impoverish vs decline

impoverish is not a noun while decline is a noun.

Word	Noun	Adjective	Verb	Adverb
impoverish	No	No	Yes	No
decline	Yes	No	Yes	No

As verbs, decline is a hypernym of impoverish; that is, decline is a word with a broader meaning than impoverish:

Other hypernyms of impoverish include worsen.

impoverish (noun)	decline (noun)
	a downward slope or bend
	a gradual decrease; as of stored charge or current
	change toward something smaller or lower
	a condition inferior to an earlier condition; a gradual falling off from a better state; decline

impoverish (verb)	decline (verb)
take away	grow smaller
make poor	grow worse
	show unwillingness towards
	inflect for number, gender, case, etc.
	go down
	fall in value
	not accept as true