Ojuice5001

I just wondered--why do new species only cause ecological problems when they are introduced by humans? Armadilloes, possums, and cattle egrets are all relative newcomers whose ranges are still expanding, but you never hear about these animals causing problems (except possums eating human garbage). Why is that?

gallo

Not true. The fauna of South America was primarily made up of marsupials until the late Miocene. The land bridge across Panama allowed the migration of placentals from the north that caused the extinction of most of the South American marsupials. The possum migrated north.

Such replacement events have probably been quite common in the past. Think punctuated equilibria.