Phase Transitions in Systems of Interacting Species

We discuss an autocatalytic reaction system: the cyclic competition model A1 + A2 --> 2 A2, A2 + A3 --> 2 A3, A3 + A4 --> 2 A4, A4 + A1 --> 2 A1), as well as its neutral counterpart. Migrations are introduced into the model. When stochastic phenomena are taken into account, a phase transition between a ``fixation'' and a ``neutral'' regime is observed. In the ``fixation'' regime, species A1 and A3 form an alliance against species A2 and A4, and the final state is one in which one of the symbiotic pairs has won. The odd--even ``coarse--grained'' systems is mapped onto the two--species neutral (Kimura) model. In the ``neutral'' regime, all four species survive for long (evolutionary) times. The analytical results are checked against computer simulations of the model. The model is generalized for n species. Also, a generalized version of the Volterra model is analysed.