In the sentence:

> They also live in Argentina, Bolivia and Ecuador where they reside mainly on estuaries, lagoons, mud flats and salt lakes.

we can distinguish two clauses. The **main clause**:

> They also live in Argentina, Bolivia and Ecuador.

and a **subordinate clause**:

> where they reside mainly on estuaries, lagoons, mud flats and salt lakes.

Note that the meaning of the main clause doesn't change if the subordinate clause is removed.

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The following is my understanding of the usage of comma and *"where"* that I have come across:

- **A comma is advised** if the *where*-clause **can be removed** without altering the meaning of the main clause. For example:

  > I first saw him in Paris, where I lived in the early sixties

- **A comma is advised** if the *where*-clause **cannot be removed**. For example:

  > This is where I live

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Hence, in the OP's quote I would use a comma:

> They also live in Argentina, Bolivia and Ecuador, where they reside mainly on estuaries, lagoons, mud flats and salt lakes.