Mirror working like instance?

 From:  Tree (TREELOY)
3159.1 
Hey all,

Just a quick little question.

After selecting several objects and mirroring them they seem to work like an instanced object. So when I select parts of the original object(s) and move them, the mirrored objects move in correspondence. Is there a way to "break" this instancing so the objects aren't linked in this way?

Thx in advance