Food never runs out entirely while the sea is around them. And in that picture I see lots of areas where one could put in a few potatoes or onions well mulched with seaweed, or plant a few boxes of lettuce, radishes, cabbage or suchlike. Might be both safer than the mainland, and if I remember correctly it was a Pagan holy place for even longer than it has been a Christian one. That might help, and Michael was supposed to be one of the protecting saints..... it is good that the causeway has been removed.