You have to thnk like that mathematics guy in Jurassic Park. Operate on the assumption that no living thing can be completely comtrolled, that it will adapt and change behaviour in a different environment and assume the worst possible outcome.
You reintroduce wolves into the UK, and they start by controlling deer population. But the deer are wild and wary and lean. The sheep, on the other hand, are fat and lazy and fenced in. Easy pickings.
Now a border collie can't take a wolf. So farmers will start using Pyrenean Sheepdogs, German Shepherds, Dobermanns and will campaign for the right to buy, and breed, Pit-Bulls. They will invest in lethal electric fences, 8-bore shotguns and petition the government to remove restrictions on high-powered rifles. Any such change in the law will mean making, importing and storing the guns and ammo for sale. Criminals will then steal said guns and ammo to equip themselves and to sell on the black market. Innocent bystanders and policemen get shot. Psychos buy the guns on the black market and shoot up schools, hospitals and shopping centres. Lots of people die.
Meanwhile, because no so-called predator will hunt if it can scavenge, wolves start to move into the towns. Now an urban fox is a relatively small nocturnal hunter. They prey on rats, rummage in bins and dumpsters, and steal your cats' food if you don't have a secure catflap. Worst they do is occasionally murder a cat or small dog that gets too territorial.
Wolves are large carnivores that operate in packs. They will break into shop warehouses and clear them out. They will kill and eat cats and dogs of all sizes. They will kill and eat homeless people they find sleeping in the open, or collapsed junkies and drunks. More people will want guns and big dogs. This willl lead inevitably to more dog attacks. Also, sooner or later, people will start using guns on each other. Mr Average will get up to go to work one morning, fnd that the neighbours' teenage son has parked across his drive for the umpteenth time, and instead of going round and knocking politely at the door to ask for it to be moved, he'll grab the rifle his wife insisted he buy because she's worried about wolves, go over there, shoot the lock off the door and kill as many of the neighbours' family as he can before their mastiff mauls him to death!
It's called the Butterfly Effect.
