List five introduced species that present a serious threat to their new communities. explain the damage done by each introduced species.

Cat (Australia); lots of cats became feral and they reproduce very fast. They became the apex predator and the native species have no defense system for them, so the cats brought tens of species on the verge of extinction (mainly placental mammals).
Fox (Australia); became an apex predator in the lack of competition, and damages the populations of native small placental mammals. 
Hair (Australia); reproduces much quicker than the native species of mammals, increased significantly in numbers, and out competes the native species for the food sources.
African bee (the Americas); much more aggressive and stronger than the native bees and systematically kills their populations.
Grey squirrel (Britain); reproduces quicker than the red squirrel, is bigger, and out competes for food, brought it on the verge of extinction.