Should endangered be capitalized in the middle of sentence?

No, "endangered" is not capitalized in the middle of a sentence unless it is the first word of the sentence.

Here are some examples:

* Correct: The endangered species is facing extinction.

* Incorrect: The Endangered species is facing extinction.

* Correct: Endangered species are often found in fragile ecosystems.

* Incorrect: Endangered Species are often found in fragile ecosystems.

Remember, "endangered" is an adjective describing a species, not a proper noun.