The BBC has an article about [alien crabs invading Norway](http://news.bbc.co.uk/2/hi/europe/4775155.stm). The giant red king crabs were introduced to the Russian Barents Sea near the border with Norway in 1960; and since they have no natural enemies in the region they’ve spread wide and far. Norway’s fishermen are calling for increased quotas to fish the crabs, something that doesn’t sound like it would hurt at first glance.
However, the point of the story is that if you mess with nature you’re going to have unintended consequences. Do it very, very carefully.