Here's why:
* Simple Subject: The main noun or pronoun that performs the action of the verb.
* The sentence is missing a verb. It needs a verb to complete the thought, such as "is" or "is the."
* "Its" is a possessive pronoun. It modifies "wilderness" but isn't the subject itself.
* "Subtropical" and "largest" are adjectives describing the wilderness.
Therefore, "wilderness" is the core noun in the sentence and the simple subject.