Here's why:
* It's a common noun: "Ecosystem" refers to a general concept, not a specific name. Like "forest" or "desert," it's not a proper noun.
* It's not a specific place: Unlike "Amazon Rainforest," "ecosystem" doesn't refer to a particular location.
Exceptions:
* If it's part of a proper noun: For example, "The Great Barrier Reef Ecosystem" would capitalize "Ecosystem" because it's part of the proper noun "Great Barrier Reef Ecosystem."
* If it's used in a title: You might capitalize "Ecosystems" in a book title or article title for stylistic reasons.
In most cases, "ecosystems" should remain lowercase.