Understanding how the collective behaviour of phytoflagellates is affected by light attenuation and diel vertical migration using individual-based modelling.

In this article, we investigate the question of the impact of the diel vertical migration (DVM) and the light attenuation by the cells on both spatial pattern and population dynamics of phytoflagellates. For these purposes, we performed a simulation study by using a spatially explicit individual-based model (IBM). The designed IBM includes 2 global mechanisms: cellular motion which is modelled through a stochastic differential equation and demographic process that is density and light intensity dependent. We showed that under no-DVM, for competitive environment, increasing the light absorption helps the creation of a strong oscillatory behaviour, that breeds aggregation-break up rhythm, which is beneficial. However, when we pass to DVM regime, it will be more advantageous for the cells to reduce their light absorption. In regards of DVM effect, we showed that it has other benefits, apart from the acquisition of resources, such as enhancing the attraction mechanism that promotes cooperation and also helping the creation of spatial voids that permit the penetration of the light. Also, we showed that the DVM reduces the predation rate, so we prove quantitatively that the DVM reduces, in general, the grazing losses. Also, we found that the DVM strategy depends on the impact of the competition and cooperation between the cells on the division and death rates. So, probably for the motile species, the local competition is not high in order to let the cells gain advantages from the DVM regarding their survivability that is explained by an annual cyclic behaviour under seasonal environment.

A 3D Individual-Based Model to Study Effects of Chemotaxis, Competition and Diffusion on the Motile-Phytoplankton Aggregation.

In this paper, we develop a 3D-individual-based model (IBM) to understand effect of various small-scale mechanisms in phytoplankton cells, on the cellular aggregation process. These mechanisms are: spatial interactions between cells due to their chemosensory abilities (chemotaxis), a molecular diffusion and a demographical process. The latter is considered as a branching process with a density-dependent death rate to take into account the local competition on resources. We implement the IBM and simulate various scenarios under real parameter values for phytoplankton cells. To quantify the effects of the different processes quoted above on the spatial and temporal distribution of phytoplankton, we used two spatial statistics: the Clark-Evans index and the group belonging percentage. Our simulation study highlights the role of the branching process with a weak-to-medium competition in reinforcing the aggregating structure that forms from attraction mechanisms (under suitable conditions for diffusion and attraction forces), and shows by contrast that aggregations cannot form when competition is high.

Decision making environment on Rift Valley fever in Ferlo (Senegal).

The Rift Valley fever (RVF), which first appeared in Kenya in 1912, is an anthropozoonosis widespread in tropical areas. In Senegal, it is particularly felt in the Ferlo area where a strong presence of ponds shared by humans, cattle and vectors is noted. As part of the studies carried out on the environmental factors which favour its start and propagation, the focus of this paper is put on the decision making process to evaluate the impacts, the interactions and to make RVF monitoring easier. The present paper proposes a model based on data mining techniques and dedicated to trade experts. This model integrates all the involved data and the results of the analyses made on the characteristics of the surrounding ponds. This approach presents some advantage in revealing the relationship between environmental factors and RVF transmission vectors for space-time epidemiology monitoring purpose.

Multi-level participatory design of land use policies in African drylands: a method to embed adaptability skills of drylands societies in a policy framework.

The participatory modelling method described here focuses on how to enable stakeholders to incorporate their own perception of environmental uncertainty and how to deal with it to design innovative environmental policies. This "self-design" approach uses role playing games and agent based modelling to let participants design their own conceptual framework, and so modelling supports, of issues. The method has a multi-scale focus I order to enable the whole multi-scale Sahelian logic to be expressed and on the other hand to encourage the players to deal with possible region-wide changes implied by their "local" policy objectives. This multi-level participatory design of land use policies has been under experimentation in Senegal since 2008 in different local and national arenas. The process has resulted in the "self-design" of a qualitative and relatively simple model of Sahelian uncertainty, which can be played like a role playing game as well a computerized model. Results are shown in perceptible autonomous organisational learning at the local level. Participants were also able to incorporate their own ideas for new rules for access to resources. They designed innovative collective rules, organised follow up and monitoring of these new land uses. Moreover, meaningful ideas for environmental policies are beginning to take shape. This work raises the epistemological question of what is meant by the term "indigenous knowledge" in environmental management, ranging from knowledge based on practical experience being included in the scholar's framing of knowledge, to a legitimate local ability to contextualize and re-arrange scientific expertise, to profoundly different worldviews which do not match ours.

On phytoplankton aggregation: a view from an IBM approach.

In this paper, we build up an individual-based model (IBM) that describes the aggregative behavior in phytoplankton. The processes in play at the individual level (an individual=a phytoplankton cell) are: a random dispersal, a displacement due to the net effect of cells present in a suitable neighborhood (spatial interactions) and a branching (cell division and death). The IBM model provides a virtual world where phytoplankton cells appear to form clusters. Using this model, we explore the spatial structure of phytoplankton and present some numerical simulations that help the understanding of the aggregation phenomenon.

Fuelwood harvesting in Niger and a generalization of Faustmann's formula.

In some forests of Niger where 'controlled rural markets' have been organized, fuelwood is harvested following a policy of the form: every T year, cut the dead trees and those live trees which have a diameter greater than D. Dead trees generally form the main part of the harvest. In this paper, we present a simple continuous time model for the management of these uneven-aged stands subject to a high natural death rate alpha, and we derive a formula for the cycle length and the diameter optimizing the discounted income over an infinite horizon. Faustmann's classical formula for even-aged stands corresponds to the limit alpha --> 0 and D = 0 (clear-cut).