Explanation:

- Context: The passage highlights the role of elephants in forest ecosystems.

- Sentence Meaning: The sentence suggests that certain forests lack something essential when elephants are absent.

- Sentence Analysis:

- "Forests without _____ have been observed to not have young _____ at all."

- The blanks imply that something crucial is missing that affects the presence of young entities in forests.

- Options:

- 1, engineers: Not applicable; engineers don't fit the context of seed dispersal in ecosystems.

- 2, elephants: Correct Answer; Elephants play a vital role in seed dispersion, affecting the presence of young trees.

- 3, conservation: Doesn't fit; conservation is a practice or concept, not something that directly disperses seeds.

- 4, seeds: Incorrect here; the passage is about elephants affecting seeds, not seeds affecting themselves.