in all eco systems there is a balance between predators and prey, so if you bring in a new plant or animal there will not be the means to keep it in check and they will take over the position of native species very quickly, for example the Austalian Possum in New Zealand has no natural predator and is destroying the forests, the rabbit in Australia has destroyed thousands of acres of land because there are not enough predetors to catch them, the grey squirrel from the USA which was introduced into the UK is taking over and wiping out the native Red Squirrel which is now an endangered species, the same goes for plants aswell.