques années des peuplements monospécifiques. L'abandon des terres s'est produit il y a 30 à 40 ans et maintenant ce processus est arrêté. Nous avons pensé qu'un outil de simulation pourrait apporter une aide appréciable 1) en reconstruisant le processus de colonisation dans son ensemble (y compris la phase initiale) 2) en permettant de comprendre l'origine des structures en classes d'âge unimodales et resserrées fréquemment observées dans les peuplements. De plus, un tel simulateur pourrait aider le gestionnaire à prédire l'extension du pin ainsi que la croissance et l'évolution des peuplements. Un modèle spatialisé et individu-centré est donc présenté. Le modèle prend en compte dans l'espace et le temps, la croissance, la production des graines et leur dissémination, la mortalité et la compétition entre individus. L'influence des paramètres initiaux est analysée et des éléments de validation fournis. Le modèle est ensuite utilisé pour prédire la colonisation et la formation d'un peuplement à partir de conditions initiales observées pour un peuplement sélectionné sur le terrain et dont les caractéristiques avant abandon étaient connues (nombre et âge des arbres, localisation, durée depuis l'abandon). Trois simulations ont été effectuées en utilisant ces mêmes conditions initiales mais suivant différents scénarios pour le processus de recrutement. Le scénario d'une fluctuation de la résistance à la colonisation de la végétation en place (qui contrôle la susceptibilité du milieu à être colonisé) apparaît comme un des plus probables pour expliquer les caractéristiques actuelles. Nous concluons que les modèles de dynamique pourraient être améliorés en considérant la résistance de la végétation en place comme un attribut variable et non pas statique et permanent dans le temps

Growth models of silver birch (Betula pendula Roth.) on two volcanic mountains in the French Massif Central

Abstract Silver birch woodlands of two volcanic mountains (altitude 850 m and 1450 m) were studied in order to establish a growth model of birch. Height, radial increment and crown width were measured on both sites taking into account two situations: isolated birch or birch within a stand. For the latter case two categories were sampled considering the competition status of the tree: birch suffering the most severe competition (suppressed tree) were distinguished from trees facing the lowest competition (dominant tree). Measures of competition were also made using plots where each tree was located by its coordinates and its height, trunk circumference and crown width recorded. Examination of radial growth curves showed similar patterns for the two sites for the same category of tree. Radial growth was always inferior for the site located at the highest altitude but tree lifespan was about twofold longer. Radial increment data were used to calculate circumference as function of tree age. Models predicting circumference with time were then established for each category of tree (dominant, suppressed or isolated) using Chapman-Richards'model. Height and crown width models were produced using circumference as the predictor. Competition indices based on vertical or horizontal angles weighted by the distance were calculated for birches in stands. Indices using vertical angles appeared to be more resistant and robust to characterize local competition. Competition index was then associated with the parameters of Chapman-Richards' growth circumference model for the two sites and models predicting the circumference from the age of the tree and its competition index are proposed

Organisms modeling: The question of radial basis function networks

There exists usually a gap between bio-inspired computational techniques and what biologists can do with these techniques in their current researches. Although biology is the root of system-theory and artifical neural networks, computer scientists are tempted to build their own systems independently of biological issues. This publication is a first-step re-evalution of an usual machine learning technique (radial basis funtion(RBF) networks) in the context of systems and biological reactive organisms