The mechanisms that maintain biodiversity have long been a central focus of ecology, and this area of research has taken on increased urgency as the pace of global change accelerates. I will discuss the importance of habitat-forming foundation species as key mediators of biodiversity patterns in space and time using one classical ecological concept and one recent extreme event. In classic work conducted on rocky shores, UW's own Bob Paine famously demonstrated that a keystone predator, Pisaster, promoted diversity by preventing a competitively dominant mussel from monopolizing space.