How important is the theme of usurption in Act 1 of The Tempest by William Shakespeare?

The major theme of The Tempest by William Shakespeare is usurpation. Prospero and Miranda are stranded on the island because Antonio, brother of Prospero, has usurped the dukedom of Milan. Sebastian, encouraged by Antonio, is attempting to usurp the Kindom of Naples from Alonso, who aided Antonio in the usurpation of Milan. Prospero himself sends the storm as revenge for usurpation of his dukedom. Ironically, the island itself on which Prospero was exiled is not rightfully his, but instead he has usurped it from Caliban and enslaved Caliban and Ariel to do his bidding. Caliban points out:

This island's mine, by Sycorax my mother,
Which thou tak'st from me.