Not everything that affects, modifies or refines a noun is an adjective, as BillJ mentions in the comments. Some nouns can take arguments, as a verb does, and these can be called *complements*. It's not that they are *changing* the meaning of the noun, as an adjective does, but they are *completing* it (hence 'complement').

*Decline*, on its own, raises the question "decline in **what**?". In this case, that question is answered with "response to a specific stimulus". Likewise, *response* prompts the question "response to what?" - and that question is answered with "a specific stimulus". *Specific*, on the other hand, is simply an adjective modifying *stimulus*. Thus, *stimulus* is a noun modified with an adjective to form the noun phrase *specific stimulus*. *Response* is a noun, completed by the noun phrase *a specific stimulus* (linked with *to*), and *decline* is a noun, completed by the noun phrase *response to a specific stimulus* (linked with *in*).