  In this paper we present and analyse a simple two populations model with
migrations among two different environments. The populations interact by
competing for resources. Equilibria are investigated. A proof for the
boundedness of the populations is provided. A kind of competitive exclusion
principle for metapopulation systems is obtained. At the same time we show that
the competitive exclusion principle at the local patch level may be prevented
to hold by the migration phenomenon, i.e. two competing populations may
coexist, provided that only one of them is allowed to freely move or that
migrations for both occur just in one direction.
