An increasing number of studies of invasion effects have pointed to a phenomenon called "invasional meltdown"| in which two or more introduced species interact in such a way that the probability of survival and/or the impact of at least one of them is enhanced (Simberloff and Von Holle 1999). In the above example of an introduced faucet snail (Bithynia tentaculata), vectoring a native trematode parasite of ducks and thereby expanding the trematode's range, a recent twist is the arrival of a European trematode (Leyogonimus polyoon). Bithynia also vectors this species, which has turned out also to be lethal to ducks (Cole and Friend 1999). So in this instance,.