One thing people don't usually think about when it comes to the environment is invasive species. These are the outsider animals brought to a new environment and do a little too well harming local animals--like stray cats in Hawaii. Invasive plants and animals can do astounding amounts of harm to any ecosystem. Invasive species sometimes are brought to a new area accidentally (like rats) or on purpose (like rabbits in Australia brought for purposes of providing something to hunt). Over on io9 they had a list of the worst of the worst invasive species to wreak havoc.
10 of the World's Most Invasive Species