Great advice.

You'll always find that the "downers" are the ones who are likely to stay with the company, while the real talent leaves in search for something less demotivating and more a pleasant work environment. It's pathetic.

But that said, it's also up to the leader to identify the troublemakers and try and rectify the situation - not just bury their head in the sand and hope it'll all go away. The problem might even stem from the leaders, if they're the reason (via poor leadership) that the downers became downers in the first place! Ultimately, it is up to the person in charge to deal with the situation in the best way possible, with the fewest upsets and negative consequences involved...