Invasion of foreign animals and plants take on many forms. Things as innocuous as small mussels which hitch a ride in the ballast of foreign ships coming into port multiply like wildfire, costing utilities millions of dollars every year to clean their water discharge facilities. Foreign plants do the same and end up spreading into freshwater lakes and streams, choking out native plants and creating navigational hazards to boaters and residents along the waterways contend with tons of floating plants crowding their shores. Parakeets, parrots and foreign bird species are now found in dozens of warmer climate states, competing with native birds and spreading disease, all because someone decided to release them into the wild instead of disposing of them properly.
Another example - fire ants from South America came on a ship to Mobile, AL in the 1950s (IIRC). I remember them as a child in the 1960s in lower AL and now they are tormenting me in south eastern NC.
Zebra mussels came to the great lakes years ago. In Lake St. Clair, they exploded and were a big problem for a number of years but their number tapered off and native fish such as perch were found eating the smaller ones.
They also have had a major positive effect on the clarity of the lake water since they filter out the particles that make the lake dirty........