Answer:
Predation
Step-by-step explanation:
The way the question is worded, gives me the effect that the nonnative species of cattails predated the native species and ultimately, lowered their numbers.
I was torn between competition or predation, but it cannot be competition due to the effect it's having on the native species. In competition, there's obvious tension between the two, though in this scenario, the species is just lowered completely. There's a chance it could be competition though, I'm not really understanding the wording of the scenario given; that's just me though.
So, ultimately, it's predation.