Cougar population figures in recent years have been compared with estimates from the early 70s as testament that lions numbers have grown out of control. While it is true that lions in North America grew during the 70s and early 80s due to protection, it is still unknown to what degree. Even historical range estimates are unreliable because they relied upon hunter field data, compared with the radio telemetry used today. There is no way to know if lions are expanding their range beyond historic estimates. It is certainly safe to assume that where deer occur in California, so do cougars.
Lions are incapable of decimating a healthy herd of deer.
They can increase mortality in an already decimated herd. Deer populations fluctuate naturally, as do cougar numbers, again according to habitat conditions and food sources. They are overpopulated Nature crops the surplus. Habitat conditions and food supply regulates the numbers. In addition, lions limit their own numbers through territorial behavior.