  Models of coordinated behavior of populations living in the same environment
are introduced for the cases when they either compete with each other, or they
both gain by mutual interactions, or finally when one hunts the other one. The
equilibria of the systems are analysed, showing that in some cases the
populations may both disappear. Coexistence leads to global asymptotic
stability for symbiotic populations, or to Hopf bifurcations for predator-prey
systems. Finally, a new very interesting phenomenon is discovered in one of
these rather simple models. Indeed tristability may be achieved in the
competition case, for which competitive exclusion is allowed to occur together
with populations coexistence.
