Here's why:

* It modifies nouns: We use "biotic" to describe something that is related to living organisms. For example, "biotic factors" (meaning factors related to living organisms) or "biotic community" (a community of living organisms).

* It doesn't act as a verb, noun, or adverb: It doesn't perform an action, stand alone as a thing, or modify a verb/adjective.

Let me know if you have any more questions about parts of speech!