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For more than 30 years we have been periodically observing multiannual outbreaks of grassland rodents, Arvicola terrestris Sherman, in Auvergne (France). This phenomenon is more and more frequent and heavy despite chemical treatment. That’s why we study the relationships between water voles and their predators within landscape and farming practices. In order to test different situations of landscape organization, we develop a simulator which will enable us to quantify the evolution of predator and vole populations. This simulator uses a spatial model based on cellular automata. Instead of the classical approach of population dynamics, we have chosen cellular automata. This technique enables us to perform spatial simulations. Simple rules apply on wide number of elements. These local and individual interactions determine complex behaviors of the system at the global level. Nowadays, high computer speeds enable this kind of approach. Maps of land use, maps of vole densities and number of various predators are the input variables of the model. Several parameters enable us to tune constitutive rules of our model (movement and reproduction of voles, hunt territory and predation, evolution of predator density, …). Outputs: for each iteration, the simulator creates maps of vole densities and calculates the number of each species. One iteration represents one year. The first results which are obtained on a real territory (municipalities of Ceyssat and Olby in the department of Puy-de-Dôme) are encouraging. We hope that our simulator (when it will be tuned and validated) will enable us to carry out a wide number of “virtual” experiments that are unfeasible in the “real” world.