"The difference in energy levels between the two teams was marked, I thought," Pardew is quoted as saying by Clubcall.com. "The emphasis of the game was never changed because of the threat.
"We've got to threaten the game more than we did. If you don't threaten the opposition then they're going to keep coming and that's what Southampton did. It was a great day for them. I thought they played very well, made it very difficult for us.
"That's a performance that we need to make sure never happens again. It was our lack of energy, wherever you were sitting in the stadium, that looked a problem."
Newcastle are back in action next weekend when they take on Manchester United.