Back in New York City, the sheer number of law enforcement officers makes it hard for big gangs to meet openly there the way they did back in the 1980s, so many gang members who have left state prison have migrated north. Less heat, more opportunity.I previously spoke about displacement in this post when I covered a chapter of the book Crime Analysis for Problem Solvers. In the example NPR spoke about, these criminal gangs have undergone geographic displacement in response to sustained law enforcement activity in their traditional territory.
I have heard of people who are skeptical about how big a factor displacement is in law enforcement. More often we see short term displacement as a response to short term enforcement efforts. However, in this case the displacement is more than just a temporary problem.
The Problem Oriented Policing Center has a good publication on this subject called Analyzing Crime Displacement and Diffusion that could help you understand this phenomena.