We are a team of five members, each working on his own tasks. These tasks are not necessarily individual, by which I mean that if someone needs help the others can help him without any problem.

Recently, a coworker got a task in which he needed to do a little modification in a task that belongs to me. However, he's leveraging that opportunity to do my whole task. 

He's not qualified to do my tasks. He's a system administrator; I'm a security engineer.

I understand that my tasks are "cooler" than his tasks, but I told him that this task is mine and he just ignores me and keeps doing it.