These are species that collectively feed at the bottom of the trophic chain. They're feeding on plant material and small biota in the water, so they're going to be very aggressive in the way they compete for those food items. They obviously will have, then, a corresponding impact on other native fish populations around them. They compete for food. They compete for space as well. They're quite a large species.
Between those two things, it's a very.... As David said, where they have become established, they have effectively taken over a lot of the biomass in those systems.