The lionfish in Florida is a great example of why animals should never be released into any habitat but their natural one. Sometimes this is accidental, but just as often it's intentional. Like the cane toad in Australia.
Anywho, as stated already, they're breeding like rabbits in the absence of natural predators, and consuming everything they can fit in their capacious maws. Reefs are just as dependent on fish as fish are on the reef. Remove all the native fish, and the reef will die.
Even large fish are at risk from the lions because the young are bite sized.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef