Scientists have discovered that Arctic foxes (Alopex lagopus) introduced to the Aleutian Islands in the 18th century caused a drastic change in the ecosystem, according to this story at ScienceDaily. Islands that once had lush grasslands supported by the "fertilizer" (guano) of seabird populations were changed to tundra once the foxes began making meals of seabird populations. Fox-free islands in the same area have both seabirds and grasslands. The research is published in the journal Science - read the full text (!) here.
Thanks to Val C. for sending in a link to this story.