coevolution [@douglas; @sapp]; it is a rather common phenomenon in nature. For example, lichens are symbiotic complexes of algae living inside fungi, and the roots of higher plants use symbiotic associations with fungi to receive important nutrients [@paracer].

Macroscopic models derived from modifications of the Lotka-Volterra competition equations have been employed to model symbiotic relations for decades [@rockwood; @yukalov]. Such model however neglect stochastic effects, relevant due to the discrete nature of the individuals and in spatially extended systems [@discrete]. More recently, the effects of mutualistic interactions in one-dimensional stepping stone models were studied by Korolev and Nelson [@korolev], and by Dall’Asta et.al. [@asta], who found that fluctuations and spatial structure favors symmetric mutualism (in which species benefit equally from the interaction). The fixation(absorbing)-coexistence(active) phase transition was found to belong to the voter model universality class if mutualism is symmetric, and to the directed percolation class if asymmetric. Lavrentovich and Nelson extended the results of [@lav] to asymmetric interactions in two and three dimensions, finding that the mutualist phase is more accessible in higher dimensional range expansions. Pigolotti et. al [@pigolotti] studied competition and cooperation between two species when the population size is not constrained as it is in stepping-stone 