The intermediate disturbance hypothesis (IDH), that biodiversity peaks at intermediate disturbance, is now contested, because the famous humped curve shows up in only a minority of real systems and its standard mechanism is disputed. What survives is sturdier: some disturbance is a necessary condition for diversity wherever one competitor can take over, and the shape of the curve is set by a single ratio, how often disturbance hits divided by how long the strongest competitor needs to win. ...