If the police and firemen’s pension system established under the provisions of section 86.583 requires a contribution from the members of the organized police or fire department, then at least one voting member of the board of trustees shall be a member of the organized police department or fire department elected by the contributing members, and if the system serves both the police and the fire departments, a member of both the organized police department and of the fire department shall be voting members of the board of trustees elected by the contributing members.