Well, from sources like canals, for example.... The Welland Canal still can function as a source for allowing species, once they get into the St. Lawrence River, to bypass Niagara Falls and get in, just like the sea lamprey, the alewife, and a few other things did. Canals breach barriers that would otherwise contain species, unless we've found other ways to help them get out.
On April 25th, 2012. See this statement in context.