California Code, Government Code - GOV § 45007

After inclusion in the system, any departments or appointive officers or employees shall not be withdrawn, either by an outright repeal of the civil service ordinance or otherwise, unless the withdrawal has been submitted to the city electors at a special or regular municipal election and approved by two-thirds of those voting on the proposition, except that regular full-time city department heads, may be withdrawn by a majority vote of the city council.