  In nature self-organized systems as flock of birds, school of fishes or herd
of sheeps have to deal with the presence of external agents such as predators
or leaders which modify their internal dynamic. Such situations take into
account a large number of individuals with their own social behavior which
interact with a few number of other individuals acting as external point source
forces. Starting from the microscopic description we derive the kinetic model
through a mean-field limit and finally the macroscopic system through a
suitable hydrodynamic limit.
