A new formalism, Grid Systems, aimed at modelling population dynamics is presented. The formalism is inspired by concepts of Membrane Computing (P Systems) and spatiality dynamics of Cellular Automata. The semantics of Grid Systems describes how stochasticity is exploited for reaction duration as well as reaction selection. Grid Systems perform reactions in maximally parallel manner, imitating natural processes. Environmental events that change population behaviour can be defined in Grid Systems as rewrite rules. A population model of a species of mosquitoes, Aedes albopictus, is presented. The model considers three types of external events: temperature change, rainfall, and desiccation. The events change the behaviour of the species directly or indirectly. Each individual in the population can move around in the ecosystem. The simulation of the model was performed by using a semantics based tool. © Springer-Verlag Berlin Heidelberg 2014.