TOEFL Grammar Question

[tajansu]

Hi, I am working on paper-based grammar exercises now. While practicing, I came across the following question.

Rare species, unlike endangered species, exist in relatively low numbers but are not necessarily in immediate danger of extinction.
My question is --Why do we use "necessarily" instead of "necessary"?

Thank you so much.

[Alan]

Because an adverb, not an adjective, is required in this sentence position...