  The collective excitations in the Bose-Hubbard model in a trap are studied by
means of numerical diagonalization in one dimension. The strength function is
calculated for monopole and dipole perturbations, and moments of the strength
function are utilized in order to obtain information about the collective
behavior under external forces. In the superfluid regime, the spectrum is found
to be exhausted by one single frequency, while in systems that contain a Mott
insulating plateau several frequencies are excited. An explanation of recent
experimental findings in terms of a Mott plateau is suggested.
