Can part of a complete subject be contained within a prepositional phrase?
Such as: Many of the habitats have been destroyed by man. What is the subject and what is the complete subject?
Yeah, why not?
Subject: the pronoun many: 1) Many have been destroyed by man.
It may well be, that 1) is ambiguous, depending on the previous discourse.
Fire spewing dragons need a special habitat to breed successfully. Many have been destroyed. Many what? Dragons or habitats? So throw in an aside, to clarify:
2) Many (of the habitats) have been destroyed by man. The PP is an adjective, describing 'many'. The complete subject is then 'Many of the habitats' Cf:
A great many have been destroyed. Subject: many. Your 'complete subject' 'A great many', includes the indefinite article and the adjective.
'the habitats' is not the subject, because you would then write 'The habitats have been destroyed.' which implies there are none left, and we wouldn't have any fire spewing dragons left!