Learn Arabic (Modern Standard): الانقراض ليس تهديدا لهذه الفصيلة فحسب، بل للغابة كلها أيضا. - Extinction is not only a threat to this species, but also to the whole forest.

While English puts 'only' right after 'is not', Arabic often places فحسب at the very end of the negated clause. The structure ليس ... فحسب frames the entire phrase you are limiting. You could use ليس فقط earlier in the sentence, but فحسب strongly prefers to anchor the end of its clause before بل introduces the addition.

Why is تهديدا written with an alif and tanween (the accusative case) instead of the nominative case?

Because it is the predicate (khabar) of ليس. The verb ليس changes the case of its predicate to the accusative (mansub). The subject of ليس here is an implied 'it' referring back to الانقراض (extinction).

Could we drop the second لـ preposition and just say بل الغابة كلها أيضا?

It is much better and more natural to repeat the preposition. In Arabic correlative structures like ليس ... فحسب، بل ... أيضا, matching the grammatical structure in both halves is highly preferred. Since the first threat was لهذه (to this), the second should explicitly be للغابة (to the forest).

What role does كلها play here, and why does it end with the feminine suffix ـها?